Transmit/receive compensation for a dual FDD/TDD architecture

Compensation measurements can be made of the transmit and receive path circuitry of a dual FDD/TDD system which can be selectively operated in either the FDD mode or alternately in the TDD mode. BY selectively controlling the switches coupling low frequency and high frequency receivers and coupling low frequency and high frequency transmitters to a diplexer, compensation measurements can be made for both FDD and TDD modes of operation.

CROSS-REFERENCES TO RELATED APPLICATIONS: 
The invention disclosed herein is related to the copending US patent 
application by Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled 
"VERTICAL ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULITTONE SPREAD SPECTRUM 
COMMUNICATIONS SYSTEM", Ser. No. 08/806,510, filed on the same day as the 
instant patent application, assigned to AT&T Wireless Services, and 
incorporated herein by reference. 
The invention disclosed herein is related to the copending US patent 
application by Elliott Hoole, entitled "TRANSMIT / RECEIVE COMPENSATION", 
Ser. No. 08/806,508, filed on the same day as the instant patent 
application, assigned to AT&T Wireless Services, and incorporated herein 
by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to communications systems. More particularly, 
the present invention relates to wireless discrete multitone spread 
spectrum communications systems. 
2. Description of Related Art 
Wireless communications systems, such as cellular and personal 
communications systems, operate over limited spectral bandwidths and must 
make highly efficient use of the scarce bandwidth resource for providing 
quality service to a large population of users. 
In the TDD PWAN system described in the copending US patent application by 
Siavash Alamouti, Doug Stolarz, and Joel Becker, entitled "VERTICAL 
ADAPTIVE ANTENNA ARRAY FOR A DISCRETE MULTITONE SPREAD SPECTRUM 
COMMUNICATIONS SYSTEM", Time Division Duplexing (TDD) is used by a base 
station and a remote unit for transmitting data and control information in 
both directions over the same multi-tone frequency channel. Transmission 
from a base station to a remote unit is called "forward transmission" and 
transmission from a remote unit to a base station is called "reverse 
transmission". The time between recurrent transmissions from either a 
remote unit or a base station is called the TDD period. In every TDD 
period, there are four consecutive transmission bursts in each direction. 
Data is transmitted during each burst using multiple tones. A base station 
and each remote unit must synchronize and conform to the TDD timing 
structure and both a base station and a remote unit must synchronize to a 
framing structure. All remote units and base stations are globally 
synchronized so that all remote units transmit at the same time and then 
all base stations transmit at the same time. 
Further, since the TDD PWAN system uses a TDD format, the compensation 
measurements of the transmit and receive path circuitry are made during 
the respective idle times of the paths. The time domain duplexing of an 
airlink results in a 50% duty cycle for utilization of the transmit and 
receive circuits. Therefore, compensation measurements for the circuitry 
of a particular path are performed when an airlink does not require its 
use. Use of the transmit/receive duty cycle of the forward and reverse 
circuits for making transmit/receive compensation measurements frees 
system bandwidth and provides much greater measurement flexibility. 
In the FDD PWAN system described in the copending US patent application by 
Alamouti, Michelson, et al., entitled "Method for Frequency Division 
Duplex Communications", Frequency Division Duplexing (FDD) is used by a 
base station and a remote unit for transmitting data and control 
information in both directions over different multi-tone frequency 
channels. The remote stations and the base station are frequency division 
duplexed (FDD) by transmitting their respective signals on different sets 
of discrete frequency tones in two different frequency bands separated by 
80 MHZ. The FDD PWAN system needs to make compensation measurements of the 
transmit and receive path circuitry. 
What is needed is a way to perform compensation measurements of the 
transmit and receive path circuitry of a dual FDD/TDD system which can be 
selectively operated in either the FDD mode or alternately in the TDD 
mode. 
SUMMARY OF THE INVENTION 
Compensation measurements can be made of the transmit and receive path 
circuitry of a dual FDD/TDD system which can be selectively operated in 
either the FDD mode or alternately in the TDD mode. By selectively 
controlling the switches coupling low frequency and high frequency 
receivers and coupling low frequency and high frequency transmitters to a 
diplexer, compensation measurements can be made for both FDD and TDD modes 
of operation. 
The invention disclosed herein is a new technique to make the most 
efficient use of the scarce spectral bandwidth. The invention is a method 
for updating the transmit and receive compensation weights in a discrete 
multitone spread spectrum (DMT-SS) communications station, such as a base 
station. The frequency spectrum of the DMT-SS tones is divided into two 
portions, an upper frequency band and a lower frequency band. A separate 
transmit and receive path is used for each band, for each antenna at the 
base station. In accordance with the inventive method, the first step is 
receiving in a lower frequency receive path and a higher frequency receive 
path at the base station during a first time period a first spread signal 
comprising a first data signal spread over a plurality of discrete tones 
including a lower tone in the lower frequency path and a higher tone in 
the higher frequency path., The next step is compensating for drift in the 
lower frequency receive path and a higher frequency receive path during 
the first time period by applying receive compensation weights to the 
first spread signal. The third step is testing a lower frequency transmit 
path and a higher frequency transmit path at the base station during the 
first time period and compiling transmit compensation weights. The next 
step is spreading a second data signal at the base station with a 
spreading code that distributes the second data signal over a plurality of 
discrete tones during a second time period. The next step is applying the 
transmit compensation weights to the second data signal during the second 
time period. The next step is transmitting the second spread signal over 
the lower frequency transmit path and the higher frequency transmit path 
during the second time period. The last step is testing the lower 
frequency receive path and the higher frequency receive path at the base 
station during the second time period and compiling new receive 
compensation weights. In this manner, compensation weights are computed 
and applied for both the lower frequency paths and the upper frequency 
paths. 
The invention includes a dual frequency division duplex (FDD) mode and time 
division duplex (TDD) mode. In the FDD mode, the low frequency receiver 
unit RX1 is set to terminal A of the diplexer and the high frequency 
transmitter unit TX2 is set to terminal B of the diplexer. In TDD mode two 
different TDD channels are provided. The first TDD channel uses the low 
frequency and the switch is reset on every TDD cycle to connect either the 
transmitter TX1 or the receiver RX1 to the A terminal of the diplexer. The 
second TDD channel uses the high frequency and the switch is reset on 
every TDD cycle to connect either the transmitter TX2 or the receiver RX2 
to the B terminal of the diplexer. This architecture allows the 
transmit/receive compensation procedure to be applied to the TDD mode 
without disabling normal communications. When either TDD channel is in the 
receive portion of its TDD cycle, then the transmit portion can be tested. 
Likewise, when either TDD channel is in the transmit portion of its TDD 
cycle, then the receive portion can be tested. 
The invention enables compensation measurements of the transmit and receive 
path circuitry of a dual FDD/TDD system which can be selectively operated 
in either the FDD mode or alternately in the TDD mode. By selectively 
controlling the switches coupling the receivers RX1 and RX2 and the 
transmitters TX1 and TX2 to the diplexer, compensation measurements can be 
made for both FDD and TDD modes of operation. 
Currently, the invention has advantageous applications in the field of 
wireless communications, such as cellular communications or personal 
communications, where bandwidth is scarce compared to the number of the 
users and their needs. Such applications may be effected in mobile, fixed, 
or minimally mobile systems. However, the invention may be advantageously 
applied to other, non-wireless, communications systems as well.

DETAILED DESCRIPTION 
FIG. 1 is a block diagram showing an overview of a system providing 
transmit and receive compensation for a base station 100 of a personal 
wireless access network (PWAN), as is described in the referenced Hoole 
patent application. Base station 100 has multiple antennas 115 for 
spatial, as well as spectral, spreading and despreading of discrete 
multitone spread spectrum (DMT-SS) communications. Each antenna 115 has an 
associated transmit/receive module having transmission path components and 
receive path components. Base station 100 also includes a digital signal 
processor (DSP) 101 connected to each of the four transmit/receive modules 
102, 103, 104 and 105. DSP 101 applies spreading and despreading weights 
for DMT-SS signals for the transmit path and the receive path, 
respectively, for each antenna 115. Each module 102, 103, 104 and 105 is 
configured to have the same transmit and receive components, of which only 
the components for module 102 and the connection to DSP 101 are shown and 
described. Module 102 includes a transmit path and a receive path that are 
connected in parallel between DSP 101 and a transmit/receive (T/R) switch 
106. T/R switch 106 is connected to an antenna 115. The transmit path 
includes a transmit compensation weight buffer 111, a packet forming 
buffer 112, a multitone frequency modulator 113, and a transmitter 114. 
When module 102 is in a transmit mode, signal flow through the transmit 
path is from DSP 101 to transmit compensation weight buffer 111, to packet 
forming buffer 112, to multitone frequency modulator 113, to transmitter 
114, and lastly through T/R switch 106 to antenna 115. The receive path 
includes a receiver 107, a multitone frequency demodulator 108, a packet 
buffer 109 and a receiver compensation weight buffer 110. When module 102 
is in a receive mode, signal flow is from antenna 115 through T/R switch 
106 to receiver 107, to multitone frequency demodulator 108, to packet 
buffer 109, to receive compensation weight buffer 110 and to DSP 101. 
The characteristics of the components forming the transmit and receive 
paths of the respective modules have different values based on component 
tolerances and that tend to drift over time and in changes in ambient 
temperature. To compensate for the component tolerances and for drift, the 
transmission and receive paths for each respective antenna is sequentially 
tested for measuring the drift of the transmit path components and the 
receive path components. Compensating weights for each path are generated 
that are then applied to signals processed in each path. To accomplish 
this, base station 100 includes a test controller 116 that is connected to 
DSP 101. Test controller 116 is connected to a test transmitter 117 and a 
test receiver 118. Test controller 116 is shown in FIG. 1 as two blocks 
for illustrative convenience. Test transmitter 117 is connected to each 
module 102, 103, 104 and 105 through a switch 119,so that a receive test 
signal can be sequentially applied to receiver 107 on each module. Test 
transmitter 117 receives an output signal from multitone frequency 
modulator 113 through a switch 121. Test receiver 118 is connected to each 
module 102, 103, 104 and 105 through a switch 120 so that a transmit 
signal can be sequentially applied from transmitter 114 to receiver 118 on 
each transmit/receive module. Test receiver 118 applies an output signal 
to multitone frequency demodulator 108 through a switch 122. 
FIG. 2 is a timing diagram for transmit/receive compensation timing for the 
base station. In a time division duplex (TDD) system, compensation 
measurements 21 are sequentially made for the receive circuitry of each 
module during the transmit portion 22 of the TDD cycle 23 and compensation 
measurements 24 are made for the transmission circuitry during the receive 
portion 26 of the TDD cycle 23. This approach does not use system 
bandwidth because when the transmitter portion of a module is active, the 
receiver portion is being tested and compensated for. Conversely, when the 
receiver portion of a module is active, the transmitter portion is being 
tested and compensated for. 
In a first TDD interval, test controller 16 uses a TDD timing signal from 
DSP 101 for first testing the receive path of a first antenna during the 
base station transmission period. Referring to transmit/receiver module 
102, test controller 116 directs a multitone test signal output from 
frequency modulator 113 in the transmit path and applies it to test 
transmitter 117. Switch 119 is controlled so that a multitone signal 
output from test transmitter 117 is applied to the input of receive 
amplifier 107 in the receive path of module 102. DSP 101 processes the 
received test signal output by receive amplifier 107 and compiles receive 
path compensation weights that are stored in receive path compensation 
buffer 110. The stored receive path compensation weights are then applied 
to the DMT-SS signals received in all subsequent TDD receive periods until 
the receive path test for module 102 are repeated. 
In the base receive period of the first TDD interval, test controller 116 
uses the TDD timing signal from DSP 101 for testing the transmission path 
of module 102. To test the transmission path, a multitone test signal 
output from frequency modulator 113 is used for testing the transmit path 
of module 102. Test controller 116 controls switch 120 so that the 
resultant signal output from transmitter 114 is directed to the input of 
test receiver 118. The output of test receiver 118 is then applied to 
multitone frequency demodulator 108 in the receive path, for a short 
interval so not to overlap actual DMT-SS signals that are output by 
receive amplifier 107 during the receive period. DSP 101 processes the 
received test signal from test receiver 118 and compiles transmit path 
compensation weights that are stored in transmit path compensation buffer 
111. The stored transmit path compensation weights are then applied to 
DMT-SS signals transmitted in all subsequent TDD receive periods until the 
transmit path test for module 102 is repeated. 
Test controller 116 then moves on to transmit/receive module 103 in the 
next consecutive (second) TDD interval. Test controller 116 uses the TDD 
timing signal from DSP 101 for first testing the receive path of module 
103 during base station transmission period, and then for testing the 
transmit path of module 103 during base station receive period. Once 
module 103 has been tested, test controller 116 moves on to 
transmit/receive module 104. When module 104 has been tested, test 
controller tests module 105. When module 105 has been tested, module 102 
is retested, and so on. 
The invention includes a dual frequency division duplex (FDD) mode and time 
division duplex (TDD) mode. FIG. 3 shows a block diagram of the 
transmit/receive scheme for a dual FDD/TDD architecture according to the 
present invention. In the FDD mode, the low frequency receiver unit RX1 is 
set to terminal A of the diplexer and the high frequency transmitter unit 
TX2 is set to terminal B of the diplexer. In TDD mode two different TDD 
channels are provided. The first TDD channel uses the low frequency and 
the switch is reset on every TDD cycle to connect either the transmitter 
TX1 or the receiver RX1 to the A terminal of the diplexer. The second TDD 
channel uses the high frequency and the switch is reset on every TDD cycle 
to connect either the transmitter TX2 or the receiver RX2 to the B 
terminal of the diplexer. This architecture allows the transmit/receive 
compensation procedure to be applied to the TDD mode without disabling 
normal communications. When either TDD channel is in the receive portion 
of its TDD cycle, then the transmit portion can be tested. Likewise, when 
either TDD channel is in the transmit portion of its TDD cycle, then the 
receive portion can be tested. 
FIG. 2 is a timing diagram for transmit/receive compensation timing for a 
base station. In a time division duplex (TDD) system, compensation 
measurements are made for the transmission circuitry during the receive 
portion of the TDD cycle and compensation measurements are made for the 
receive circuitry during the transmit portion of the TDD cycle. A base 
station has multiple antennas for spatial, as well as spectral, spreading 
and despreading of discrete multitone spread spectrum (DMT-SS) 
communications. Each antenna has its own transmission path components and 
receive path components. The transmit amplifier, for example, in the 
transmit path and the receive amplifier, for example, in the receive path 
tend to drift in their characteristics over time. The invention manages 
the sequential testing of each respective transmission path and receive 
path for each antenna. The invention measures the drift of the transmit 
path components and the receive path components and prepares compensating 
weights to be applied to signals processed in each path. 
The base station's digital signal processor (DSP) applies the spreading and 
despreading weights for the DMT-SS signals for the transmit path and the 
receive path, respectively, for each antenna. In a first TDD interval, a 
test controller coupled to the DSP, uses the TDD timing signal from the 
DSP to first test the receive path of a first antenna (during the base 
station transmission period). To test the receive path, the test 
controller takes a multitone test signal output from the frequency 
modulator in the transmit path and applies it to a test transmitter that 
directs the multitone signal to the input of the receive amplifier in the 
receive path. The DSP processes the received test signal output by the 
receive amplifier and compiles receive path compensation weights that are 
stored in a receive path compensation buffer. The receive path 
compensation weights are then applied to the DMT-SS signals received in 
all later TDD receive periods, until the receive path test for that 
antenna are repeated. 
In the base receive period of the first TDD interval, the test controller 
coupled to the DSP, uses the TDD timing signal from the DSP to test the 
transmission path of the first antenna. To test the transmission path, the 
test controller takes applies a multitone test signal output from the 
frequency modulator in the transmit path to the transmitter in the 
transmit path. The test controller then directs the resultant signal 
output from the transmitter in the transmit path to the input of a test 
receiver. The output of the test receiver is than applied to the multitone 
frequency demodulator in the receive path, in a short interval so as to 
not overlap the DMT-SS signals being output by the receive amplifier 
during the receive period. The DSP processes the received test signal 
applied by the test receiver and compiles transmit path compensation 
weights that are stored in a transmit path compensation buffer. The 
transmit path compensation weights are then applied to the DMT-SS signals 
transmitted in all later TDD receive periods, until the transmit path test 
for that antenna is repeated. 
The test controller then moves on to the second antenna in the next 
consecutive (second) TDD interval. The test controller coupled to the DSP, 
uses the TDD timing signal from the DSP to first test the receive path of 
a second antenna (during base station transmission period) and then to 
test the transmit path of the second antenna (during base station receive 
period). 
Separate physical circuitry exists for transmit and receive. With a TDD 
system, each one of those circuit paths are being used half the time. If 
the transmitter is idle half the time and transmit/receive compensation 
measurements have to be performed on that circuitry then those 
measurements can be performed during the idle time of the transmitter. 
This would utilize none of the system band width since at that point in 
time the receiver would be active. Conversely, when the receiver is idle, 
the receiver can be compensated. This takes advantage of the duty cycle of 
the circuitry. 
Transmit and receive compensation uses a set of weights that compensates 
one of the links for the difference in the two paths. The retrodirectivity 
principle relies on the fact that the transmitter receive paths are 
identical. The same circuitry used in both directions and the path to the 
base station electronics are not identical. This is making measurements of 
the transfer function of the circuitry and producing a set of compensating 
weights to apply to the transmitted data so that at the antenna the 
forward and reverse path look identical. 
The de-spreading weights at the receiving node (the base station) are with 
minor modification use as spreading weights on transmission. If the link 
medium were truly identical, there would be identical weights. The trouble 
is that either through drift, tolerances in the electronics, and other 
real life variations the links are not absolutely identical. This 
compensation will give an additive or more applicable factor that will 
make the effect of the de-spread weights and the spread weights the same. 
In a TDD, in receive mode at the base station and the transmit site is 
idle, the measurement is performed by sending out a predetermined set of 
tones out through the transmitter. This is received by a probe antenna and 
demodulated and investigated. This gives you a measurement of the base 
station transmitter probe receiver transfer function. The probe antenna is 
coupled off near the antenna on the base station. From the point where the 
path between the PA and each antennae have to be phase matched or 
identical. On measures the base station transmitter probe receiver 
transfer function and also the base station receiver probe transmitter 
transfer function. The probe itself has to be measured, the probe 
transmitter probe receiver transfer function. 
The compensation measurements of the receive and transmit circuitry are 
made during the idle time of the circuitry. The time domain duplexing of 
the airlink results in a 50% duty cycle for the utilization of the 
transmit and receive circuits. Therefore compensation measurements can be 
performed with the circuitry when the airlink dies not require their use. 
Use of the T/R duty cycle of the forward and reverse circuits to make T/R 
compensation measurements frees system bandwidth and provides much greater 
measurment flexibility. 
FIG. 3 shows receiver 107 including the low frequency band receiver RX1 and 
the high frequency band receiver RX2. FIG. 3 also shows transmitter 114 
including the low frequency band transmitter TX1 and the high frequency 
band transmitter TX2. FIG. 3 shows that in FDD mode, the low frequency 
receiver unit RX1 is set to terminal A of the diplexer by switch setting 
AR and the high frequency transmitter unit TX2 is set to terminal B of the 
diplexer by switch setting BT. In TDD mode two different TDD channels are 
provided. The first TDD channel uses the low frequency and the switch A is 
reset on every TDD cycle to connect either the transmitter TX1 (AT 
setting) or the receiver RX1 (AR setting) to the A terminal of the 
diplexer. The second TDD channel uses the high frequency and the switch B 
is reset on every TDD cycle to connect either the transmitter TX2 (BT 
setting) or the receiver RX2 (BR setting) to the B terminal of the 
diplexer. This architecture allows the transmit/receive compensation 
procedure to be applied to the TDD mode without disabling normal 
communications. When either TDD channel is in the receive portion of its 
TDD cycle, then the transmit portion can be tested. Likewise, when either 
TDD channel is in the transmit portion of its TDD cycle, then the receive 
portion can be tested. 
The PWAN system in the PCS spectrum has two radio frequency bands that are 
separated by 80 MHz from one another. The PWAN system is able to operate 
in a frequency duplex mode (FDD type system--Frequency Division Duplex) or 
it can be operated as a time division duplex (TDD). 
FIG. 3 shows a block diagram of a transmit/receive scheme for a dual 
FDD/TDD architecture according to the present invention. In TDD, there is 
a transmitter and a receiver, a switch at the receiver on both sides and a 
transmitter on both sides. Each chain has a TRC. In FDD mode, one would 
pull the switch A to the AR setting to receive and the switch B to the BT 
setting to transmit and leave them there. The low frequency F(LOW) chain 
operates as "receive only" and the high frequency chain F(HIGH) operates 
as "transmit only". This leaves two idle chains all the time. To swap 
modes so that the receiver is working in the frequency band, switch the TR 
switches to engage the power amplifier in the one band and the low noise 
amplifier in the other band. For FDD mode, move the switch and leave it 
there. 
To operate in TDD mode, switch both A and B on every TDD frame. Switch them 
both (A to AR and B to BR) to receive and receive both frequencies. Then 
switch them both (A to AT and B to BT) to power and transmit on both 
frequencies. 
The topology is basically an up and down conversion radio. The principle 
that is that we have two transmit/receive chains, one is dedicated to the 
lower frequency band and one is dedicated to the higher frequency band, 
whereas, a typical radio would have an up conversion and a down conversion 
or a transmit/receive path. Typically, systems are built to do either TDD 
of FDD. They do not have two complete chains, they only have one. Normally 
they would dedicate: receive to one frequency band, transmit to the other. 
Or they would just have one frequency band that they work in. This 
architecture is built to work in the case where there are diverse 
frequencies and time division duplex. 
In FDD, one has to send for high frequency (the BT setting for B) and 
receive for low frequency (the AR setting for A). To put this same radio 
in FDD, the schematic of FIG. 3 looks exactly the same. The only thing 
different is that one frequency is chosen to receive only. Throw the 
switch (settings BT for B and AR for A) over and leave it. Choose high 
frequency to be the transmit. Throw that switch over there and leave it. 
This gives the opportunity for TR compensation. Power tones can be 
injected by means of the switches A and B. A power tone can be injected 
into the system. One would be the injection signal and one would be rating 
the power. While in the transmit mode (the BT setting) this receive chain 
is idle(AR setting). In TDD mode, during the transmit portion (the AT and 
BT settings), the receive is idle (the AR and BR settings). Testing can be 
done on RX1 and RX2 at this time. 
While the present invention has been described in connection with the 
illustrated embodiments, it will be appreciated and understood that 
modifications may be made without departing from the true spirit and scope 
of the invention.