WO2002101962A2

WO2002101962A2 - Method and system for increased bandwidth efficiency in multiple input - multiple output channels

Info

Publication number: WO2002101962A2
Application number: PCT/US2002/021993
Authority: WO
Inventors: John W. Ketchum
Original assignee: Qualcomm Incorporated
Priority date: 2001-01-05
Filing date: 2002-01-02
Publication date: 2002-12-19
Also published as: CN100586050C; US20040179626A1; US7881360B2; CN1498473A; WO2002101962A3; AU2002330872A1; KR100887909B1; US20030165183A1; US6731668B2; EP1396106A2; TW576034B; KR20030070912A; BR0206314A; JP5248535B2; JP2005502232A; JP2010114940A

Abstract

In one disclosed embodiment, an input bit stream (110) is supplied to a trellis code block (102) . For example, the trellis code block can perform convolutional coding using a rate 6/7 code. The output of the trellis code block is then modulated using, for example, trellis coded quadrature amplitude modulation (104) with 128 signal points or modulation symbols. The sequence of modulation symbols thus generated can be diversity encoded. The diversity encoding can be either a space time encoding, for example, or a space frequency encoding. The sequence of modulation symbols, or the sequence of diversity encoded modulation symbols, is fed to two or more orthogonal Walsh covers (110, 112, 114, 116) for example, replicas of the modulation symbol sequences can be provided to increase diversity, or multiplexing the modulation symbol sequences can be used to increase data transmission rate or 'throughput'. The outputs of the Walsh covers are fed as separate inputs into a communication channel.

Description

METHOD AND SYSTEM FOR INCREASED BANDWIDTH

EFFICIENCY IN MULTIPLE INPUT - MULTIPLE

OUTPUT CHANNELS

BACKGROUND

1. FIELD OF THE INVENTION

The present invention generally relates to the field of wireless

communication systems. More specifically, the invention relates to transmission

for wideband code division multiple access communication systems using

multiple input multiple output channels.

2. RELATED ART

In wireless communication systems several users share a common

communication channel. To avoid conflicts arising from several users

transmitting information over the communication channel at the same time, some

regulation on allocating the available channel capacity to the users is required.

Regulation of user access to the communication channel is achieved by various

forms of multiple access protocols. One form of protocol is known as code

division multiple access (CDMA). In addition to providing multiple access

allocation to a channel of limited capacity, a protocol can serve other functions,

for example, providing isolation of users from each other, i.e. limiting

interference between users, and providing security by making interception and

decoding difficult for a non-intended receiver, also referred to as low probability

of intercept.

In CDMA systems each signal is separated from those of other users by coding the signal. Each user uniquely encodes its information signal into a

transmission signal. The intended receiver, knowing the code sequences of the

user, can decode the transmission signal to receive the information. The

encoding of the information signal spreads its spectrum so that the bandwidth of

the encoded transmission signal is much greater than the original bandwidth of

the information signal. For this reason CDMA is also referred to as "spread

spectrum" modulation or coding. The energy of each user's signal is spread

across the channel bandwidth so that each user's signal appears as noise to the

other users. So long as the decoding process can achieve an adequate signal to

noise ratio, i.e. separation of the desired user's signal from the "noise" of the

other users' signals, the information in the signal can be recovered. Other factors

which affect information recovery of the user's signal are different conditions in

the environment for each subscriber, such as fading due to shadowing and

multipath. Briefly, shadowing is interference caused by a physical object

interrupting the signal transmission path between the transmitter and receiver, for

example, a large building. Multipath is a signal distortion which occurs as a

result of the signal traversing multiple paths of different lengths and arriving at

the receiver at different times. Multipath is also referred to as "time dispersion"

of the communication channel. Multipath fading may also vary with time. For

example, in a communication unit being carried in a moving car, the amount of

multipath fading can vary rapidly.

A number of methods have been implemented to provide effective coding

and decoding of spread spectrum signals. The methods include error detection

and correction codes, and convolutional codes. In wireless communications, especially in voice communications, it is desirable to provide communication

between two users in both directions simultaneously, referred to as duplexing or

full-duplexing. One method used to provide duplexing with CDMA is frequency

division duplexing. In frequency division duplexing, one frequency band is used

for communication from a base station to a mobile user, called the forward

channel, and another frequency band is used for communication from the mobile

user to the base station, called the reverse channel. A forward channel may also

be referred to as a downlink channel, and a reverse channel may also be referred

to as an uplink channel. Specific implementation of coding and modulation may

differ between forward and reverse channels.

The information in the user's signal in the form of digital data is coded to

protect it from errors. Errors may arise, for example, as a result of the effects of

time-varying multipath fading, as discussed above. The coding protects the

digital data from errors by introducing redundancy into the information signal.

Codes used to detect errors are called error detection codes, and codes which are

capable of detecting and correcting errors are called error correction codes. Two

basic types of error detection and correction codes are block codes and

convolutional codes.

Convolutional codes operate by mapping a continuous information

sequence of bits from the digital information of the user's signal into a

continuous encoded sequence of bits for transmission. By way of contrast,

convolutional codes are different from block codes in that information sequences

are not first grouped into distinct blocks and encoded. A convolutional code is

generated by passing the information sequence through a shift register. The shift register contains, in general, N stages with k bits in each stage and n function

generators. The information sequence is shifted through the N stages k bits at a

time, and for each k bits of the information sequence the n function generators

produce n bits of the encoded sequence. The rate of the code is defined as R = k /

n, and is equal to the input rate of user information being coded divided by the

output rate of coded information being transmitted. The number N is called the

constraint length of the code; complexity - or computing cost - of the code

increases exponentially with the constraint length. A convolutional code of

constraint length 9 and code rate 3/4, for example, is used in some CDMA

systems.

The highly structured nature of the mapping of the continuous information

sequence of bits into continuous encoded sequence of bits enables the use of

decoding algorithms for convolutional codes which are considerably different

from those used for block codes. The coding performed by a particular

convolutional code can be represented in various ways. For example, the coding

may be represented by generator polynomials, logic tables, state diagrams, or

trellis diagrams. If the coding is represented by a trellis diagram, for example,

the particular trellis diagram representation will depend on the particular

convolutional code being represented. The trellis diagram representation

depends on the convolutional code in such a way that decoding of the encoded

sequence can be performed if the trellis diagram representation is known.

For signal transmission, convolutional coding may be combined with

modulation in a technique referred to as "trellis coded modulation". Trellis

coded modulation integrates the convolutional coding with signal modulation in such a way that the increased benefit of coding more than offsets the additional

cost of modulating the more complex signal. One way to compare different

methods of signal transmission is to compare the bandwidth efficiency.

Bandwidth efficiency is typically measured by comparing the amount of

information transmitted for a given bandwidth, referred to as the "normalized

data rate", to the SNR per bit. The maximum normalized data rate that can

possibly be achieved for a given SNR per bit is the theoretical maximum capacity

of the channel, referred to as the "Shannon capacity" of the channel. The more

bandwidth efficient a method of signal transmission is, the more nearly it is able

to use the full Shannon capacity of the channel. A channel with multiple transmit

antennas and multiple receive antennas which uses all possible signal paths

between each pair of transmit and receive antennas, referred to as a multiple

input multiple output ("MIMO") channel, is known to have a higher Shannon

capacity under certain channel conditions than a similar channel which uses only

one transmit-receive antenna pair.

For signal reception, the signal must be demodulated and decoded. There

are many methods of decoding convolutional codes, also referred to as

"detection." One method of decoding convolutional codes that uses the trellis

diagram representation is Viterbi decoding. In the trellis diagram, each path

through the trellis corresponds to a possible encoded sequence from the

convolutional coder and the original information sequence that generated the

encoded sequence. The Viterbi algorithm uses the encoded sequence actually

received to determine a value of a metric for some of the paths through the trellis

and to eliminate other paths from consideration. Finally, the decoder chooses a path through the trellis with the most favorable value of the metric, and the

corresponding information sequence is thereby decoded. Thus, the Viterbi

decoder provides maximum likelihood detection, as known in the art.

As stated above, one of the objects of coding is to protect the information

in the user's signal from errors caused by various phenomena, for example,

multipath fading. Another collection of techniques which can be used to increase

signal reliability is referred to as "diversity." Simply stated, diversity exploits the

random nature of radio propagation by supplying to the receiver several replicas

of the same information signal transmitted over independently fading - i.e. highly

uncorrelated - signal paths for communication. For example, if one radio signal

path undergoes a deep fade, another independent path may have a strong signal.

By having more than one path to select from the signal to noise ratio of the

information signal can be improved. One implementation of diversity is the

RAKE receiver, which employs several antennas at the receiver to provide a

selection of different signal paths. A shortcoming of the RAKE receiver is that

its effectiveness breaks down at high data rates. One means of counteracting the

effects of time dispersion or multipath is the use of orthogonal frequency division

multiplexing ("OFDM") as known in the art. OFDM works well at high data

rates and thus avoids the shortcoming of ineffectiveness at high data rates with

the RAKE receiver.

A further collection of techniques which can be used to increase signal

reliability is referred to as "power control". Simply stated, power control adjusts

the power of the signal at the transmitter while the signal is being transmitted in

order to compensate for varying conditions in the communication channel, such as relative movement of different users and multipath fading. Power control

relies on the transmission of information regarding the condition of the channel,

or "channel state mformation" (CSI) from the receiving unit back to the

transmitter. Thus, power control is a CSI technique. There are other CSI

techniques which involve, for example, the use of separate "pilot signals" and

"training periods" of signal transmission. Diversity techniques, on the other hand

are non-CSI techniques, in that no separate transmission of channel condition

information is required for their implementation. In general, non-CSI techniques

can be simpler and less costly to implement because non-CSI techniques avoid

the complexity of transmitting channel state information.

Moreover, non-CSI techniques have an advantage over CSI techniques in

that non-CSI techniques avoid incurring the "overhead" of transmitting channel

state information, i.e. non-user information, on the channel. To the extent that

channel capacity, i.e. the Shannon capacity for a given SNR per bit, is used to

transmit non-user information, i.e. CSI, less channel capacity is available for

transmitting user information, and the effective bandwidth efficiency of the

transmission is, therefore, reduced. A channel which is not stable or for which

channel conditions do not change slowly enough can require transmitting channel

state information at high data rates to keep up with changes in channel condition

in order for the transmitter to be able to make effective use of the channel state

information. Thus, non-CSI techniques can provide an advantage for mobile

communications where channel conditions are subject to rapid change.

The advantages of increased channel capacities of MIMO channels have

been used in conjunction with a number of CSI techniques. The use of non-CSI techniques such as coding, diversity, and OFDM can also be used to improve the

error performance and "throughput", i.e. the data rate of user information, for

wireless communications. Thus, there is a need in the art for taking advantage of

the increased capacity of MIMO channels by increasing the effective bandwidth

efficiency of transmission in MIMO channels while avoiding the disadvantages

of transmitting channel state information. There is also a need in the art to

provide improvements in error performance, data rate, and capacity of wireless

communications in MIMO channels by exploiting increased bandwidth

efficiency.

SUMMARY

The present invention is directed to method and system for increased

bandwidth efficiency in multiple input - multiple output channels. The invention

overcomes the need in the art for increasing bandwidth efficiency while avoiding

the disadvantages of transmitting channel state information. Also the invention

provides improvements in error performance, data rate, and capacity of wireless

communications in multiple input multiple output channels.

In one aspect of the invention an input bit stream is supplied to a trellis

code block. For example, the trellis code block can perform convolutional

coding using a rate 6/7 code. The output of the trellis code block is then

modulated using, for example, trellis coded quadrature amplitude modulation

with 128 signal points or modulation symbols. The sequence of modulation

symbols thus generated can be diversity encoded. The diversity encoding can be

either a space time encoding, for example, or a space frequency encoding. The sequence of modulation symbols, or the sequence of diversity encoded

modulation symbols, is fed to two or more orthogonal Walsh covers. For

example, replicas of the modulation symbol sequences can be provided to

increase diversity, or demultiplexing the modulation symbol sequences can be

used to increase data transmission rate or "throughput". The outputs of the

Walsh covers are fed as separate inputs into a communication channel. The

communication channel can be, for example, a multiple input multiple output

channel in a WCDMA communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates, in block diagram form, one example of

communication channel multiple input orthogonality for one embodiment of the

present invention in an exemplary communication system.

Figure 2 illustrates, in block diagram form, an example of communication

channel multiple input orthogonality for another embodiment of the present

invention in an exemplary communication system.

Figure 3 illustrates in block diagram form, one example of communication

channel multiple input orthogonality with Alamouti transmit diversity for one

embodiment of the present invention in an exemplary communication system.

Figure 4 illustrates, in block diagram form, an example of communication

channel multiple input orthogonality with Alamouti transmit diversity for another

embodiment of the present invention in an exemplary communication system.

Figure 5 illustrates, in block diagram form, an example of receiver

processing for use in conjunction with the examples of multiple input orthogonality given in either of Figures 1 or 2.

Figure 6 illustrates, in block diagram form, an example of receiver

processing for use in conjunction with the examples of multiple input

orthogonality with Alamouti transmit diversity given in either of Figures 3 or 4.

Figure 7 illustrates, in block diagram form, an example of communication

channel multiple input orthogonality and diversity using orthogonal frequency

division multiplexing for one embodiment of the present invention in an

exemplary communication system.

Figure 8 illustrates, in block diagram form, an example of receiver

processing for use in conjunction with the example of multiple input

orthogonality and diversity using orthogonal frequency division multiplexing

given in Figure 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The presently disclosed embodiments are directed to method and system

for increased bandwidth efficiency in multiple input - multiple output channels.

The following description contains specific information pertaining to the

implementation of the presently disclosed embodiments. One skilled in the art

will recognize that the presently disclosed embodiments may be implemented in

a manner different from that specifically discussed in the present application.

Moreover, some of the specific details of the disclosed embodiments are not

discussed in order not to obscure the invention. The specific details not

described in the present application are within the knowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detailed

description are directed to merely example embodiments. To maintain brevity,

other embodiments which use the principles of the present invention are not

specifically described in the present application and are not specifically

illustrated by the present drawings.

Figure 1 illustrates an example of orthogonality of multiple inputs to a

communication channel in accordance with one embodiment. Exemplary system

100 shown in Figure 1 constitutes part of a transmitter which may generally

reside in a base station, gateway, or satellite repeater when communication is

taking place in a forward channel. Exemplary system 100 can be part of a base

station transmitter, for example, in a wideband code division multiple access

(WCDMA) communication system. A WCDMA communication system is also

referred to as a "spread spectrum communication system".

In exemplary system 100 shown in Figure 1, input bit stream 101 contains

the user's signal which includes information that is to be transmitted across the

communication channel. The communication channel can be, for example, radio

frequency transmission between transmit and receive antennas in a wireless

communication system, including all the signal paths between multiple transmit

antennas and multiple receive antennas. In this example, a transmit antenna is

referred to as an input to the communication channel and a receive antenna is

referred to as an output of the communication channel. A communication system

in which there is more than one input or more than one output to a

communication channel is also referred to as a multiple input multiple output ("MIMO") system.

Continuing with Figure 1, input bit stream 101 is supplied to "trellis code"

block 102. Trellis code block 102 performs convolutional coding, as described

above, on input bit stream 101. In one embodiment trellis code block 102

performs convolutional coding with a code rate of the form (n -l)/n, also referred

to as a "rate (n -l)/n trellis code". As stated above, the code rate is equal to the

input rate of information being coded divided by the output rate of coded

information, or equivalently the ratio of the number of bits of input to the number

of bits of output. For example, in one embodiment a rate 6/7 trellis code is used,

thus, there are 7 bits of output for each 6 bits of input to trellis code block 102.

Trellis code block 102 works in conjunction with QAM (quadrature

amplitude modulation) block 104. The combined effect of trellis code block 102

and QAM block 104 is to convert input bit stream 101 into a sequence of

modulation symbols, also referred to as a "modulation symbol sequence".

Modulation symbols can be represented as signal points in a complex phase

space as described in an article titled "Channel Coding with Multilevel/Phase

Signals" by G. Ungerboeck, IEEE Transactions in Information Theory, vol. IT-

28, pp 55 - 67, January 1982. In one embodiment, a trellis coded quadrature

amplitude modulation with 128 modulation symbols, or signal points, is used. A

trellis coded quadrature amplitude modulation with 128 modulation symbols is

also referred to as 128-QAM. Other quadrature amplitude modulations can be

used, for example, 8-QAM and 32-QAM. Moreover, other types of

multilevel/phase modulations can be used, such as pulse amplitude modulation

("PAM"), phase shift keying ("PSK"), or differential phase shift keying ("DPSK"). By using different modulation schemes, that is, varying the

modulation scheme during operation of the system, different data rates or

throughputs can be supported at a tradeoff in reliability. For example, 8-QAM

can be used to increase the error reliability of signal transmission at a given

signal transmit power but at a lower data transmission rate in comparison to 128-

QAM or 32-QAM, and conversely, 32-QAM can be used to increase throughput

or data rate at the cost of decreased reliability in error performance for the same

signal power compared to 8-QAM. The choice of modulation can be used in

conjunction with choice of Walsh functions, discussed below, to further enhance

the different combinations of throughput and reliability which can be achieved.

As shown in Figure 1, the modulation symbol sequence from QAM block

104 is fed to each of Walsh covers 110, 112, 114, and 116. Walsh cover 110 is

labeled "Walsh 1"; Walsh cover 112 is labeled "Walsh 2"; Walsh cover 114 is

labeled "Walsh 3"; and Walsh cover 116is labeled "Walsh 4". Each of Walsh

covers 110, 112, 114, and 116 is labeled differently to indicate that a distinct

Walsh function is used by each Walsh cover to achieve orthogonality of the four

outputs of Walsh covers 110, 112, 114, and 116.

By way of background, in WCDMA communications, each distinct signal

is spread to allow many signals to simultaneously occupy the same bandwidth

without significantly interfering with one another. The signals can originate from

different users, or as in the present example shown in Figure 1, the signal can be

replicated, for example, for the purpose of achieving diversity, as explained

above. One means of spreading is the application of distinct "orthogonal"

spreading codes or functions, such as Walsh functions, to each distinct signal. "Orthogonality" refers to lack of correlation between the spreading functions. In

a given spread spectrum communication system using Walsh functions (also

called Walsh code sequences), a pre-defined Walsh function matrix having n

rows of n chips each is established in advance to define the different Walsh

functions to be used to distinguish different distinct signals. In the present

example, each distinct replica of the modulation symbol sequence is assigned a

distinct Walsh function. In other words, each distinct replica of the output signal

from QAM block 104 is coded by a distinct Walsh code sequence in order to

separate each distinct signal from the others.

The general principles of CDMA communication systems, and in

particular the general principles for generation of spread spectrum signals for

transmission over a communication channel is described in U.S. patent 4,901,307

entitled "Spread Spectrum Multiple Access Communication System Using

Satellite or Terrestrial Repeaters" and assigned to the assignee of the present

invention. The disclosure in that patent, i.e. U.S. patent 4,901,307, is hereby

fully incorporated by reference into the present application. Moreover, U.S.

patent 5,103,459 entitled "System and Method for Generating Signal Waveforms

in a CDMA Cellular Telephone System" and assigned to the assignee of the

present invention, discloses principles related to PN spreading, Walsh covering,

and techniques to generate CDMA spread spectrum communication signals. The

disclosure in that patent, i.e. U.S. patent 5,103,459, is also hereby fully

incoφorated by reference into the present application. Further, the presently

disclosed embodiment utilizes time multiplexing of data and various principles

related to "high data rate" communication systems, and the presently disclosed embodiments can be used in "high data rate" communication systems, such as

that disclosed in U.S. patent application entitled "Method and Apparatus for High

Rate Packet Data Transmission" Serial No. 08/963,386 filed on November 3,

1997, and assigned to the assignee of the present invention. The disclosure in

that patent application is also hereby fully incorporated by reference into the

present application.

A Walsh function having n rows of n chips each is also referred to as a

Walsh function matrix of order n. An example of a Walsh function matrix where

n is equal to 4, i.e. a Walsh function matrix of order 4, is shown below:

In this example, there are 4 Walsh functions, each function having 4 chips.

Each Walsh function is one row in the above Walsh function matrix. For

example, the second row of the Walsh function matrix is the Walsh function

having the sequence 1, 0, 1, 0. It is seen that each Walsh function, i.e. each row

in the above matrix, has zero correlation with any other row in the matrix. Stated

differently, exactly half of the chips in every Walsh function differ from those in

every other Walsh function in the matrix.

Application of distinct orthogonal spreading functions, such as Walsh

functions, to each distinct signal results in a transformation of each modulation

symbol in each distinct signal into a respective spread sequence of output chips,

where each spread sequence of output chips is orthogonal with every other spread sequence of output chips. Using Walsh functions, the transformation can be

performed by XOR'ing each modulation symbol in each distinct signal with a

sequence of chips in a particular Walsh function. Using the second Walsh

function in the above example, i.e. the second row of the matrix, and XOR'ing a

modulation symbol of "a" with the second row of the matrix results in the spread

sequence of output chips: a , a , a , a , where "a" denotes the binary

complement of a. Thus, in this illustrative example, each modulation symbol is

spread into a spread sequence of output chips having a length of 4. The number

of output chips produced for each input modulation symbol is called the

spreading factor; in this illustrative example, the spreading factor is 4. In

practice, Walsh functions of length 2 to 512 (i.e. Walsh functions having from 2

to 512chips in each Walsh code sequence) are used. Thus, spreading factors may

range from 2 to 512 in practice.

Output 120 of Walsh cover 110, output 122 of Walsh cover 112, output

124 of Walsh cover 114, and output 126 of Walsh cover 116 are, thus, spread

sequences of output chips which are mutually orthogonal. Each of outputs 120,

122, 124, and 126 is input separately to the communication channel. For

example, each spread sequence of output chips can be passed on to an FIR

("finite duration impulse response") filter used for pulse shaping signals prior to

their transmission using a separate antenna for each signal input into the

communication channel. A receiver or receivers at the output of the

communication channel, which may use a single antenna or multiple antennas,

receives the signals. For example, the received signals can be passed through a

receive FIR filter and then to a Walsh de-cover. The Walsh de-cover de-spreads the distinct spread sequences of output chips - i.e., undoes the Walsh function

spreading - by applying the distinct inverse functions of the original distinct

Walsh function spreadings. Returning to the example used above (which has a

spreading factor of 4), the second row of the matrix, 1, 0, 1, 0, is again XOR'ed

to the sequence of output chips, a , a , a , a , to produce the data symbol

sequence a , a , a , a , and thus the de-spread data symbol is "a", the original

input data symbol. Thus, the original modulation symbol sequence can be

recovered.

The modulation symbol sequence is input to a maximum likelihood

decoder, which can be, for example, a Viterbi decoder. Due to imperfections in

the communication channel, the received modulation symbol sequence may not

be exactly identical to the original modulation symbol sequence that was input to

the communication channel. Simply stated, maximum likelihood detection

determines a valid modulation symbol sequence that is most likely to have

produced the modulation symbol sequence that is received by the maximum

likelihood decoder. Thus, given a modulation symbol sequence, the maximum

likelihood decoder determines a "best estimate" of the original modulation

symbol sequence and decodes the best estimate into an output information

sequence. Because the best estimate is used, the output information sequence

contains a minimal number of errors in comparison to the original information

transmitted.

Thus, Figure 1 shows one example of a system that can be used to provide

greater reliability of communications through use of orthogonal transmit

diversity in a multiple input multiple output communication channel. Figure 2 illustrates an example of orthogonality of multiple inputs to a

communication channel in accordance with another embodiment. Exemplary

system 200 shown in Figure 2 constitutes part of a transmitter which may

generally reside in a base station, gateway, or satellite repeater when

communication is taking place in a forward channel. Exemplary system 200 can

be part of a base station transmitter, for example, in a WCDMA communication

system or spread spectrum communication system.

In exemplary system 200 shown in Figure 2, input bit stream 201 contains

the signal of one or more users. The signal includes information that is to be

transmitted across the communication channel. The communication channel can

be, for example, radio frequency transmission between transmit and receive

antennas in a wireless communication system comprising a multiple input

multiple output, or MIMO, channel as described above.

Continuing with Figure 2, input bit stream 201 is supplied to "trellis code"

block 202. Trellis code block 202 performs convolutional coding, as described

above, on input bit stream 201. In one embodiment described here, trellis code

block 202 performs rate (n -l)/n trellis coding on input bit stream 201. For

example, in one embodiment a rate 6/7 trellis code is used.

Trellis code block 202 works in conjunction with QAM block 204. The

combined effect of trellis code block 202 and QAM block 204 is to convert input

bit stream 201 into a modulation symbol sequence, in which modulation symbols

can be represented as signal points in a complex phase space as stated above. In

one embodiment, a trellis coded quadrature amplitude modulation with 128

modulation symbols, or 128-QAM, is used. As discussed above, other modulations can be used, and in conjunction with different Walsh functions,

multiple combinations of throughput and reliability can be supported.

As shown in Figure 2, a portion of the modulation symbol sequence from

QAM block 204 is fed to each of Walsh covers 210, 212, 214, and 216 in turn, by

time demultiplexing the modulation symbol sequence. That is, a separate

modulation symbol sequence is simultaneously fed to each of Walsh covers 210,

212, 214, and 216. Because the capacity of the communication channel is still

the same as for the example of Figure 1, each separate Walsh cover can still input

spread sequences to the communication channel at the same rate. To compensate

for the increased amount of information being fed to the communication channel,

the output rate of trellis code block 202 and QAM block 204 is increased, or

"speeds up", by a factor of four to feed each separate Walsh cover its distinct

modulation symbol sequence. Thus, the maximum information throughput rate

in the example of Figure 2 is increased by a factor of four over that in the

example of Figure 1. There is a corresponding loss, however, of diversity in the

example of Figure 2 compared to the example of Figure 1 where four replicas of

the modulation symbol sequence are simultaneously transmitted over the

communication channel. In other words, some of the increased reliability in the

example of Figure 1 is traded for increased information throughput, or data rate,

in the example of Figure 2.

Walsh cover 210 is labeled "Walsh 1"; Walsh cover 212 is labeled "Walsh

2"; Walsh cover 214 is labeled "Walsh 3"; and Walsh cover 216is labeled

"Walsh 4". Each of Walsh covers 210, 212, 214, and 216 is labeled differently to

indicate that a distinct Walsh function is used by each Walsh cover to achieve orthogonality of the four outputs of Walsh covers 210, 212, 214, and 216.

Output 220 of Walsh cover 210, output 222 of Walsh cover 212, output 224 of

Walsh cover 214, and output 226 of Walsh cover 216 are, thus, spread sequences

of output chips which are mutually orthogonal. Each of outputs 220, 222, 224,

and 226 is input separately to the communication channel. For example, each

spread sequence of output chips can be passed on to an FIR filter used for pulse

shaping signals prior to their transmission using a separate antenna for each

signal input into the communication channel. A receiver or receivers at the

output of the communication channel, which may use a single antenna or

multiple antennas, receives the signals, passes the signals through receive FIR

filters, Walsh de-covers the signals, and decodes the modulation symbol

sequence using maximum likelihood decoding as described above.

Thus, Figure 2 shows an example of a system that can be used to provide

greater reliability of communications and increased data rate, with a trade off

between increased reliability and increased data rate, through use of orthogonal

transmit diversity in a multiple input multiple output communication channel.

Figure 3 illustrates an example of orthogonality with Alamouti transmit

diversity of multiple inputs to a communication channel in accordance with one

embodiment. Exemplary system 300 shown in Figure 3 constitutes part of a

transmitter which may generally reside in a base station, gateway, or satellite

repeater when communication is taking place in a forward channel. Exemplary

system 300 can be part of a base station transmitter, for example, in a WCDMA

communication system or spread spectrum communication system.

In exemplary system 300 shown in Figure 3, input bit stream 301 contains the user's signal which includes information that is to be transmitted across the

communication channel. The communication channel can be, for example, radio

frequency transmission between transmit and receive antennas in a wireless

communication system comprising a multiple input multiple output, or MIMO,

channel as described above.

Continuing with Figure 3, input bit stream 301 is supplied to "trellis code"

block 302. Trellis code block 302 performs convolutional coding, as described

above, on input bit stream 301. In one embodiment described here, trellis code

block 302 performs rate (n -l)/n trellis coding on input bit stream 301. For

example, in one embodiment a rate 6/7 trellis code is used.

Trellis code block 302 works in conjunction with QAM block 304. The

combined effect of trellis code block 302 and QAM block 304 is to convert input

bit stream 301 into a modulation symbol sequence, in which modulation symbols

one embodiment, a trellis coded quadrature amplitude modulation with 128

modulation symbols, or 128-QAM, is used. As discussed above, other

modulations can be used, and in conjunction with different Walsh functions,

multiple combinations of throughput and reliability can be supported.

As shown in Figure 3, the modulation symbol sequence from QAM block

304 is provided to each of Alamouti blocks 306 and 308. The input and output of

each of Alamouti blocks 306 and 308 is identical. As indicated in Figure 3, the

modulation symbol sequence from QAM block 304 is alternately passed to the

"A" inputs of Alamouti blocks 306 and 308 and then to the "B" inputs of

Alamouti blocks 306 and 308. Thus, the modulation symbol sequence from QAM block 304 is effectively grouped into input groups of symbols, each input

group containing 2 symbols, referred to in the present example as an "A symbol"

and a "B symbol". Each of Alamouti blocks 306 and 308 has a first and a second

output. For each input group of modulation symbols, the first output of each of

Alamouti blocks 306 and 308 is the A symbol followed in sequence by the

complex conjugate of the B symbol, indicated in Figure 3 by the notation "(A,

B*)". Recall that each modulation symbol is represented as a signal point in

complex phase space, i.e. as a complex number. For each input group of

modulation symbols, the second output of each of Alamouti blocks 306 and 308

is the B symbol followed in sequence by the negative complex conjugate of the A

symbol, indicated in Figure 3 by the notation "(B, -A*)". The theoretical

justification and advantages of this scheme are described in an article titled "A

Simple Transmit Diversity Technique for Wireless Communications" by S. M.

Alamouti, IEEE Journal on Select Areas in Communications, vol. 16, no. 8, pp

1451 - 58, October 1998. The scheme of diversity encoding described in the

present example is referred to as space time encoding. As described in the article

referenced here, space frequency encoding can also be used in the present

example, as is apparent to a person of ordinary skill in the art. The first output of

Alamouti block 306 is fed to Walsh cover 310. The second output of Alamouti

block 306 is fed to Walsh cover 312. The first output of Alamouti block 308 is

fed to Walsh cover 314. The second output of Alamouti block 308 is fed to

Walsh cover 316.

Walsh cover 310 is labeled "Walsh 1"; Walsh cover 312 is also labeled

"Walsh 1"; Walsh cover 314 is labeled "Walsh 2"; and Walsh cover 316is also labeled "Walsh 2". Each of Walsh covers 310 and 312, is labeled identically,

"Walsh 1", to indicate that the same Walsh function is used to spread both

outputs of Alamouti block 306. Similarly, each of Walsh covers 314 and 316, is

labeled identically, "Walsh 2", to indicate that the same Walsh function is used to

spread both outputs of Alamouti block 308. In other words, both Walsh covers in

each pair of Walsh covers use the same Walsh function. Walsh covers 310 and

312 are labeled differently from Walsh covers 314 and 316 to indicate that

distinct Walsh functions are used by each pair of Walsh covers to achieve

pairwise mutual orthogonality of the four outputs of Walsh covers 310, 312, 314,

and 316. In other words, pairwise mutual orthogonality refers to the condition

that each pair of Walsh covers is orthogonal with every other pair of Walsh

covers. Output 320 of Walsh cover 310, output 322 of Walsh cover 312, output

324 of Walsh cover 314, and output 326 of Walsh cover 316 are, thus, spread

sequences of output chips for which the pair of outputs 320 and 322 are

orthogonal to the pair of outputs 324 and 326. Each of outputs 320, 322, 324,

and 326 is input separately to the communication channel. For example, each

shaping signals prior to their transmission using a separate antenna for each

signal input into the communication channel. A receiver or receivers at the

output of the communication channel, which may use a single antenna or

multiple antennas, receives the signals, passes the signals through receive FIR

filters, Walsh de-covers the signals, and decodes the modulation symbol

sequence using maximum likelihood decoding as described above.

The maximum information throughput rate in the example of Figure 3 is increased by a factor of two over that in the example of Figure 1. Moreover, due

to the use of Alamouti diversity encoding in the example of Figure 3, there is an

improvement in diversity compared to the example of Figure 2. Thus, Figure 3

shows an example of a system that can be used to provide greater reliability of

communications and increased data rate, with a trade off between increased

reliability and increased data rate, through use of orthogonal transmit diversity in

a multiple input multiple output communication channel.

Figure 4 illustrates an example of orthogonality with Alamouti transmit

diversity of multiple inputs to a communication channel in accordance with

another embodiment. Exemplary system 400 shown in Figure 4 constitutes part

of a transmitter which may generally reside in a base station, gateway, or satellite

repeater when communication is taking place in a forward channel. Exemplary

system 400 can be part of a base station transmitter, for example, in a WCDMA

communication system or spread spectrum communication system.

In exemplary system 400 shown in Figure 4, input bit stream 401 contains

the signal of one or more users which includes information that is to be

transmitted across the communication channel. The communication channel can

be, for example, radio frequency transmission between transmit and receive

antennas in a wireless communication system comprising a multiple input

multiple output, or MIMO, channel as described above.

Continuing with Figure 4, input bit stream 401 is supplied to "trellis code"

block 402. Trellis code block 402 performs convolutional coding, as described

above, on input bit stream 401. In one embodiment described here, trellis code

block 402 performs rate (n -l)/n trellis coding on input bit stream 401. For example, in one embodiment a rate 6/7 trellis code is used.

Trellis code block 402 works in conjunction with QAM block 404. The

combined effect of trellis code block 402 and QAM block 404 is to convert input

bit stream 401 into a modulation symbol sequence, in which modulation symbols

one embodiment, a trellis coded quadrature amplitude modulation with 128

modulation symbols, or 128-QAM, is used. As discussed above, other

modulations can be used, and in conjunction with different Walsh functions,

multiple combinations of throughput and reliability can be supported.

As shown in Figure 4, a portion of the modulation symbol sequence from

QAM block 404 is fed to each of Alamouti blocks 406 and 408 in turn, by time

demultiplexing the modulation symbol sequence. That is, a separate modulation

symbol sequence is simultaneously fed to each input of each of Alamouti blocks

406 and 408. As indicated in Figure 4, the modulation symbol sequence from

QAM block 404 is alternately passed to the "A" and "B" inputs of Alamouti

block 406 and then to the "C" and "D" inputs of Alamouti block 408. Thus, the

modulation symbol sequence from QAM block 404 is effectively grouped into

input groups of symbols, each input group containing 4 symbols, referred to in

the present example as an "A symbol", a "B symbol", a "C symbol", and a "D

symbol".

Each of Alamouti blocks 406 and 408 has a first and a second output. For

each input group of modulation symbols, the first output of Alamouti block 406

is the A symbol followed in sequence by the complex conjugate of the B symbol,

indicated in Figure 4 by the notation "(A, B*)". Recall that each modulation symbol is represented as a signal point in complex phase space, i.e. as a complex

number. For each input group of modulation symbols, the second output of

Alamouti block 406 is the B symbol followed in sequence by the negative

complex conjugate of the A symbol, indicated in Figure 4 by the notation "(B, -

A*)". For each input group of modulation symbols, the first output of Alamouti

block 408 is the C symbol followed in sequence by the complex conjugate of the

D symbol, indicated in Figure 4 by the notation "(C, D*)". For each input group

of modulation symbols, the second output of Alamouti block 408 is the D symbol

followed in sequence by the negative complex conjugate of the C symbol,

indicated in Figure 4 by the notation "(D, -C*)". The scheme of diversity

encoding described in the present example is thus a variant of the space time

encoding described in the above-referenced article by S. M. Alamouti. As

described in the above-referenced article, space frequency encoding can also be

used in the present example, as is apparent to a person of ordinary skill in the art.

The first output of Alamouti block 406 is fed to Walsh cover 410. The second

output of Alamouti block 406 is fed to Walsh cover 412. The first output of

Alamouti block 408 is fed to Walsh cover 414. The second output of Alamouti

block 408 is fed to Walsh cover 416.

Walsh cover 410 is labeled "Walsh 1"; Walsh cover 412 is also labeled

"Walsh 1"; Walsh cover 414 is labeled "Walsh 2"; and Walsh cover 416is also

labeled "Walsh 2". Each of Walsh covers 410 and 412, is labeled identically,

"Walsh 1", to indicate that the same Walsh function is used to spread both

outputs of Alamouti block 406. Similarly, each of Walsh covers 414 and 416, is

labeled identically, "Walsh 2", to indicate that the same Walsh function is used to spread both outputs of Alamouti block 408. Walsh covers 410 and 412 are

labeled differently from Walsh covers 414 and 416 to indicate that distinct Walsh

functions are used by each pair of Walsh covers to achieve pairwise

orthogonality of the four outputs of Walsh covers 410, 412, 414, and 416.

Output 420 of Walsh cover 410, output 422 of Walsh cover 412, output 424 of

Walsh cover 414, and output 426 of Walsh cover 416 are, thus, spread sequences

of output chips for which the pair of outputs 420 and 422 are orthogonal to the

pair of outputs 424 and 426. Each of outputs 420, 422, 424, and 426 is input

separately to the communication channel. For example, each spread sequence of

output chips can be passed on to an FIR filter used for pulse shaping signals prior

to their transmission using a separate antenna for each signal input into the

communication channel. A receiver or receivers at the output of the

communication channel, which may use a single antenna or multiple antennas,

receives the signals, passes the signals through receive FIR filters, Walsh de-

covers the signals, and decodes the modulation symbol sequence using maximum

likelihood decoding as described above.

The maximum information throughput rate in the example of Figure 4 is

increased by a factor of two over that in the example of Figure 3. Thus, the

information throughput rate in the example of Figure 4 is the equal of the

information throughput rate in the example of Figure 2. Moreover, due to the use

of Alamouti diversity encoding in the example of Figure 4, there is an

improvement in diversity compared to the example of Figure 2. Thus, Figure 4

shows an example of a system that can be used to provide greater reliability of

communications and increased data rate, with a trade off between increased reliability and increased data rate, through use of orthogonal transmit diversity in

a multiple input multiple output communication channel.

Figure 5 illustrates an example of receiver processing in accordance with

one embodiment. Exemplary system 500 shown in Figure 5 constitutes part of a

receiver which may generally reside in a subscriber unit or mobile unit when

communication is taking place in a forward channel. Exemplary system 500 can

be part of a subscriber unit receiver, for example, in a WCDMA communication

system or spread spectrum communication system.

In exemplary system 500 shown in Figure 5, input signal 501 is received

from a communication channel with 4 inputs. The communication channel can

be, for example, radio frequency transmission between multiple transmit and

receive antennas in a wireless communication system comprising a multiple input

multiple output, or MIMO, channel as described above. For a channel with 4

receive antennas, for example, a "copy" of exemplary system 500 would be

present for each separate receive antenna.

Continuing with Figure 5, the receive processing shown in Figure 5 is

intended to be used with either of the transmit processes used in Figure 1 or

Figure 2. Thus, input signal 501 comprises a composite signal which has been

Walsh covered by each of Walsh 1, Walsh 2, Walsh 3, and Walsh 4 of either

Figure 1 or Figure 2. Input signal 501 is passed to each of Walsh decovers 512,

514, 516, and 518.

Walsh decover 512 is labeled "Walsh 1"; Walsh decover 514 is labeled

"Walsh 2"; Walsh decover 516 is labeled "Walsh 3"; and Walsh decover 518

labeled "Walsh 4". Each of Walsh decovers 512, 514, 516, and 518 is labeled so as to indicate that the same distinct Walsh function is used by each Walsh

decover to decover the corresponding transmit signal from Figure 1 or Figure 2.

Thus, the four outputs of Walsh decovers 512, 514, 516, and 518 correspond to 4

replicas of the single modulation symbol sequence that was transmitted in the

case of Figure 1, or alternatively, to 4 distinct modulation symbol sequences that

were transmitted in the case of Figure 2. The replica or distinct modulation

symbol sequence output by Walsh decover 512 is passed to metric generation

block 522, the output of Walsh decover 514 is passed to metric generation block

524, and so forth as indicated in Figure 5.

Each of metric generation blocks 522, 524, 526, and 528 performs metric

generation, also referred to as "path metric" generation or "branch metric"

generation, for input to a trellis decoder, which can be a Viterbi decoder, for

example. For example, each of metric generation blocks 522, 524, 526, and 528

can multiply each output of the Walsh decover operation by the complex

conjugate of each possible transmitted modulation symbol, then multiply by the

complex conjugate of the estimated complex path gain, take twice the real part of

the result, and subtract a bias term, comprising the square of the magnitude of the

path gain of the channel times the square of the magnitude of the possible

transmitted modulation symbol, to produce a metric value for input to the trellis

decoder. As shown in Figure 5, the metric values output by metric generation

blocks 522, 524, 526, and 528 are all fed into trellis decode block 532. Output

533 of trellis decode block 532 is a maximum likelihood estimate, as discussed

above, of the original input bit stream of one or more users, that is, input bit

stream 101 in the case of Figure 1 or input bit stream 201 in the case of Figure 2. Thus, Figure 5 shows an example of receiver processing for utilization of signal

orthogonality in a multiple input multiple output communication channel.

Figure 6 illustrates an example of receiver processing in accordance with

one embodiment. Exemplary system 600 shown in Figure 6 constitutes part of a

receiver which may generally reside in a subscriber unit or mobile unit when

communication is taking place in a forward channel. Exemplary system 600 can

be part of a subscriber unit receiver, for example, in a WCDMA communication

system or spread spectrum communication system.

In exemplary system 600 shown in Figure 6, input signal 601 is received

from a communication channel with 4 inputs. The communication channel can

be, for example, radio frequency transmission between multiple transmit and

receive antennas in a wireless communication system comprising a multiple input

multiple output, or MIMO, channel as described above. For a channel with 4

receive antennas, for example, a "copy" of exemplary system 600 would be

present for each separate receive antenna.

Continuing with Figure 6, the receive processing shown in Figure 6 is

intended to be used with either of the transmit processes used in Figure 3 or

Figure 4. Thus, input signal 601 comprises a composite signal which has been

Walsh covered by each of Walsh 1, and Walsh 2 of either Figure 3 or Figure 4.

Input signal 601 is passed to each of Walsh decovers 602 and 604.

Walsh decover 602 is labeled "Walsh 1", and Walsh decover 604 is

labeled "Walsh 2". Each of Walsh decovers 602 and 604 is labeled so as to

indicate that the same distinct Walsh function is used by each Walsh decover to

decover the corresponding transmit signal from Figure 3 or Figure 4. Thus, the two outputs of Walsh decovers 602 and 604 correspond to a received symbol pair

from which each symbol of the received symbol pair can be estimated using the

Alamouti technique referenced above.

By way of brief illustration, the symbol pair (A, B*) shown in Figure 3

can be transmitted with a channel path gain of hi so that the pair "becomes" (hiA,

hiB*) and the symbol pair (B, -A*) shown in Figure 3 can be transmitted with a

channel path gain of h₂ so that the pair "becomes" (h₂B, - h₂A*). During the first

time interval a certain amount of noise n(l) from the channel gets added to the

received signal in the first time interval, referred to as r(l). The received signal

in the first time interval is then:

r(l) = h₁A + h₂B + n(l).

During the second time interval a certain amount of noise n(2) from the

channel gets added to the received signal in the second time interval, referred to

as r(2). The received signal in the second time interval is then:

With the aid of signal delay elements, for example, to make the values of

r(l) and r(2) available at the same point in time, the blocks marked "first symbol

Alamouti estimate" perform the following algebraic operations on r(l) and r(2).

First symbol estimate = hj*r(l) - h₂r*(2).

Similarly, the blocks marked "second symbol Alamouti estimate" perform

the following algebraic operations on r(l) and r(2).

Second symbol estimate = h₂*r(l) + l^r*^).

It can be shown using the algebra of complex numbers that:

First symbol estimate = (|h_!|² + |h₂|²)A + hι*n(l) + h₂n(2), and Second symbol estimate = (|h_ϊ|² + |h₂|²)B + h₂*n(l) + hjn(2).

Thus, "first symbol estimate" is an estimate of the symbol "A" and

"second symbol estimate" is an estimate of the symbol "B". Each estimate

contains a bias, (|hι|² + | h₂|²), which can be offset by metric generation blocks

622, 624, 626, and 628.

In the case where the transmit process of Figure 3 is used, both Walsh 1

and Walsh 2 cover the symbol pair A, B. Thus, output symbol S_x of first symbol

Alamouti estimate block 612 shown in Figure 6 is an estimate of symbol "A";

output symbol S₂ of second symbol Alamouti estimate block 614is an estimate of

symbol "B"; output symbol S₃ is an estimate of symbol "A"; and output symbol

S₄ is an estimate of symbol "B", in the case of Figure 3.

In the case where the transmit process of Figure 4 is used, Walsh 1 covers

the symbol pair A, B and Walsh 2 covers the symbol pair C, D. Thus, output

symbol Si of first symbol Alamouti estimate block 612 shown in Figure 6 is an

estimate of symbol "A"; output symbol S₂ of second symbol Alamouti estimate

block 614 is an estimate of symbol "B"; output symbol S₃ of first symbol

Alamouti estimate block 616 is an estimate of symbol "C"; and output symbol S

of second symbol Alamouti estimate block 618 is an estimate of symbol "D", in

the case of Figure 4.

The symbol estimates are passed to each of metric generation blocks 622,

624, 626, and 628 respectively as indicated in Figure 6. Each of metric

generation blocks 622, 624, 626, and 628 performs metric generation, also

referred to as "path metric" generation or "branch metric" generation, for input to

a trellis decoder, which can be a Viterbi decoder, for example. For example, each of metric generation blocks 622, 624, 626, and 628 can multiply each

modulation symbol estimate in the sequence of symbol estimates generated by

each of the Alamouti estimate blocks by the complex conjugate of each possible

transmitted modulation symbol, take twice the real part of the result, and subtract

a bias term, comprising ( i ² + |h₂|²) times the square of the magnitude of the

possible transmitted modulation symbol, to produce a metric value for input to

the trellis decoder. As shown in Figure 6, the metric values output by metric

generation blocks 622, 624, 626, and 628 are all fed into trellis decode block 632.

Output 633 of trellis decode block 632 is a maximum likelihood estimate, as

discussed above, of either the original user input bit stream in the case of Figure

3 or multiple users' input bit streams in the case of Figure 4. Thus, Figure 6

shows an example of receiver processing for utilization of signal orthogonality

with Alamouti transmit diversity in a multiple input multiple output

communication channel.

Figure 7 shows an example of how OFDM can be used in one

embodiment. Exemplary system 700 shown in Figure 7 constitutes part of a

transmitter which may generally reside in a base station, gateway, or satellite

repeater when communication is taking place in a forward channel. Exemplary

system 700 can be part of a base station transmitter, for example, in a WCDMA

communication system or spread spectrum communication system.

In exemplary system 700 shown in Figure 7, input bit stream 701 contains

the signal of one or more users. The signal includes information that is to be

transmitted across the communication channel. The communication channel can

be, for example, radio frequency transmission between transmit and receive antennas in a wireless communication system comprising a multiple input

multiple output, or MIMO, channel as described above. As shown in Figure 7,

input bit stream 701 is supplied to "trellis coded QAM" block 702. Trellis coded

QAM block 202 performs convolutional coding and quadrature amplitude

modulation, as described above in relation to Figures 1 and 2, on input bit stream

701.

Continuing with Figure 7, the modulation symbol sequence from trellis

coded QAM block 202 is fed to "frequency coding" block 704. Frequency

coding block 704 feeds the modulation symbol sequence to each of

Walsh/Alamouti blocks 712, 714, and 716 by time demultiplexing the

modulation symbol sequence. That is, a portion of the modulation symbol

sequence is simultaneously fed to each of Walsh Alamouti blocks 712, 714, and

716 at a separate distinct frequency for each Walsh Alamouti block. Each

distinct separate frequency is also referred to as a "frequency bin". The example

shown in Figure 7 uses only 3 separate frequencies or frequency bins for the sake

of simplicity and brevity in the present illustrative example. Thus, the example

of Figure 7 shows only 3 Walsh/Alamouti blocks, one for each frequency bin. In

practice any desired number of frequency bins can be used based on practical

considerations. The number of Walsh/Alamouti blocks can be adjusted

accordingly, as is apparent to a person of ordinary skill in the art.

Moreover, the present example is based on straightforward demultiplexing

of the modulation symbol sequence. That is, a symbol from the modulation

symbol sequence is coded into the first frequency bin, the subsequent symbol is

coded into the second frequency bin, the next subsequent symbol is coded into the third frequency bin, and the next subsequent symbol is coded once again into

the first frequency bin, and so forth for each subsequent symbol. Many

variations on this scheme are possible, for example, a replica of the entire

modulation symbol sequence can be coded into each frequency bin. Any of a

number of other techniques known in the art can also be used, such as symbol

repetition and symbol interleaving, for example. The details of implementing

these techniques are apparent to a person of ordinary skill in the art, and are

therefore not presented here.

Continuing with Figure 7, each of Walsh/Alamouti blocks performs the

processing subsequent to trellis coded modulation as described above in

connection with any of Figures 1 through 4. For example, if the processing of

Figure 4 were chosen for each of Walsh/Alamouti blocks 712, 714, and 716, then

each of Walsh/Alamouti blocks 712, 714, and 716 performs the processing

indicated in Figure 4 for Alamouti blocks 406 and 408, and Walsh covers 410,

412, 414, and 416. For example, in the case of Walsh/Alamouti block 712, the

modulation symbol sequence that is input to Walsh/Alamouti block 712 is first

demultiplexed into two Alamouti blocks corresponding to Alamouti block 406

and Alamouti block 408 of Figure 4. The output of Walsh/Alamouti block 712

is, thus, the output of 4 Walsh covers corresponding to Walsh covers 410, 412,

414, and 416. Thus, the output of Walsh/Alamouti block 712 is 4 pairwise

orthogonal spread sequences of output chips as described above in relation to

Figure 4. Thus, the orthogonal transmit diversity technique, in any of the forms

described above, is applied to the modulation symbol sequence for each

frequency bin. The four outputs of each of Walsh/Alamouti blocks 712, 714, and 716 are

fed to each of "inverse FFT and cyclic prefix" blocks 722, 724, 726, and 728.

That is, each of the four inverse FFT and cyclic prefix blocks 722, 724, 726, and

728 has 3 distinct spread sequences of output chips, one for each frequency bin,

as input. The present example, as with the examples of Figures 1 through 4,

assumes that four separate antennas are used as inputs to the multiple input

multiple output communication channel. It is manifest that any practical number

of inputs to the multiple input multiple output channel can be used. For example,

a channel with 8 inputs would require 8 inverse FFT and cyclic prefix blocks,

and each Walsh/Alamouti block would be required to provide 8 outputs. The

required changes to the examples given are apparent to a person of ordinary skill

in the art. Thus, each inverse FFT and cyclic prefix block performs inverse FFT

processing on the output of all of the frequency bins, i.e. the spread sequences of

output chips. The inverse FFT operation is performed once for each chip period

of the spread sequences of output chips. The cyclic prefix is a technique, known

in the art, which adds a certain amount of known samples to the inverse FFT to

account for time dispersion in the channel. The amount of cyclic prefix is

determined based on the maximum time dispersion among all signal paths in the

multiple input multiple output channel. Thus, each of inverse FFT and cyclic

prefix blocks 722, 724, 726, and 728 transform the spread sequences of output

chips from all of the frequency bins from the frequency domain into the time

domain.

Output 723 of inverse FFT and cyclic prefix block 722, output 725 of

inverse FFT and cyclic prefix block 724, output 727 of inverse FFT and cyclic prefix block 726, output 729 of inverse FFT and cyclic prefix block 728 are, thus,

spread sequences of output chips in the time domain. Each of outputs 723, 725,

727, and 729 is input separately to the communication channel. For example,

each spread sequence of output chips can be passed on to an FIR filter used for

pulse shaping signals prior to their transmission using a separate antenna for each

signal input into the communication channel. A receiver or receivers at the

output of the communication channel, which may use a single antenna or

multiple antennas, receives the signals, passes the signals through receive FIR

filters, and performs other processing as described below in relation to Figurer 8.

Thus, Figure 7 shows an example of a system that can be used to

counteract the effects of time dispersion in a multiple input multiple output

communication channel through use of OFDM combined with orthogonal

transmit diversity. Thus, the system provides greater reliability of

communications and increased data rate for increased bandwidth efficiency of

signal transmission in a multiple input multiple output channel.

Figure 8 illustrates an example of receiver processing which incorporates

OFDM in accordance with one embodiment Exemplary system 800 shown in

Figure 8 constitutes part of a receiver which may generally reside in a subscriber

unit or mobile unit when communication is taking place in a forward channel.

Exemplary system 800 can be part of a subscriber unit receiver, for example, in a

WCDMA communication system or spread spectrum communication system.

In exemplary system 800 shown in Figure 8, input signals 801, 803, 805,

and 807 are received from a multiple input multiple output communication

channel. The communication channel can be, for example, radio frequency transmission between multiple transmit and receive antennas in a wireless

communication system comprising a multiple input multiple output, or MIMO,

channel as described above. For the example used in the present application, a

channel with 4 transmit antennas and 4 receive antennas is illustrated. Thus each

of input signals 801, 803, 805, and 807 is received on a separate receive antenna.

Continuing with Figure 8, the receive processing shown in Figure 8 is

intended to be used with the transmit processes used in Figure 7. The transmit

process of Figure 7 can incorporate any of the transmit processes described in

connection with Figure 1, Figure 2, Figure 3, or Figure 4, as noted above. Thus,

input signals 801, 803, 805, and 807 comprise time domain spread sequences of

output chips from all of the frequency bins. Input signal 801 is passed to FFT

block 802, input signal 803 is passed to FFT block 804, input signal 805 is

passed to FFT block 806, and input signal 807 is passed to FFT block 808. Each

of FFT blocks 802, 804, 806, and 808 performs an FFT operation once for each

chip period of the spread sequences of output chips to transform the spread

sequences of output chips from the time domain into the frequency domain to fill

all of the frequency bins. The spread sequences of output chips in the frequency

domain are then passed to "Walsh/Alamouti and metric generation" blocks 812,

814, and 816. There is one Walsh/Alamouti and metric generation block for each

frequency bin. Recall that the example used in the present application has 3

frequency bins, thus there are 3 Walsh/Alamouti and metric generation blocks.

Each block performs receiver processing for generation of metric values as

described above in relation to Figures 5 and 6. The processing of Figure 5 or

Figure 6, respectively, is used depending on whether the transmit processing scheme used is that of Figures 1 or 2 or that of Figures 3 or 4. The metric values

output by each of Walsh/Alamouti and metric generation blocks 812, 814, and

816 are passed to "frequency decode" block 822. Frequency decode block 822

time multiplexes the metric values to undo the time demultiplexing performed in

the example used in the present application, which was discussed above in

relation to Figure 7. If other techniques, for example, interleaving, were used,

then frequency decode block 822 would include the corresponding

deinterleaving, for example, as is apparent to a person of ordinary skill in the art.

The frequency decoded metric values are then passed to trellis decode block 832,

which can comprise a Viterbi decoder, for example, to decode the modulation

symbol sequence using maximum likelihood decoding as described above. Thus,

Figure 8 shows an example of receiver processing for utilization of signal

orthogonality with Alamouti transmit diversity combined with orthogonal

frequency division multiplexing in a multiple input multiple output

communication channel.

It is appreciated by the above description that the disclosed embodiments

provide a method and system for increased bandwidth efficiency in multiple

input multiple - output channels using multiple input multiple output techniques

for wireless communications in a WCDMA system. According to an

embodiment described above, user's information is transmitted between multiple

transmit antennas and multiple receive antennas while maintaining orthogonality

between antennas as well as between users. Moreover, according to an

embodiment described above, diversity is also achieved while maintaining

orthogonality between antennas as well as between users.. Although the disclosed embodiments are described as applied to communications in a

WCDMA system, it will be readily apparent to a person of ordinary skill in the

art how to apply the disclosed embodiments in similar situations where

orthogonal transmit diversity is needed for use with multiple transmit and receive

antennas or multiple communication inputs and outputs.

From the above description, it is manifest that various techniques can be

used for implementing the concepts of the presently disclosed embodiments

without departing from its scope. Moreover, while the presently disclosed

embodiments have been described with specific reference to certain

embodiments, a person of ordinary skill in the art would recognize that changes

can be made in form and detail without departing from the spirit and the scope of

the presently disclosed embodiments. For example, the quadrature amplitude

modulation QAM presented in an embodiment described here can be replaced by

other types of modulation such as phase shift keying ("PSK"). Also, for example,

the space time diversity encoding, presented in one embodiment described here,

can be replaced by space frequency diversity encoding. The described

embodiments are to be considered in all respects as illustrative and not

restrictive. It should also be understood that the presently disclosed

embodiments are not limited to the particular embodiments described herein, but

is capable of many rearrangements, modifications, and substitutions without

departing from the scope of the invention.

Thus, method and system for increased bandwidth efficiency in multiple

input - multiple output channels have been described.

WHAT IS CLAIMED IS:

Claims

1. A method comprising steps of:

supplying an input bit stream to a trellis code block;

modulating an output of said trellis code block so as to provide a

modulation symbol sequence;

feeding said modulation symbol sequence to a plurality of Walsh covers,

wherein each of said plurality of Walsh covers outputs one of a plurality of

spread sequences of output chips;

transmitting said plurality of spread sequences of output chips over a

channel.

2. The method of claim 1 wherein said feeding step comprises feeding

a replica of said modulation symbol sequence to each of said plurality of Walsh

covers.

3. The method of claim 1 wherein said feeding step comprises time

demultiplexing said modulation symbol sequence so as to simultaneously provide

a portion of said modulation symbol sequence to each of said plurality of Walsh

covers.

4. The method of claim 1 wherein said plurality of Walsh covers comprise mutually orthogonal Walsh covers.

5. The method of claim 1 wherein said modulating step comprises

modulating said output of said trellis code block using trellis coded quadrature

amplitude modulation.

6. The method of claim 1 wherein said trellis code block performs rate

(n -l)/n trellis coding on said input bit stream.

7. The method of claim 1 further comprising steps of:

frequency coding said modulation symbol sequence after said modulating

step and before said feeding step;

performing inverse FFT and cyclic prefix processing after said feeding

step and before said transmitting step.

8. The method of claim 1 wherein said channel comprises a multiple

input multiple output channel.

9. A method comprising steps of:

supplying an input bit stream to a trellis code block;

modulating an output of said trellis code block so as to provide a first

modulation symbol sequence;

diversity encoding said first modulation symbol sequence so as to generate

a second modulation symbol sequence; feeding said second modulation symbol sequence to a plurality of Walsh

covers, wherein each of said plurality of Walsh covers outputs one of a plurality

of spread sequences of output chips;

transmitting said plurality of spread sequences of output chips over a

channel.

10. The method of claim 9 wherein said diversity encoding is space

time encoding.

11. The method of claim 9 wherein said diversity encoding is space

frequency encoding.

12. The method of claim 9 wherein said feeding step comprises feeding

a replica of said second modulation symbol sequence to a pair of said plurality of

Walsh covers.

13. The method of claim 9 wherein said diversity encoding step

comprises demultiplexing said first modulation symbol sequence, whereby a

portion of said first modulation symbol sequence corresponding to a pair of said

plurality of Walsh covers is simultaneously diversity encoded, so as to provide a

portion of said second modulation symbol sequence to each of said plurality of

Walsh covers.

14. The method of claim 9 wherein said plurality of Walsh covers comprise pairwise mutually orthogonal Walsh covers.

15. The method of claim 9 wherein said modulating step comprises

amplitude modulation.

16. The method of claim 1 further comprising steps of:

frequency coding said modulation symbol sequence after said modulating

step and before said feeding step;

performing inverse FFT and cyclic prefix processing after said feeding

step and before said transmitting step.

17. The method of claim 9 wherein said channel comprises a multiple

input multiple output channel.

18. A system comprising:

a trellis code block configured to encode an input bit stream;

a modulator configured to receive an output of said trellis code block to

provide a modulation symbol sequence;

a plurality of Walsh covers, wherein said modulation symbol sequence is

fed to said plurality of Walsh covers and each of said plurality of Walsh covers

outputs one of a plurality of spread sequences of output chips;

said system configured to transmit said plurality of spread sequences of

output chips over a channel.

19. The system of claim 18 wherein a replica of said modulation

symbol sequence is fed to each of said plurality of Walsh covers.

20. The system of claim 18 wherein said modulation symbol sequence

is time demultiplexed so as to simultaneously provide a portion of said

modulation symbol sequence to each of said plurality of Walsh covers.

21. The system of claim 18 wherein said plurality of Walsh covers

comprise mutually orthogonal Walsh covers.

22. The system of claim 18 wherein said modulator is configured to

modulate said output of said trellis code block using trellis coded quadrature

amplitude modulation.

23. The system of claim 18 wherein said trellis code block performs

rate (n -l)/n trellis coding on said input bit stream.

24. The system of claim 18 further comprising:

a frequency coder configured to frequency code said modulation symbol

sequence so as to simultaneously provide a portion of said modulation symbol

sequence to each of said plurality of Walsh covers ;

an inverse FFT processor configured to transform said output of each of

said plurality of Walsh covers from the frequency domain into the time domain so as to provide said plurality of spread sequences of output chips.

25. The system of claim 18 wherein said channel comprises a multiple

input multiple output channel.

26. A system comprising:

a trellis code block configured to encode an input bit stream;

a modulator configured to receive an output of said trellis code block to

provide a first modulation symbol sequence;

an Alamouti block configured to diversity encode said first modulation

symbol sequence so as to generate a second modulation symbol sequence;

a plurality of Walsh covers, wherein said second modulation symbol

sequence is fed to said plurality of Walsh covers and each of said plurality of

Walsh covers outputs one of a plurality of spread sequences of output chips;

said system configured to transmit said plurality of spread sequences of

output chips over a channel.

27. The system of claim 26 wherein said Alamouti block is configured

to diversity encode using space time encoding.

28. The system of claim 26 wherein said Alamouti block is configured

to diversity encode using space frequency encoding.

29. The system of claim 26 wherein a replica of said second modulation symbol sequence is fed to a pair of said plurality of Walsh covers.

30. The system of claim 26 wherein said first modulation symbol

sequence is demultiplexed, whereby a portion of said first modulation symbol

sequence corresponding to a pair of said plurality of Walsh covers is

simultaneously diversity encoded, so as to provide a portion of said second

modulation symbol sequence to each of said plurality of Walsh covers.

31. The system of claim 26 wherein said plurality of Walsh covers

comprises pairwise mutually orthogonal Walsh covers.

32. The system of claim 26 wherein said modulating step comprises

amplitude modulation.

33. The system of claim 26 further comprising:

a frequency coder configured to frequency code said modulation symbol

sequence so as to simultaneously provide a portion of said modulation symbol

sequence to each of said plurality of Walsh covers ;

an inverse FFT processor configured to transform said output of each of

said plurality of Walsh covers from the frequency domain into the time domain

so as to provide said plurality of spread sequences of output chips.

34. The system of claim 26 wherein said channel comprises a multiple input multiple output channel.