WO1999009769A1

WO1999009769A1 - Methods and apparatus for improved base station transceivers

Info

Publication number: WO1999009769A1
Application number: PCT/US1998/016984
Authority: WO
Inventors: Ronald S. Ogaz; Gerald E. Stark
Original assignee: Interwave Communications International Ltd.
Priority date: 1997-08-20
Filing date: 1998-08-17
Publication date: 1999-02-25
Also published as: EP1005766A1; CN1276133A; AU8909298A; AU752200B2; CA2298638A1; US6101400A; CN1192654C

Abstract

A base transceiver station (BTS) in a cellular communication system having a base station controller (BSC) and a mobile services switching center (MSC). The BSC is configured for facilitating communication between the BSC and a plurality of mobile stations (MS's). The communication with the plurality of the MS's is accomplished via radio frequency (RF) medium. The BTS includes a first interface circuit for coupling the BTS with the BSC. The BTS further includes a processor coupled to the first interface circuit for processing first digital data received from the BSC to form second digital data. There is further included a first central transceiver (CTRX) circuit coupled to the processor. The first CTRX circuit is co-resident with the processor and the first interface circuit. The BTS further includes a first remote transceiver (RTRX) circuit coupled to the first CTRX circuit. The first RTRX circuit includes a first antenna circuit for communicating with a first MS of the plurality of MS's via the RF medium, wherein the first RTRX circuit is implemented in a geographically remote manner from the first CTRX circuit.

Description

METHODS AND APPARATUS FOR IMPROVED BASE STATION

TRANSCEIVERS

Background of the Invention

The present invention relates to apparatus and methods for improving cellular

communication networks. More particularly, the present invention relates to improved base

transceiver stations (BTS) architectures in a cellular communication network.

Cellular communication systems are well known in the art. In a typical cellular

communication, the mobile stations (MS's) may transmit and receive voice and/or data with

the cellular network and one another utilizing radio waves. To facilitate discussion, Fig. 1

depicts the architecture of a cellular communication network 100 that implements the well-

known Global System for Mobile Communication (GSM) standard. Although the GSM

cellular network is chosen herein for illustration purposes, it should be borne in mind that

the invention disclosed herein is not limited to any particular standard.

In Fig. 1, there is shown a plurality of mobile stations (MS's) 102, 104, and 106,

representing the mobile interface with the cellular users. In a typical network, MS's 102,

104 and 106 may be, for example, the mobile handsets or the fixed mobile stations mounted

in vehicles. Mobile stations 102, 104, and 106 typically include radio and processing

functions for exchanging voice and data via radio waves with transceivers (TRX's) in base

transceiver stations (BTS's) 114 and 116. The transceivers (TRX's) are shown in Fig. 1 as

transceivers 114a, 114b, 114c, 116a, and 116b. The BTS's may be thought of, in one sense,

as the counterpart to the MS's within the cellular network, and its main role is to connect the

mobile stations with the rest of cellular communication network 100. There is also shown in Fig. 1 a base station controller (BSC) 118, whose function is

to monitor and control the BTS's. There may be any number of BSC 118 in a network,

whose responsibility includes, among other responsibilities, radio interface management,

e.g., the allocation and release of radio channels and handover management. Mobile

Services Switching Center (MSC) 120 controls one or more BSC's 118 and provides the

basic switching function within the cellular network, including setting-up of calls to and

from the MS's. MSC 120 also provides the interface between the cellular network users (via

the BSC and BTS) with external networks (e.g., PSTN or public switched telephone

network). The components of GSM cellular network 100 are well known to those skilled in

the art and are not discussed in great detail here for brevity's sake. Additional information

pertaining to GSM and the cellular networks implementing the GSM standard may be found

in many existing references including, for example, Redl, Weber & Oliphant, An

Introduction to GSM (Artech House Publishers, 1995).

In the prior art, the radio circuitries of the TRX's are typically implemented such that

they co-locate with other circuits of the BTS. By way of example, Fig. 2 illustrates in

greater detail exemplary prior art BTS 114 of Fig. 1, including TRX's 114a, 114b, and 114c.

As is typical, the antennas of the prior art TRX's co-locate with the BTS such that the BTS

defines the cell. Although one antenna is shown to facilitate simplicity of illustration,

separate transmit and receive antennas may be provided for each TRX, as is well known.

Other major functional blocks of BTS 114 includes ABIS interface 202, which implements

the circuitry necessary for interfacing between BTS 114 and its BSC. CPU circuit 204

implements the call processing functions, including for example LAPDm processing, speech

framing, channel coding, interleaving, burst formatting, ciphering, modulation, and the like. The architecture of the prior art BTS is well known and is not discussed here in great detail

for simplicity's sake.

It has been found, however, that the conventional BTS architecture has many

disadvantages. By way of example, the integration of the radio circuitries of the TRX's and

the processing circuitries of the BTS in one unit results in a complex and maintenance-

intensive electronic subsystem. Yet prior art BTS's are often installed in locations selected

primarily for optimum radio transmission quality such as on top of buildings and other

outdoor structures instead of ease of access. These locations, being exposed to the elements,

are typically hostile to the delicate and complex electronic circuits of the prior art BTS.

Accordingly, these factors tend to render the installation, maintenance, and upgrade of prior

art BTS's difficult and expensive.

The integration of the radio circuitries of the TRX's in the prior art BTS also limits

the flexibility with which the cell can be modified to accommodate changes in capacity. In

the prior art, the BTS, which contains the co-resident TRX antennas, essentially defines the

cell. Although some cell shaping may be accomplished by, for example, employing

directional antennas, the cell is more or less limited by the transmit power of the antennas in

the BTS. Scaling the transmit power upward increases the cell size at the expense of

capacity since the use of larger cells reduces the ability to reuse frequencies among

neighboring cells. Increasing the transmit power also increases the amount of heat

generated, thereby reducing the reliability of the circuitries in the prior art BTS unless fans

and/or additional heat dissipation techniques are employed.

In view of the foregoing, there are desired improved BTS architectures for

overcoming the disadvantages associated with prior art BTS's. In particular, there are desired BTS architectures which offer improved reliability and simplified maintenance, as

well as increase the flexibility with which the cell can be modified to accommodate changes

in capacity.

Summary of the Invention

The invention relates, in one embodiment, to a base transceiver station (BTS) in a

cellular communication system having a base station controller (BSC) and a mobile services

switching center (MSC). The BSC is configured for facilitating communication between the

BSC and a plurality of mobile stations (MS's). The communication with the plurality of the

MS's is accomplished via radio frequency (RF) medium. The BTS includes a first interface

circuit for coupling the BTS with the BSC. The BTS further includes a processor coupled

to the first interface circuit for processing first digital data received from the BSC to form

second digital data. There is further included a first central transceiver (CTRX) circuit

coupled to the processor. The first CTRX circuit is co-resident with the processor and the

first interface circuit. The BTS further includes a first remote transceiver (RTRX) circuit

coupled to the first CTRX circuit. The first RTRX circuit includes a first antenna circuit for

communicating with a first MS of the plurality of MS's via the RF medium, wherein the

first RTRX circuit is implemented in a geographically remote manner from the first CTRX

circuit.

In another embodiment, the BTS further includes a second remote transceiver

(RTRX) circuit coupled to the first CTRX circuit. The second RTRX circuit includes a

second antenna circuit for communicating with the first MS via the RF medium. The first

CTRX includes an RF quality selection circuit for selecting one of the first RTRX and the

second RTRX for use in communicating with the first MS.

These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. Brief Description of the Drawings

The invention, together with further advantages thereof, may best be understood by

reference to the following description taken in conjunction with the accompanying drawings in

which like reference numbers refer to like structures/items and in which:

To facilitate discussion, Fig. 1 depicts the architecture of a conventional cellular

communication network.

Fig. 2 illustrates in greater detail the prior art BTS of Fig. 1.

Fig. 3 illustrates, in accordance with one embodiment of the invention, a concentrated

BTS (CBTS), including the remote transceivers (TRX's).

Fig. 4 illustrates, in accordance with one embodiment of the present invention, a CBTS

having dynamic configuration capability.

Fig. 5 shows, in accordance with one embodiment of the present invention, a logic block

diagram of a remote TRX .

Fig. 6 is a block diagram illustration of the data flow within a prior art GSM BTS

module during transmit and receive.

Figs. 7A, 7B and 7C illustrate, in accordance with various embodiments of the present

invention, the data flow within the inventive CBTS during transmit and receive. Detailed Description of the Preferred Embodiments

The present invention will now be described in detail with reference to a few

preferred embodiments thereof as illustrated in the accompanying drawings. In the

following description, numerous specific details are set forth to provide a thorough

understanding of the present invention. It will be obvious, however, to one skilled in the

art, that the present invention may be practiced without some or all of these specific details.

In other instances, well known structures and process steps have not been described in detail

to not unnecessarily obscure the present invention.

In accordance with one aspect of the present invention, there is provided a novel and

nonobvious concentrated BTS (CBTS) architecture in which the TRX is divided into two

TRX subsystems: a central TRX subsystem which co-resides with the CBTS and a remote

TRX subsystem which is geographically remote from the CBTS and the central TRX. In

accordance with this aspect of the present invention, the remote TRX includes the RF

antenna circuitries that are employed for transmitting and receiving signaling and user data

with the MS's via RF signals. As the term is employed hereinafter, user data refers to non-

voice call data (such as short messages, text, graphics, or voice files, and the like) as well as

voice calls between cellular users.

The antenna circuitries in the remote TRX convert the data from a digital format into

RF signals for transmission to the MS's and convert RF signals from the MS's into digital

data for processing by the cellular network. Although additional processing capabilities may

be built into the remote TRX if desired, it is in general preferable to keep the circuitries

within the remote TRX simple to simplify maintenance and upgrade. Additionally, since the remote TRX may be implemented in hard-to-reach locations (e.g., locations which offer

optimal transmission quality such as the top of building or other structures) or be exposed to

the elements, simple remote TRX designs promote ruggedness, which reduces maintenance

costs.

The remote TRX is coupled to the central TRX at the CBTS through an appropriate

transmission medium such as a conductive or a fiber optic medium. Since the CBTS no

longer contains the antenna circuitries, the transmission quality no longer depends on the

location of the CBTS. Accordingly, it becomes possible to locate the CBTS inside to offer

the BTS circuitries greater protection from the elements or to locate the CBTS at a location

that is selected for convenient access for maintenance and upgrade.

On the selected transmission medium, an appropriate physical layer protocol is then

implemented. In one embodiment, the physical layer represents one of the well known El,

E2, or E3 protocols although any other suitable physical layer protocol may also be

employed. Other embodiments could be HDSL, OC3, or OC3 - Optical.

For actual transmission between the remote TRX and its corresponding central TRX

at the CBTS, an appropriate transport protocol is also implemented on top of the physical

layer protocol. The transport protocol may represent any suitable protocol and may in some

cases represent packet-switching or TDM protocols. In the one embodiment, an

Asynchronous Transmission Mode (ATM) protocol is employed. In the another

embodiment, a Frame Relay protocol is employed.

The central TRX, which is co-resident with the CBTS, includes interface circuits

necessary for communicating with the remote TRX's using the aforementioned transport and

physical layer protocols. The CBTS preferably also includes the bulk of the processing circuitries necessary

for preparing the digital data from the cellular network for transmission via the air interface.

Such CBTS circuits include, for example, the well known circuits for processing the

messages received from the MSC and BSC for call setup and system control, speech

processing and ABIS interface circuits. One embodiment also includes channel coding,

interleaving, burst formatting, ciphering, and others. In general, the bulk of the circuitries

preferably resides in the CBTS since, as mentioned earlier, it is usually desirable to keep the

remote TRX design simple and rugged. If maintenance and/or upgrade is required, the fact

that the bulk of the circuitries is located with the CBTS (i.e., out of the elements or in an

easily accessible location) substantially simplifies these tasks.

The features and advantages of the present invention may be better understood with

reference to the figures below. Fig. 3 illustrates, in accordance with one embodiment of the

invention, a concentrated BTS (CBTS) 300, including ABIS interface 202. From the

perspective of the remainder of the cellular network, e.g., the BSC's and the MSC's that are

above CBTS 300, CBTS 300 preferably appears substantially similar to a prior art BTS. As

discussed hereinbelow, however, CPU subsystem 302 and the central and remote TRX's of

CBTS 300 are substantially different from the CPU subsystem and the TRX's of the prior

art.

In CBTS 300, the antenna circuitries are implemented in remote TRX subsystems.

With reference to Fig. 3, the antennas are shown on remote TRX's 306, 308, 310, 312, and

314. Although each remote TRX is shown with a single antenna, a remote TRX may be

implemented with separate transmit and one or more receive antennas, as is known. Each

remote TRX preferably includes the antenna circuits, e.g., the radio interface circuitry, as

well as circuitries necessary to process, in the uplink direction, the received RF signals into binary data bits to be sent to the central TRX (discussed hereinbelow). Additionally, each

remote TRX preferably includes circuitries necessary to process the downlink binary data

bits received from the cellular network (via the central TRX) into RF signals to be

transmitted to the MS's.

A plurality of central TRX's 322 and 324 are implemented in CBTS 300. Each

central TRX is coupled at any given time to a unique set of remote TRX's. In the

implementation shown, remote TRX's 306 and 308 are coupled to central TRX 322 while

remote TRX's 310, 312, and 314 are coupled to central TRX 324. The coupling between a

remote TRX and its central TRX may take place through any appropriate transmission

medium including, for example, twisted pairs, co-axial cables, or fiber optics. In one

embodiment, the transmission medium represents a twisted pair, and the traffic data, the

radio control and status are passed between the central TRX and the remote TRX through an

Asynchronous Transfer Mode (ATM) link using a digital baseband physical layer protocol

(Tl, El, E2, E3, DS1, DS3, or the like). Although each set of remote TRX's is shown in

Fig. 3 to be in a daisy-chain arrangement, individual remote TRX's may be coupled to its

associated central TRX in parallel if desired.

In general, any number of remote TRX's may be coupled to a central TRX, and data

from each remote TRX may bear an appropriate identifier to permit the central TRX to

identify the remote TRX from which the data is sent. In practice, the number of remote

TRX's may be limited to a reasonable number to suit the processing capabilities of the

central TRX and/or to avoid overwhelming the transmission channel between the remote

TRX's and the central TRX (e.g., to avoid overwhelming transmission channel 350 in Fig.

3). If the physical layer framing on transmission channel 350 is El (30 DSOs), about 5 or 6

(or more if capacity permits) remote TRX's works well. For E2 physical layer framing, about 22 (or more if capacity permits) remote TRX's may be daisy-chained to a central

TRX. For E3 physical layer framing, a greater number (e.g., 88 or even more) remote

TRX's may be daisy chained due to the greater bit rate on the transmission channel.

It should be appreciated that since it is no longer necessary to position CBTS 300 for

optimum RF transmission with the MS's, CBTS 300 may be located anywhere convenient,

e.g., in the back office. The remote TRX's, being smaller, simpler, and more rugged, may

advantageously be positioned throughout the cell for optimum transmission quality and

capacity.

As the antenna circuits are remoted from the CBTS (e.g., via cabling), the CBTS

needs not be considered the base of the cell. With the present invention, each central TRX

now effectively defines an aggregate cell, which is made up of the radio cells of the

associated remote TRX's. The remote TRX's themselves, being remoted from the central

TRX (e.g., via cabling) may be dispersed anywhere within the cell and may even be

interspersed among remote TRX's which are associated with other central TRX's. It should

be appreciated that the multiplicity of sets of remote TRX's, as well as their ease of

positioning, offers the service provider flexibility in cell shaping in a manner that is simply

unattainable in the prior art.

The individual radio cell may of course be shaped further using traditional antenna

techniques, e.g., using directional antennas or increasing the transmit power. If transmit

power is increased, the additional heat and power generated do not pose a danger to the

processing circuitries of the CBTS as in the case of the prior art BTS circuitries, which are

co-resident with the antennas of the prior art TRX's. On the other hand, it is typically the

case that a given area previously covered by a high power TRX may be covered as well by multiple remote TRX's, each transmitting at a lower power level. In this manner, a given

area may be covered with .an array of simple, rugged and lower power remote TRX's,

thereby substantially reducing the costs of implementing the BTS's, as well as minimizing

the potential for cell-to-cell interference, and/or improving frequency reuse. The ability to

employ lower power antennas while offering equivalent or better coverage in a given area is

yet another advantage of the present invention.

In an antenna diversity application, each central TRX essentially represents a

separate RF channel during use, data from a given MS in the cell may be picked up by more

than one remote TRX's. Accordingly, the central TRX preferably includes RF quality

selection circuitries for selecting the remote TRX whose transmission quality is the best so

that the selected remote TRX may be employed for communication with the MS during the

call. In the antenna diversity application, since each central TRX and its associated set of

remote TRX's represent an RF channel at any given time, an MS communicating on that

channel may be picked up by more than one remote TRX. With reference to Fig. 3, for

example, RF signals from MS 330 may be picked up by the antennas within the set of

remote TRX's containing remote TRX's 310, 312, and 314. Central TRX 324 preferably

includes RF quality selection circuitries for selecting among the set of remote TRX's 310,

312, and 314 the remote TRX which offers the best transmission quality. If remote TRX

312 is found to offer the best transmission quality, central TRX 324 may employ the

antennas within remote TRX 312 for subsequent communication with MS 330. Selection

may be made, in one embodiment, by comparing the receive equalizer quality and the

Receive Signal Indicator Strength (RSSI) from the various remote TRX's and selecting the

best from those received. In a TDMA system (GSM) this selection may be done for each

TDMA burst. In Fig. 3, each set of remote TRX's is shown directly coupled to its associated central

TRX via the appropriate transmission medium. In one embodiment, routing resources may

be provided in both the remote TRX's and the CBTS to facilitate dynamic assignment of, for

example, CBTS Digital Signal Processing (DSP) resources to remote TRXs of the aggregate

cells. Fig. 4 shows such an implementation wherein remote TRX's 402, 404, 406, 408, and

410 of CBTS 400 are daisy-chained to routing circuit 412. In one embodiment, routing

circuit 412 represents an Asynchronous Transfer Mode (ATM) routing circuit. A database,

table, or intelligent algorithm controlling routing circuit 412 determines which remote TRX

is assigned to which of central TRX's 420, 422, and 424. In this case, each remote TRX's

may be associated with a unique ATM address and provided with appropriate ATM framing

circuits to packetize the demodulated RF data for transmission to routing circuit 412 or to

depacketize the ATM data packets sent from routing circuit 412. Traffic data, radio control,

and status data may be packed, in one embodiment, into the ATM cells for transmission

between a remote TRX and its associated central TRX at up to about two bursts per cell.

Analogous techniques may be employed if a Frame Relay Protocol is used.

The use of routing circuit 412 advantageously facilitates dynamic DSP assignment of

the aggregate cells associated with central TRX's 420 and 422 to handle changes in capacity

requirements. The dynamic configuration may be accomplished by simply changing the

routing table or database that routing circuit 412 uses to route data between the central

TRX's and the remote TRX's. By way of example, at time T₀ remote TRX 402 may be

routed to central TRX 420, remote TRX 404 may be routed to central TRX 422, and remote

TRX's 406, 408, and 410 may be routed to central TRX 424. If capacity in the vicinity of

remote TRX's 408 and 410 increases at time T„ either tempor.arily or permanently, dynamic

DSP assignment may be employed to route data from remote TRX 410 to its own central TRX 424, remote TRX 408 to its own central TRX 422, and redistribute the remaining

remote TRX's 402, 404, and 406 to central TRX 420. Since the area in the vicinity of

remote TRX 408 and the area in the vicinity of remote TRX 410 do not have to share central

TRX's after reconfiguration, more capacity may be handled in these areas. As can be

appreciated by those skilled in the art, dynamic DSP assignment substantially simplifies the

task of redistributing capacity when remote and/or central TRX's are added or removed from

the CBTS.

In general, there is no limit to the number of central TRX's that may be implemented

in a given CBTS. In practice, the number of central TRX's are scaled corresponding to the

processing (and routing if the Dynamic DSP Assignment implementation is desired)

capabilities of the CBTS. Since each additional central TRX adds an additional RF channel

to the BTS, it is desirable to endow CPU 302 of Figs. 3 and 4 with sufficient processing

capabilities to ensure that all calls are properly handled. By way of example, in one non-

antenna diversity application, each remote radio may provide one GSM frequency channel (

8 traffic channels). Depending on the design of the CTRX, one CTRX can process any

number of RTRXs (around 8 to 16 in one example). In some embodiments, parallel

processing techniques may be employed to enable the CBTS to handle a large number of

central TRX's. Such CPU scaling exercise is within the skills of those skilled in the art

given this disclosure.

Fig. 5 shows, in accordance with one embodiment of the present invention, a logic

block diagram of a remote TRX, e.g., remote TRX 402 of Fig. 4.

Remote TRX 402 includes a radio subcircuit 502, whose basic function is to transmit

and receive user data over the air. Two antennas are shown coupled to radio subcircuit 502: a transmit antenna 504 and a receive antenna 506. Although two antennas are shown herein,

both receive and transmit antennas may be integrated into a single antenna, as is known.

Radio interface subcircuit 508 receives packetized data from physical layer

framing/transport framing interface subcircuit 510 and formats the downlink data into bursts

for transmission to radio subcircuit 502, along with the frequency and power information for

the burst. In one embodiment the frequency information itself is recovered from a Global

Positioning System (GPS) terminal and synchronized to the proper specification (e.g., GSM

specification) using timing phase locked loop (PLL) subcircuit 512. On the receive side,

radio interface subcircuit 508 takes the demodulated data out of receive antenna 506 and

packetizes it for transmission to the central TRX via the daisy-chain transmission medium

518 (which may implement, for example, the El protocol). In one embodiment, radio

interface subcircuit 508 is implemented using a field programmable gate array (FPGA).

Digital Signal Processing (DSP) subcircuit 514 performs the modulation of the

binary signals received from the cellular network for transmission via transmit antenna 504,

demodulation of the RF signal received via receive antenna 506, as well as signal

equalization. In one embodiment, DSP subcircuit 514 is implemented by a general purpose

Digital Signal Processor (DSP).

A read-only memory subcircuit 520 stores configuration data for configuring remote

TRX 402 upon start up. Microcontroller 522 provides house-keeping functions such as

status checks, diagnostics, and power management. In one embodiment, microcontroller 522

may be employed to take remote TRX 402 out of the daisy-chained loop if remote TRX 402

is found to be defective, e.g., by connecting the input and output of the remote TRX

together. Microcontroller 552 also oversees the communication between the remote TRX and its central counterpart. In one embodiment, all communication between the remote TRX

and the central TRX is performed via the aforementioned physical layer framing/transport

framing interface subcircuit 510. Physical layer framing/transport framing interface

subcircuit 510 performs the tasks of formatting and driving data to the CBTS; receiving and

deformatting data received from the CBTS; and supervising, controlling and monitoring the

health of the connection.

The transport framing protocol is, in one embodiment, a packetized protocol where

the data packet contains a preamble, or header, containing an address of the receiver. It may

also contain a packet type identifier, serial number, time stamp, and other control

information. The termination of the packet contains error correcting fields. Data flow

between RTRX and CBTS is mapped to the transport protocol packets so that one transport

packet typically corresponds to one radio TDMA burst. Frequency, power and user data

may be included in each downlink (RTRX to MS) transport packet. Receive quality status,

signal strength and user data may be part of the uplink (MS to RTRX) transport packet.

The packetized transport protocol also allows system-defined transport packets that

are not transmitted over the air. The definition of the system messages may depend on the

partition of functions between the CTRX and RTRX. An exemplary type of system message

may be a specially defined timing packet. This timing packet is broadcasted to all remote

radios and is used to synchronize the TDMA frames of all remote radios. One embodiment

may use the Global Positioning Satellite network as a time reference. A time stamp may be

passed to remote radios and each remote radio may calculate the time delay through the

network from a local GPS reading relative to the time stamp. Another system transport

packet may be call setup information that is transported only once per mobile call session. An example would be the encryption key to cipher the user data. Remote radio status .and

control information would be another example of a system transport packet.

The use of the packet transport protocol between remote radios and CBTS allows the

service provider great flexibility in physical connectivity of the radio network. The capacity

of the communication medium becomes, in one embodiment, a function of traffic load,

rather than control demands of the physical radios. This flexibility was previously

unavailable in the prior art.

As can be appreciated from Fig. 5, the design of a remote TRX is deliberately kept

simple to promote ruggedness and simplify maintenance. In one embodiment, the remote

TRX preferably includes the subcircuits necessary for interfacing the remote TRX with the

physical layer framing and transport framing protocols (e.g., El and ATM), digital signal

processing subcircuits for equalization and demodulation of the received RF data, and the RF

radio subcircuits themselves. All other circuits traditionally associated with the prior art

TRX's and BTS are preferably kept in the concentrated BTS (CBTS). In one embodiment,

the CBTS may include circuits for interfacing the central TRX with the physical layer

framing and transport framing protocols (e.g., El and ATM) to communicate with the

remote TRX, digital signal processing subcircuits for convolution encoding, decoding, bit

interleaving, and burst formatting. Additionally, the CBTS may also include the interface to

the BSC.

The division of labor between the remote TRX and the central TRX within the CBTS

may be better understood with reference to the exemplary implementations of Figs. 6 and 7.

Fig. 6 is a block diagram illustration of the data flow within a prior art GSM BTS module

during transmit and receive. With reference to Fig. 6, data to be transmitted to the MS's are received from the BSC via an ABIS interface 602. Speech De-framing block 604 extracts

the digital data from the ABIS frame and passes it to Channel Coding block 606, whose

purpose is to package the extracted digital data for eventual transmission using the RF

medium. The data is then interleaved on a multiplicity of bursts to minimize the risk of

losing consecutive bits during transmission in Interleaving block 608. The bursts are then

formatted in Burst Formatting block 610 and optionally ciphered for security in Cipher block

612. Thereafter, the digital data is modulated (e.g., using Gaussian Minimum-Shift Keying)

for transmission to the MS's as RF signals via Tx Radio block 616.

On the receive side, RF signals from the MS's are received at Rx Radio block 650,

and demodulated into digital data and equalized in Demodulation/Equalization block 652.

Deciphering, if any, is performed in Deciphering block 654. Burst Formatting block 656

extracts user data and from the speech frame and builds it with the appropriated midamble

into a 148 bit burst. Thereafter, De-Interleaving block 658 reassembles the data from groups

of bursts. The digital data is then stripped using Channel Decoding block 660 to extract the

user data. Speech Framing block 662 frames the stripped digital data into ABIS frames for

transmission to the BSC. The functional blocks of Fig. 6 are well known to those skilled in

the GSM art. As mentioned, circuitries necessary for implementing the functional blocks of

Fig. 6 are typically implemented in one box in the prior art, i.e., these circuits are co-resident

in the prior art BTS.

To contrast, Fig. 7 A illustrates, in accordance with one embodiment of the present

invention, the data flow within the inventive CBTS during transmit and receive. In Fig. 7A,

ABIS interface block 602, as well as blocks 604, 606, 608, 610, and 612 on the downlink

path and blocks 654, 656, 658, 660, and 662 in the uplink path perform substantially the

same functions as corresponding blocks in Fig. 6. As these blocks have been discussed earlier, they will not be repeated here for brevity's sake. To facilitate remote communication

between the central TRX in the CBTS and the remote TRX, additional physical layer

framing and transport framing functional blocks have been added. On the transmit side,

these are shown as transport framing interface blocks 702 and 708, with transport framing

interface block 702 being co-resident with the CBTS and its counterpart transport framing

interface block 708 being implemented in the remote TRX. The transport framing may

employ the well known ATM protocol or Frame Relay protocol, as mentioned earlier.

Also on the transmit side, physical layer framing for communication between the

remote TRX and the central TRX is implemented via physical layer framing interface blocks

704 and 706, with physical layer framing interface block 704 being co-resident with the

CBTS and its counterpart physical layer framing interface block 706 being implemented in

the remote TRX. In one embodiment, as the ciphered digital data is outputted from Cipher

block 612, it is packetized into ATM frames (block 702) and framed for transmission via the

El protocol (block 704). This data is transmitted to the remote TRX via the transport link

710. Upon receiving the data, the remote TRX extracts the data from the El frame (block

706) and de-packetizes the data (block 708). The de-packetized data is then modulated

(block 614) and transmitted out to the MS's via transmit radio block 616.

Note that line 720 signifies the demarcation between the functional blocks

implemented in the remote TRX and those implemented in the CBTS/central TRX. In

practice, the functional blocks above line 720 in Fig. 7A are typically implemented in a

single CBTS/central TRX box (and even on the same backplane in some cases). The

functional blocks below line 720 of Fig. 7 A are implemented in the remote TRX, which is

typically located some distance away from the CBTS/central TRX. This is in contrast to the prior art BTS of Fig. 6 wherein all the major blocks shown therein are co-resident with the

BTS.

On the receive side of Fig. 7 A, RF signals from the MS's are received by Rx radio

block 650 and demodulated into digital data in Demodulation/Equalization block 652. The

demodulated data, as well as the RF quality data obtained in the equalization process, is then

transmitted to the central TRX in the CBTS via transport framing interface blocks 760 and

766. Again, physical layer framing interface blocks 762 and 764 are provided to facilitate

framing of the demodulated data for transmission via the physical layer protocol. If multiple

remote TRX's are provided in an antenna diversity application, for example, optional RF

quality selection block 768 may be employed to select among the remote TRX's one which

offers the best transmission quality. Note that this selection may be performed prior to any

transmission of user data, e.g., as the called MS answers a page from the network.

Thereafter, data from the selected remote TRX is deciphered (if desired), formatted, de-

interleaved, channel decoded, and framed via blocks 654, 656, 658, 660, and 662 in the

manner discussed in connection with Fig. 6 prior to being transmitted to the BSC via ABIS

interface block 602.

Although the division between the remote TRX and the central TRX occurs in

between the ciphering and modulation blocks in the transmit direction and the deciphering

and demodulation/equalization blocks in the receive direction, such is not a limitation of the

invention. In fact, it is contemplated that the split between the remote and central TRX's

may occur anywhere in the transmit and receive paths. Fig. 7B illustrates one exemplary

application wherein the split between the remote and central TRX's occurs between other

blocks of the BTS. In Fig. 7B, the split between the remote and central TRX's occurs

between the speech de-framing and channel coding blocks in the transmit path and between the speech framing and channel decoding blocks in the receive paths. Of course the split

may take place between any other blocks if desired.

Note that it is not necessary that the split between the remote and central TRX's be

symmetrical in the transmit and receive paths. Fig. 7C illustrates one exemplary application

wherein the split between the remote and central TRX's is asymmetrical such that there is

more transmit path circuitries in the central TRX than there are receive path circuitries.

Again, the exact locations in the transmit and receive paths where the remote and central

TRX's may be split are implementation specific.

As can be appreciated from the foregoing, the invention allows the remote TRX's to

be implemented as simple, rugged, and low-maintenance remote antennas. These remote

TRX's may then be deployed throughout the area to be covered, with the bulk of the delicate

and expensive circuitries being implemented in the CBTS and located out of the elements

and/or in an easy-to-access location for maintenance and upgrade. In the antenna diversity

application, the use of multiple simple remote TRX's advantageously minimizes duplication

of logic, as only simple remote radios (instead of the entire TRX as in the case of the prior

art) need to be duplicated. The logic in the CBTS may be shared by multiple radios.

Since multiple remote TRX's may be coupled to a single central TRX in the antenna

diversity application, the inventive architecture offers great flexibility in configuring the cell.

Cell shaping is no longer limited to modifying antenna shape and transmit range around the

BTS. With the inventive CBTS architecture, cabling can be run from a central TRX to any

number of geographically dispersed remote TRX's to form an aggregate cell out of the

geographically dispersed radio cells. With multiple central TRX's per CBTS, the service

provider is given great latitude in configuring the cell. As mentioned, multiple inexpensive low-power remote TRX's may now be

employed in place of the high power TRX of the prior art to cover the same area. Beside

reducing the costs of the radio circuits, the invention also promotes frequency reuse since

each radio cell (associated with each remote TRX) may be made smaller. Also as discussed,

the ability to dynamically associate one or more remote TRX with a given central TRX

offers the service provider great flexibility in reconfiguring the cell to adapt to changes in

capacity using the existing set of remote/central TRX's or additional remote/central TRX's.

While this invention has been described in terms of several preferred embodiments,

there are alterations, permutations, and equivalents which fall within the scope of this

invention. It should also be noted that there are many alternative ways of implementing the

methods and apparatuses of the present invention. It is therefore intended that the following

appended claims be interpreted as including all such alterations, permutations, and

equivalents as fall within the true spirit and scope of the present invention.

Claims

1. In a cellular communication system having a base station controller (BSC)

and a mobile services switching center (MSC), a base transceiver station (BTS) for

facilitating commumcation between said BSC and a plurality of mobile stations (MS's),

said communication with said plurality of said MS's being accomplished via radio

frequency (RF) medium, comprising:

a first interface circuit for coupling said BTS with said BSC;

a processor coupled to said first interface circuit for processing first digital data

received from said BSC to form second digital data;

a first central transceiver (CTRX) circuit coupled to said processor, said first CTRX

circuit being co-resident with said processor and said first interface circuit; and

a first remote transceiver (RTRX) circuit coupled to said first CTRX circuit, said

first RTRX circuit including a first antenna circuit for communicating with a first MS of

said plurality of MS's via said RF medium, wherein said first RTRX circuit is implemented

in a geographically remote manner from said first CTRX circuit.

2. The base transceiver station of claim 1 further comprising:

a second remote transceiver (RTRX) circuit coupled to said first CTRX circuit, said

second RTRX circuit including a second antenna circuit for communicating with said first MS via said RF medium, said first CTRX includes an RF quality selection circuit for

selecting one of said first RTRX and said

second RTRX for use in communicating with said first MS.

3. The base transceiver station of claim 2 wherein said second RTRX circuit

and said first RTRX circuit .are daisy-chained with said first RTRX directly coupled to said

first CTRX and said second RTRX directly coupled to said first RTRX.

4. The base transceiver station of claim 2 further comprising:

a second central transceiver (CTRX) circuit coupled to said processor, said second

CTRX circuit being co-resident with said processor and said first interface circuit;

a third remote transceiver (RTRX) circuit coupled to said second CTRX circuit, said

third RTRX circuit including a third antenna circuit for communicating with a second MS

of said plurality of MS's via said RF medium, wherein said third RTRX circuit is

implemented in a geographically remote manner from said second CTRX circuit.

5. The base transceiver station of claim 4 wherein said first RTRX circuit and

said third RTRX circuit are coupled to said first CTRX and said second CTRX respectively

via a routing circuit.

6. The base transceiver station of claim 5 wherein said routing circuit is an Asynchronous Mode Transfer (ATM) routing circuit.

7. The base transceiver station of claim 5 wherein said routing circuit is a Frame Relay routing circuit.