WO2002032000A1 - Method and apparatus employing a remote wireless repeater for calibrating a wireless base station having an adaptive antenna array - Google Patents

Abstract

A method and apparatus for calibration of an adaptive antenna array in a wireless communication system having at least one remote device (300) for communication with a base transceiver station (BTS) (100) and a plurality of mobile stations can determine calibration factors for respective antenna transmit and receive paths without the need for any additional or external equipment for calibration. Calibration factors for respective transmit and receive paths associated with each of the antenna elements are determined by transmitting a downlink signal from the BTS (100) to the remote device (300) the downlink signals back to the BTS (100) as at least one uplink signal, and receiving the uplink signal at the BTS (100).

Description

METHOD AND APPARATUS EMPLOYING A REMOTE WIRELESS

REPEATER FOR CALIBRATING A WIRELESS BASE STATION

HAVING AN ADAPTIVE ANTENNA ARRAY

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to the field of RF communication systems, and more

particularly to a system and method for calibration of adaptive antenna arrays.

Description of the Related Art

In order to remain competitive in an increasingly crowded market, wireless

equipment manufacturers experience constant pressure to reduce their costs

and improve performance. One way to reduce the overall cost of a cellular

phone system is to redesign individual system components or software so that

the system may operate more efficiently. For example, it would be desirable to

supply more users while maintaining an acceptable signal quality. One method

for increasing the efficiency and performance of a cellular system is through the

use of adaptive antenna arrays ("adaptive arrays") . Until recently, adaptive

arrays had been used almost exclusively for a variety of military applications,

such as phased-array radar and direction-finding systems. Non-military uses

were limited by losses and signal degradation resulting from the combining of a

large number of analog signals (in transmit mode) and splitting the large number

of analog signals (in receive mode) required in multi-channel communication

systems. The losses and degradation noted was generally more than offset by

any advantage gained through employing an adaptive antenna array. However, with the advent of high-speed digital signal processing, losses and signal degradation associated with analog processing to accomplish the same result

are almost entirely avoided.

Adaptive antenna array systems provide greater range over traditional technologies due to increased antenna gain. As a carrier wave propagates through space, the signal power decreases. Since mobile subscribers cannot detect signals below a minimum threshold level of power, increasing antenna

gain extends the distance a carrier wave can travel. Thus, an adaptive antenna array can increase the cell size that a given base transceiver station (BTS) can

serve.

An adaptive antenna array can also increase user capacity over traditional antenna technology by amplifying the signal coming from and going to the mobile user while dampening other signals coming from sources disposed in other directions. This ability is commonly referred to as "digital beamforming. " By steering a beam and positioning multiple nulls, an adaptive array is able to reduce co-channel and adjacent channel interference. This allows each cell to

use all frequencies within an operator's licensed band and may even make it possible to use single carrier frequencies more than once within a given cell.

Separating multiple signals having the same frequency is possible using an adaptive array, provided the signals arrive from angles or otherwise have distinctive propagation paths. In the same manner, multipath arrivals of a

desired signal, which in typical systems degrades signal quality due to intersymbol interference, is used to define the spatial signature, thus isolating

and enhancing the signal from interferers.

An antenna array consists of N identical antenna elements arranged in a

particular geometry. The geometry of the array determines spatial resolution of

the signals transmitted or received, i.e. the amount of coverage in a given

spatial region. Commonly used array types are the uniform linear and circular

arrays.

For any given geometry, the phases and amplitudes of the currents

exciting the array elements as well as the number of array elements determine

the gain of the array in a certain direction. The phases and amplitudes of the

currents on the antenna array elements can be electronically adjusted such that

received signals from a certain direction add in phase, and maximum gain is

achieved in that direction. Due to the reciprocal nature of adaptive antennas,

this approach is also generally applicable to focus the direction of transmitted

energy from the antenna array for transmission as well.

To adjust the amplitude and phases of the individual array currents,

complex weights are placed in the signal path of each antenna element. The

weighted signals are combined and the output is fed to a control unit that

operates on the individual signals and their combined output to update the

weights. Weight updating is usually accomplished adaptively to satisfy a

chosen optimization criteria. There are several commonly used adaptive algorithms available for updating the weights. These include gradient based algorithms, recursive

methods, and others such as the constant modulus method (CMA).

For the phases and amplitudes of the various currents of the antenna array transmit elements to be controlled properly, the individual (unweighted)

array currents must be emitted by the various antenna elements at the same instant with a known phase and amplitude. Consequently, the relative phase

and amplitude offsets introduced for each of the complete transmit paths associated with each array element must be determined and compensated for to provide precise beam steering. Since the direction from the base station for

transmitting a beam is determined partly from information provided by the receive signal, relative phase and amplitude offsets in each of the complete receive paths associated with each array element must also be determined in order to understand the effect of the receive signal path providing angle-of-

arrival information. In digital systems, the complete transmit and receive paths extend between the respective digital signal processors and the respective antenna elements.

Each adaptive array antenna element requires a separate transceiver chain for operation of the adaptive array. Thus, each antenna element is provided a

dedicated receiver apparatus chain and transmit apparatus chain. For example, a receive apparatus chain may include an antenna element, cables, filters, RF electronics, physical connections, and an analog-to-digital converter, assuming the processing is digital. Due to normal variances in the manufacture of the

antenna array elements, connecting cables, and transmit and receive electronics

chains, there will be differing errors and non-linearities introduced by signal

paths comprising combinations of these components. Thus, for example,

identical signals for transmission passing through the different transmit

apparatus chains will emerge with different amplitudes and phases.

These composite amplitude and phase errors in a given antenna signal

path can be captured for a fixed reference angle from the base station (for

example, north) and a set of receive and transmit calibration factors calculated

to equalize the transfer functions of the various receive and transmit apparatus

chains. Each antenna array element along with its corresponding cables and the

corresponding receive electronics in the path from each antenna element to its

respective digital signal processor shall be referred to as the "receive apparatus

chain" for each antenna element. Similarly, each antenna array element along

with its corresponding cables and the corresponding transmit electronics from

the respective digital signal processor to the respective antenna element shall be

referred to as the "transmit apparatus chain" for each antenna element. These

calibration factors can be used to transmute the actual signals that are actually

seen at the end of each antenna array element chain into corresponding signals

that would be expected at the end of each chain if all signal path components

behaved identically. Because the transmit signals and the receive signals follow somewhat different hardware paths, the adaptive antenna system will have both transmit

and a receive calibration factors. It should also be noted that the phase and

amplitude shifts that occur in the receive and transmit apparatus chains are, in

general, frequency dependent. Thus, in broadband applications, calibration

factors must generally be determined at the plurality of carrier frequencies used,

or at least a sampling of these frequencies.

Accurate real-time calibration is required for all receive and transmit

apparatus chains. Periodically, the calibration procedure must be performed as

differences in the propagation path may vary during the day and from day to day

due to temperature and other environmental conditions. Since there are active

components involved, their responses will generally change with time and

temperature. Thus, for an adaptive antenna array to function properly, periodic

calibration of the various transmit and receive apparatus chains should be

performed.

The complete paths for the receive apparatus chains may be calibrated

using a fixed near-field or far-field transmitter calibration source to allow the

respective receive apparatus chain path delays and magnitude shifts to be

calculated and stored. The calibration of the transmit apparatus chain path

delays, phase and magnitude shifts is similar, requiring signals to be transmitted

simultaneously from each antenna array element to be received at a known near-

field or far-field reference point. Therefore, there is a need for a method to calibrate all transmit apparatus

chains and receive apparatus. Moreover, such a method should preferably not

impact the traffic capacity of the cellular system.

SUMMARY OF THE INVENTION

A method for determining calibration factors for respective transmit and receive paths associated with an adaptive antenna array having a plurality of antenna elements is accomplished. The system includes a base transceiver station (BTS) having an adaptive antenna array and at least one remote device for communicating with the BTS. Multiple downlink signals are transmitted from the BTS, preferably simultaneously, to the remote device, one signal from each of the antenna elements. The remote device retransmits the downlink signals back to the BTS, preferably as one composite uplink signal . The returned composite uplink signal contains information for each of the receive and transmit antenna array paths that is uniquely discernable. BTS then determines calibration factors for the respective antenna transmit and receive paths.

The remote device can be a repeater, preferably a translating repeater. The translating repeater can demodulate downlink calibration signals. The method can further include the step of placing the remote device in a loopback mode. The method can also include the step of storing the determined calibration factors.

The BTS can transmit the downlink signals one at a time. Alternatively, downlink signals can be transmitted by the BTS from respective antenna elements at substantially the same time.

The method can include the step of programming the remote device with a list of calibration frequencies. Alternatively, the method can include the step of the BTS providing a list of calibration frequencies to the remote device using a wireless transmission. For example, the list of calibration frequencies can be transmitted by the BTS along with neighbor list and system information. Calibration can be performed so as to not impact system traffic capacity. For example, when the communication system is a TDM system, a RACH signal can be used for the uplink signals and a SDCCH for the downlink signals.

Calibration can be performed at a plurality of carrier frequencies. In this embodiment, the method can include the step of the BTS commanding the remote device to a frequency other than the first calibration frequency.

A self-calibrating communications system not requiring any additional or external equipment for calibration includes a base transceiver station (BTS) having an antenna array with a plurality of antenna elements and at least one remote device for communicating with a plurality of mobile stations and the BTS. The remote device is adapted to be configured in a loopback mode, wherein calibration signals transmitted by the BTS are retransmitted by the remote device back to the BTS during calibration. Thus, antenna array calibration factors for the respective antenna transmit and receive paths can be determined without the need for any other additional or external equipment for calibration.

The remote device can be a repeater, preferably a translating repeater. The remote device can include a structure for demodulating the calibration transmitted by the

BTS.

BRIEF DESCRIPTION OF THE DRAWINGS

There are presently shown in the drawings embodiments which are

presently preferred, it being understood, however, that the invention is not

limited to the precise arrangements and instrumentalities shown.

Fig. 1 shows a simplified block diagram of a wideband digital

beamforming base station transceiver (BTS) .

Fig. 2 shows a block diagram of a wireless communications system

deploying a plurality of wireless translating repeaters and base transceiver

stations.

Fig. 3 shows the various components comprising the total signal shift for

the round trip of a loopback signal transmitted by a base transceiver station

(BTS) to a remote device and back to the BTS.

Fig. 4 is a flow chart for a calibration method according to an embodiment

of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 , a simplified block diagram of a digital beamforming

base transceiver station (BTS) is shown. BTS 1 00 shown includes a 4 element

antenna array 1 1 0-1 1 3 (hereafter 1 1 0). Although a 4 element array is shown,

BTS 1 00 can have any number of antenna elements provided at least two

elements are provided . Each antenna element has a dedicated receive apparatus chain comprising filter/duplexer 1 20-1 23 (hereafter 1 20), broadband digital transceiver 140-143

(hereafter 140), channelizer/combiner 1 50-1 53 (hereafter XMUX 1 50) and associated connectors inclusive of digital signal processors 1 70-1 73 (hereafter 1 70) . In the receive mode, XMUX 1 50 operates as a channelizer. In a

broadband application, digital signal processor boards (DSP) 1 70 comprise a plurality of individual digital signal processors. Filter/duplexer 1 20 performs amplification, filtering and down conversion to IF of received RF signals. In broadband applications, assuming multiple frequency channels are active at any

given instant, received signals are multi-channel signals.

The broadband digital transceiver 140 performs A/D conversion then digitally down-converts received signals. The multi-channel digital signal output by the broadband digital transceiver 140 is separated by channelizer 1 50 into

baseband digital signals having an I and Q representation for each active

channel. There is preferably one channelizer 1 50 provided for each antenna element 1 10. In the preferred embodiment, the channelizer 1 50 is a FFT channelizer. The baseband digital signals, respectively associated with each antenna element 1 10, can then communicated to a processor, such as digital

array processor 160. Although shown as a separate module and positioned on

the XMUX 1 50 side of control and timing bus 1 62 and switching bus 1 64, digital array processor 1 60 can be positioned on the other side of buses 1 62 and 1 64 with DSP 1 70. Moreover, in the preferred embodiment, digital array

processor 1 60 can be positioned on the same board as DSP 1 70.

Digital array processor 1 60 can be used to store the various adaptive

array calibration factors and angular weighting factors. Calibration factors and

angular weighting factors are preferably stored separately. Calibration factors

are used to compensate for relative path delay and amplitude variations that

occur when signals traverse the various transmit and receive apparatus chains,

with reference to a fixed reference angle (for example, north) . The angular

weighting factors are used to point the antenna beam and nulls into the desired

directions (for both transmit and receive) relative to the reference angle (for

example, 4- 23 degrees north) . The phases and amplitudes of the calibration

factors and angular weighting factors are effectively added together to result in

a net weighting factor that is applied to signals traveling in each antenna

apparatus chain for each signal frequency (and time slot in TDM systems) .

Digital array processor 1 60 may be used to calculate the various

calibration factors and angular weighting factors and store these factors.

Through application of net weighting factors to each of the respective apparatus

chains, digital array processor 1 60 can adjust the baseband digital signals

received from and for transmission by each antenna element 1 10 to beamform

each active channel. The net weighting factors can be determined and applied

using "Super DSP" cards, where one processor does the beamforming for a

single RF carrier (all 8 time slots in GSM, for example) and a second processor does the signal processing (modem functions) . Having both processors on the

same board reduces signal interconnectivity requirements and improves system

reliability. Combining the processors on a single board also generally reduces

system cost compared to separate board implementations when implementing

adaptive antenna arrays.

For the receive path, the respective phase and amplitude net weighting

factors are preferably applied after the XMUX 1 50 (digital channelizer) and

before the DSP 1 70. DSP 1 70 receives the signal components adjusted with

respective net weighting factors from each receive apparatus chain output by

the digital array processor 1 60 and demodulates these signals to recover the

combined beamformed communication signal . The recovered communication

signal is then communicated to the communication system via a suitable

interface (not shown) .

System operation in the transmit direction is quite similar to the receive

direction . Each antenna element has a dedicated transmit apparatus chain

comprising filter/duplexer 1 20, multi-carrier power amplifier (MCPA) 1 30- 1 33

(hereafter 1 30), broadband digital transceiver 1 40- 1 43, XMUX 1 50 (using the

combiner) and associated connectors inclusive of respective digital signal

processors 1 70. DSPs 1 70 are associated with specific antenna elements 1 10

and a specific frequency channel for processing respective ones of a plurality of

incoming (voice/data) communication signals to be transmitted over respective

frequency channels. Their processed (modulated and encoded) outputs are supplied to the digital array processor 1 60, which can apply appropriate net

weighting factors to each active channel and for each transmit apparatus chain

associated with each antenna element 1 10. For the transmit path, the phase

and amplitude net weighting factors are preferably applied after the DSP 1 70

(modulation) and before the XMUX 1 50 (digital combiner) . At these points in

the signal path, each signal (RF frequency and time slot) is a baseband signal

having an I and Q representation.

The outputs of the digital array processor 1 60 for each transmit apparatus

chain are input into a combiner 1 50. In the preferred embodiment, there is one

combiner 1 50 for each antenna element 1 1 0 and the combiner is an inverse FFT

combiner. The combiner 1 50 forms a multichannel digital signal which is input

to the broadband digital transceiver 140, where it is upconverted to IF, D/A

converted, and amplified by a high power multi-carrier power amplifier (MCPA)

1 30. The composite multi-frequency signal is then supplied to RF elements 1 20

for amplification, filtering and up conversion from IF to RF. The antenna

elements 1 1 0 then each transmit the beamformed multi-frequency

communication signal.

Although a 4 element antenna array embodiment is shown in Fig. 1 , the

invention is not limited to 4 antenna elements. Note that although four DSP

boards 1 70 are shown in Fig. 1 , each may provide a plurality of digital signal

processors per board. For example, 24 digital signal processors may be

provided per DSP board 1 70. In the preferred embodiment of the invention, 96 channels are supported by BTS 1 00 through use of 1 2 RF carriers and 8 TDM

time slots. A separate digital signal processor may be dedicated to each channel

(timeslot), or a digital signal processor having sufficient processing speed may

process multiple channels, such as all eight channels (timeslots) in GSM

systems.

To obtain improved performance of the adaptive antenna array, both

transmitted and received signals must be calibrated so that unweighted signals

traveling along the various antenna apparatus chains from respective DSPs 1 70

to antenna element 1 10 each reach each antenna element 1 1 0 (transmit) and

DSPs 1 70 (receive) simultaneously (phase) and with the same amplitude.

Preferably, the various transmit and receive apparatus chains are calibrated to all

be within 1 to 2 degrees of phase and 0.1 db to 0.2 db in magnitude relative to

each other.

At the time of system installation, the physical distances between the

various antenna elements are preferably measured and recorded . This

information can be used to calculate and compensate for free space path time

delays between the various antenna elements. For example, for a triangular 4

element system where a single antenna is placed at the center of the triangle,

there are 4 unique distances to measure.

The invention uses a remote device to receive and loopback signals

transmitted by BTS 1 00 to determine calibration factors for the various receive

and transmit apparatus chains. In one embodiment of the invention, the remote device also frequency shifts the received calibration signal into the receive band

of BTS 100. In the preferred embodiment of the invention, a remotely located

wireless translating repeater ("translating repeater") may be used as the remote

device for calibration of an adaptive antenna array included with BTS 100.

As shown in the block diagram illustrated in Fig.2 a conventional wireless

communications system such as a Personal Communication System ("PCS") or

other similar system 10 can include remote repeaters, such as translating

repeaters. In this system 10, omni-directional translating repeaters 12-1...12-n

are deployed in peripheral cells surrounding broadband base transceiver stations

("BTS"), such as 100-1 ...100-m. Clusters of cells are each supported by a

plurality of translating repeaters, such as translating repeaters 12- 1...12-i. Cell

clusters are generally associated with a single BTS, such as 100-1. Translating

repeaters within a cell cluster are generally exclusively served or hosted by a

"host BTS" positioned within a given cell cluster.

Those skilled in the art will readily appreciate that non-translating

repeaters or directional or sectorized translating repeaters may replace omni¬

directional translating repeaters 12-1... 12-n in this system. The system 10 can

include translator omni-directional antennas 11-1, 11-2...11-i, ... 11-n-2, 11-n-

1 and 11-n, translating repeaters 12-1, 12-2, ... 12-i, ... 12-n-2, 12-n-1 and 12-

n, translating repeater antennas 13-1, 13-2, ... 13-i, ... 13-n-2, 13-n-1 and 13-

n, BTS directional antennas 14-1, ... 14-m, and broadband base transceiver

stations 15-1 , ... 15-m. The system 10 can further include a mobile switching center ("MSC") 1 6, one or more base station controllers 1 7 and a plurality of

mobile users 1 8- 1 and 1 8-2.

Translating repeaters 1 2 receive radio signals from mobile users 1 8

through omni-directional antennas 1 1 and forward a frequency shifted version of

the received signal to BTS 1 00 through translating repeater directional antennas

1 3. Likewise, radio signals transmitted from BTS 100re frequency shifted by

translating repeater 1 2 before being forwarded to mobile users 1 8. BTS 100

demodulates signal received from translating repeaters 1 2 through BTS antennas

14 and connects these signals to the Public Switched Telephone Network 92

(PSTN) through MSC 1 6. In addition, in the transmit direction, BTS 1 00

modulates signals received from the PSTN 92 through MSC 1 6 to format them

for transmission through BTS antennas 1 4-1 ... 14-m to their respective hosted

translating repeaters 1 2.

Thus, in systems such as 1 0, a plurality of translating repeaters may

already be installed in conjunction with one or more BTS 1 00. In this case, use

of a translating repeaters as an adaptive array calibration device results in no

added system expense. If a wireless translating repeater is not already used in

the cellular system, a reduced cost modified translating repeater version can be

used, the modified translating repeater having only backhaul receive and

transmit functions.

The total phase delay and magnitude shift (collectively the "signal shift")

for a round trip (TD) of a loopback signal transmitted by BTS 1 00 to a remote device 300, retransmitted by the remote device 300 and received by BTS 1 00

consists of the following components as shown in Fig. 3 for a n = 4 element

array:

1 . Signal shift from the respective DSP 1 70-1 73 to respective

transmit antenna elements 1 10-1 1 3 (transmit apparatus chain), shown as 31 0-

31 3; plus

2. Delay during signal travel in free space to the remote device,

shown as 320 for antenna element 21 0; plus

3. Signal shift during remote device signal shown as 330; plus

4. Delay during signal travel in free space from remote device 300

back to respective antenna elements, such as 340 for antenna element 21 0,

(note that due to symmetry 320 = 340), plus

5. Signal shift from each receiving antenna 21 0-21 3 to their

respective DSP 1 70 (receive apparatus chain), shown as 350-353.

Thus, for the 4 element antenna array BTS 1 00 shown, 1 6 distinct round

trip signal shifts can be measured from the various combinations of distinct

possible round trip paths. Thus, 1 6 independent linear equations can be derived.

During or soon after installation, the remote device 300 location is measured

and characterized with reference to the fixed angular position and distance

between the remote device 300 and the respective BTS antenna elements 21 0-

21 3. This is normally accomplished using a survey. This fixed information can

then be stored by BTS 1 00 and taken into account during calibration factor calculations to account for the differences in free space delays such as 320 and

340 between the remote device 300, such as a translating repeater, and the

various antenna elements 21 0-21 3. Since the there are only 9 unknowns being

4 transmit apparatus chains 310-31 3, 1 unknown signal shift 330 at remote

device 300 and 4 receive apparatus chain signal shifts 350-353, compilation of

data for the respective round trip paths permits solution to the above-referenced

unknowns.

A first embodiment of the invention is described with the help of the flow

chart of Fig. 4. In step 410, a remote device, such as a translating repeater, can

be signaled to enter a loopback mode by an appropriate signal, preferably a

wireless signal from BTS 1 00. In the loopback mode, the remote device 300

retransmits received signals received from the BTS 100 back to the BTS 1 00.

For example, a translating repeater may be placed in the loopback mode for one

frame (8 time slots in GSM) . Once in the loopback mode, signals received by the

translating repeater are preferably coupled off to a low level (e.g. -40 dB),

additionally attenuated, and frequency shifted from the transmit to receive band

(for example, 80 MHZ lower for GSM-1 900) and then re-transmitted back to

BTS 1 00.

In GSM systems, the BTS 100 transmitted loopback signal is preferably

GSM type access burst, the burst being any burst having a short duration

relative to the duration the applicable timeslot. In step 420, loopback signals

can be transmitted from each BTS antenna element 21 0-21 3, one at a time. In step 430, the remote device (e.g . translating repeater) receives the loopback

signals. In step 440, the remote device re-transmits the loopback signals to

BTS 1 00. In step 450, the retransmitted loopback signals are received by any or

all antenna elements 21 0-21 3 of the BTS adaptive array. In step 460, signal

shifts for each receive apparatus chain 31 0-31 3 may be measured and

calibration factors determined from resulting receive apparatus chain signal

shifts 310-31 3 measured from receipt of any one of the returned loopback

signals originally transmitted by BTS 100 from any of its antenna elements.

These values may be recorded for each antenna receive chain to permit

computation of calibration factors for the respective receive apparatus chains to

compensate for receive signal shift differences.

Similarly, in step 470, round trip signal shifts (TD) for the loopback

signals can be measured. For a 4 antenna element adaptive array, 1 6 round trip

signal shifts can be measured. In step 480, transmit apparatus chain calibration

factors 350-353 are readily be determined by subtracting the earlier determined

respective receive signal shifts 31 0-31 3 from measured round trip signal shift

data and adjusting for free space delay differences such as 320 and 340.

Transmit apparatus chain calibration factors can be used to compensate for

transmit signal shift differences from signals transmitted by each antenna

element 210-21 3. In step 490, receive and transmit apparatus signal shifts are

stored in a suitable BTS memory. Since signal shifts are generally frequency

dependent, the calibration process for both transmit and receive apparatus chains are preferably repeated for at least a representative sample of all carrier

frequencies supported by BTS 1 00.

In another embodiment of the invention, calibration factors can be

determined over a range of frequencies using a farfield calibration method. At

any given time, the transfer characteristics of the various transmit apparatus

31 0-31 3 and receive apparatus chains 350-353 can vary over the frequency

band of broadband BTS 1 00. This is primarily due to the differences in the

broadband transceiver's 240 SAW filter transfer characteristics from its band

edges to the center of the filter's band. Frequency dependence of the

calibration factors requires the various antenna paths to be analyzed at several

frequencies across the broadband BTS frequency band.

This embodiment preferably uses a translating repeater with loopback

capabilities as part of the calibration process. The repeater can be placed at (or

utilized from) a known location with respect to a BTS having an adaptive

antenna array. The BTS and translating repeater can each be programmed with

a set of calibration frequencies.

After determining calibration factors at a first carrier frequency, the BTS

100 can signal the remote device, such as a translating repeater, to step to a

next programmed calibration frequency. Likewise, the BTS 100 can step to the

next calibration frequency and the calibration process can be repeated at one or

more other carrier frequencies. Calibration information can then be saved for

each carrier frequency and used to compensate for the differences in the transfer function of the various receive apparatus chains and transmit apparatus

chains when a call is assigned having a given carrier frequency.

The above described multi-frequency calibration method required the

remote device to be pre-programmed to operate at certain predetermined

calibration frequencies for the calibration process to proceed. However, in a

preferred embodiment of the invention, the BTS 1 00 can provide wireless

calibration frequency information to the remote device 300, rather than relying

on pre-programming . The ability to provide wireless calibration frequency

information to the remote device allows the system in real-time to calibrate new

BTS carrier frequencies within a cellular provider's allocated band which may be

assigned during normal cellular system operation.

The remote device (e.g. calibration transceiver) may be co-located with a

BTS 100 for near-filed calibration. Alternatively, the calibration transceiver can

be actually used in the cellular system, such as a translating repeater while also

supporting far-field calibration.

Preferably, frequency calibration information is provided without

impacting the traffic capacity of the cellular system. For example, BTS 1 00

regularly transmits a "neighbor list" over a control channel that mobile users use

to determine the frequencies to scan adjacent cells. Scanning the received

signal strength (RSSI) of mobile users from transmissions originating from base

stations serving neighboring cells is commonly used by cellular systems to

perform mobile assisted handovers. The co-located calibration transceiver or remotely located translating repeater could also receive and use the calibration

information provided by the BTS transmitting a calibration frequency listing

along with the neighbor list information.

In addition, in some systems such as GSM, operator defined information

that is not required for the operation of a conventional wireless system can be

transmitted. In GSM, specific "System Information" supports this function.

Information that is broadcast on the BCCH is broadcast in groups called "System

Information. " There are currently 1 6 different System information messages

that are currently defined in the GSM 04.08, Mobile layer 3 Interface

Specification Standard. System information can also be used to broadcast to

"intelligent" programmable calibration units the calibration frequencies to

perform the antenna array calibration.

In a preferred embodiment of the invention, calibration of the various

transmit and receive array chains may be performed in a manner which does not

impact the traffic capacity of the cellular system. For example, idle time slots of

frames normally allocated for Standalone Dedicated Control Channels (SDCCH)

on the downlink and idle Random Access Channels (RACH) on the uplink may be

used to calibrate a GSM BTS having an adaptive antenna array using a remote

device, such as a translating repeater. The invention preferably uses a

translating repeater with loopback capabilities as part of the calibration process.

A problem with using a translating repeater with loopback for adaptive

array calibration is that two traffic channels, which may otherwise support cellular traffic, may be occupied during the calibration interval. In GSM systems, the uplink and downlink channels in TDM systems having 8 time slots

(e.g. GSM) are offset by 3 time slots (time slots are also referred to as burst periods). In GSM, from the BTS perspective, the uplink burst follows the downlink burst by 3 time slots. Alternatively, from the mobile's perspective, the downlink follows the uplink by three time slots. For example, from the mobile's

perspective, if the downlink burst uses time slot 6, the corresponding uplink burst uses time slot 3. Consequently, if traffic time slots are used for antenna array calibration, antenna array calibration using a translating repeater with loopback in a GSM system would generally require two traffic channels be allocated for the calibration.

Various options are conceivable to avoid using otherwise active traffic channels. The calibration method could wait until two appropriately spaced non- traffic time slots of a given carrier frequency become idle, but during busy times calibration could be significantly delayed. Such a delay could result in degraded beamforming performance due to adaptive array component drift.

A more efficient calibration method is to channels normally allocated for

control purposes. For example, a burst otherwise allocated for SDCCH on a downlink control channel can be used for transmission of the BTS calibration signal and a RACH signal can be used for the translating repeater uplink

calibration signal. Time slots during the selected specific frames of the multiframe used preferably align in time in the uplink and downlink direction. SDCCH are stand-alone dedicated control channels included as part of the

51 frame control multiframe. When an SDCCH is configured on a time slot

other than time slot 0, 8 SDCCHs are defined. When a mobile user accesses

the cellular network using a RACH, the BTS 1 00 will assign the mobile user one

of the 8 SDCCHs, unless the mobile is a GPRS mobile.

A GSM non-combined control carrier supports RACHs on the uplink on

time slot 0 for all frames. Downlink SDCCH is specifically assigned to time slot

3 because the downlink SDCCH is desired to align in time with the uplink RACH

which is fixed on time slot 0 in GSM systems. Time slot 0 is used exclusively

for the uplink RACH . Other control channel configurations also allow the SDCCH

to be multiplexed on time slot 0, but for purposes of this invention, this is not

preferred because the downlink SDCCH would not align with the uplink RACH .

The last 3 frames of the 51 frame control multiframe configured to

support SDCCHs are generally not used as they are intentionally left idle in GSM

systems. Accordingly, in GSM systems, calibration signals can be transmitted

by the BTS 1 00 during these idle frames, which are preferably assigned to time

slot 3. This permits the translating repeater to transmit the frequency shifted

calibration signal on time slot 0 normally allocated to a RACH, stealing the

RACH . Thus, the calibration is performed without utilizing time slots which can

otherwise be used to support traffic.

The calibration signal cannot overlap into an adjacent time slot without

being corrupted. Thus, the calibration signal should propagate from BTS 1 00 to the remote calibration unit and return so that the returned calibration signal does not overlap with an adjacent (later) timeslot. Therefore, it is preferable to use a

short duration burst, such as a burst having the length of a RACH, because it is not known the length of time it will take for the transmitted calibration signal to

return to the BTS 100. It is noted that the calibration signal is not constrained to take on specific RACH characteristics, such as, modulation, channel coding,

message content, etc.

There is a slight probability that a mobile attempting to access the system over the RACH will do so at the same time slot as the calibration signal is

looped back to the BTS by the translating repeater. However, in this situation,

the system will behave the same as if the two mobiles attempted to simultaneously access the system. In this case, the mobile will re-attempt system access at a later time. If the mobile corrupts the received looped back calibration signal such that it cannot be used, the system will simply reattempt calibration during the next multiframe.

Once the receive and transmit chain calibration factors are calculated and stored by BTS 100, the angle of arrival for each uplink channel may be calculated and stored. In practice, angle of arrival for translating repeater transmissions relative to the BTS 100 is known (e.g., based on a survey) at the

time of installation and does not change over time. Thus, the translating repeater angle of arrival can be used as an absolute reference, permitting mobile user angle of arrivals at the BTS 100 to be made relative to that fixed reference angle.

In a multi-carrier BTS utilizing 12 RF carriers and 8 TDMA time slots, 96

full duplex channels of GSM are available. In this configuration, 96X2 values of calibration factors are stored for each antenna element and its dedicated receive apparatus chain. Thus, for a duplexed system having 96 channels and 4 antennas, 768 calibration factors are stored to support beamforming in the

receive direction. After the uplink angle of arrival is determined, the downlink

steering angle is determined as the reciprocal direction. In actual operation, BTS 100 uses the receive chain apparatus calibration factors when an uplink signal is received by antenna elements 210 to determine the angular location of the signal source, such as a mobile user. Various

algorithms known in the art permit accurate identification of the mobile's location, allowing the determination of the angle of arrival of the mobile user's signal. Upon receipt of the arriving signal, the digital array processor 1 60 may be used to determine the angle of arrival of the incoming signal by measuring the signal shifts of the arriving signal after traveling the respective receive

apparatus chains of the several antenna elements 210. In this determination, receive apparatus chain calibration factors already determined and stored therein are used to compensate for differences in the various receive apparatus chains.

Using the mobile user's angle of arrival, angular weighting factors can be readily calculated using methods known in the art to narrow the beam to focus to the user's location and to position nulls to steer toward interference sources. Angular weighting factors are combined with respective calibration factors to produce appropriate net weighting factors for application to the signal paths of each receive antenna chain. Appropriate net weighting factors permit pointing a beam towards the mobile user and up to N-1 nulls (N is the number of antenna

elements) toward interference sources. Similarly, using the inverse of the angle of arrival for transmitted signals, respective transmit apparatus chain net weighting factors are determined to point a beam towards the mobile user and up to N-1 nulls toward interference sources. Translating repeaters may be used to calibrate the antenna array in either

a passive analog loopback repeat function or an active receive/demodulate remodulate/transmit function. In the passive repeat function, downlink signal are simply frequency translated and looped back to BTS 100. In the active receive function, the translating repeater can demodulate the received signal and

remodulate (and frequency shift) the received signal for retransmission. The active receive function can be used to send additional information such as RSSI (receive signal strength) of the mobile user to the BTS 100 during calibration.

Transmit calibration factors are preferably constantly updated during system operation because of component drift, principally due to environmental

factors. For example, the length of RF cables and jumpers change (and result in

corresponding shifts in time delays to traverse) with heating and cooling from the sun, day/night, weather and other environmental factors. Devices such as surface acoustic wave (SAW) filters found in both the BTS upconverter

(downlink transmit) and BTS downconverter (uplink receive) as part of

broadband digital transceiver 240 circuitry are particularly sensitive to

temperature and are known to produce significant changes in signal shift from

modest changes in temperature.

Channelizer 1 50 separates the inputted composite digital signal comprised

of all active RF carriers into separate digital signals representing each RF carrier

from a composite signal. Using the angle of arrival data determined for mobile

users for all active timeslots for each RF carrier in a TDM system, net weighting

factors may be determined by the digital array processor 1 60 and be separately

applied to each active timeslot to point a beam towards the mobile user's

location and to point one or more nulls at the most intense interference sources.

Turning to the transmit direction, signals output by DSP 1 70 to the digital

array processor 1 60 are separate digital signals for each active channel

(timeslot). Using the reciprocal of the angle of arrival for the received signal, the

digital array processor 1 60 calculates the optimum net weighting factors for the

various transmit apparatus chains and places these net weighting factors in each

of the channel's transmit signal path at the digital array processor 1 60 to point

the transmitted antenna beam and one or more nulls in the reciprocal of the

uplink signal direction.

Assuming full channel use, the digital array processor 1 60 generally

determines optimum net weighting factors for each antenna element 21 0 for each of the 96 full duplex signals. However, it is often not desirable to

beamform the dedicated control channel used as beacons, since such control

channels must be generally available throughout a given cell. Consequently, in

the absence of major blocking structures, no weighting factors will be applied to

BTS 100 transmitted control channels which function as beacons.

Calibration factors and angular weighting factors may be stored in

memory locations in the base station digital array processing card 1 60. These

factors are preferably stored separately. Neglecting control channels, for a

cellular system having 96 channels and having 4 duplexed transmit/receive

antennas elements, the number of memory locations required is 768 (96x2x4)

for calibration factors and the same number for separately stored weighting

factors. Angular weighting factors must generally be frequently updated since the cellular user may be moving and a variety of interference sources may arise.

This invention could apply to CDMA, GSM or other systems.

Additionally, the invention may be practiced with either a broadband BTS or a

narrowband BTS. It should be understood that the examples and embodiments

described herein are for illustrative purposes only and that various modifications

or changes in light thereof will be suggested to persons skilled in the art and are

to be included within the spirit and purview of this application. The invention

can take many other specific forms without departing from the spirit or essential

attributes thereof for an indication of the scope of the invention.

Claims

CLAIMSWhat is claimed is:

1 . In a wireless communication system having at least one remote

device for communicating with a base transceiver station (BTS) and a plurality of

mobile stations, said BTS having an adaptive antenna array including a plurality

of antenna elements, a method for determining calibration factors for respective

transmit and receive paths associated with each of said antenna elements,

comprising the steps of:

transmitting a downlink signal from said BTS to said remote device

from each of said antenna elements;

retransmitting from said remote device said downlink signals back

to said BTS as at least one uplink signal;

receiving said uplink signal at said BTS, and

determining calibration factors for said respective antenna transmit

and receive paths.

2. The method of claim 1 , wherein said remote device is a repeater.

3. The method of claim 1 , wherein said remote device is a translating

repeater.

4. The method of claim 1 , further comprising the step of placing said

remote device in a loopback mode.

5. The method of claim 4, further comprising the step of demodulating

said loopback signal by said translating repeater.

6. The method of claim 1 , wherein said BTS transmits said downlink

signals from each of said antenna element, one at a time.

7. The method of claim 1 , further comprising the step of programming

said remote device with a list of calibration frequencies.

8. The method of claim 1 , further comprising the step of said BTS

providing a list of calibration frequencies to said remote device using a wireless

transmission.

9. The method of claim 8, wherein said list of calibration frequencies

are transmitted by said BTS with at least one transmission selected from the

group consisting of neighbor lists and system information.

1 0. The method of claim 1 , wherein said calibration method is

performed without impacting system traffic capacity.

1 1 . The method of claim 1 0, wherein said communication system is a

TDM system and a RACH is used for said uplink signals and a SDCCH for said

downlink signals.

1 2. The method of claim 1 , further comprising the step of storing said

calibration factors.

1 3. The method of claim 1 , wherein said calibration is performed at a

first calibration frequency, further comprising the step repeating said calibration

at a calibration frequency other than said first calibration frequency.

14. The method of claim 1 3, further comprising the step of said BTS

commanding said remote device to said frequency other than said first

calibration frequency.

1 5. A self-calibrating communications system not requiring any

additional or external equipment for calibration, the system comprising:

a base transceiver station (BTS) having an antenna array with a

plurality of antenna elements;

at least one remote device for communicating with a plurality of

mobile stations and said BTS, said remote device configured to be operated in a loopback mode, wherein calibration signals transmitted by said BTS are re¬

transmitted by said remote device back to said BTS during calibration, whereby

calibration factors for said respective antenna transmit and receive paths can be

determined without the need for any additional or external equipment for

calibration.

1 6. The system of claim 1 5, wherein said remote device is a repeater.

1 7 The system of claim 1 5, wherein said remote device is a translating

repeater.

1 8. The system of claim 1 5, wherein said remote device includes

structure for demodulating said calibration signals transmitted by said BTS.