Base stations for TDD telephony and methods for operating the same

A base station for a time division duplex (TDD) cordless telephone system has a frequency division duplex (FDD) transmit signal path for outputting FDD transmit signals, the transmit signal path including a transmit signal timing generator for controlling the timing of the transmit signals, and an FDD receive signal path for processing FDD receive signals input into the base station. The receive signal path includes a demodulator for demodulating the receive signals, a bit timer responsive to bit clock timing in the receive signals for controlling the timing of the receive signals in dependance on the receive signal bit clock timing and a data processor responsive to the demodulator and the bit timer for correspondingly varying the timing of the receive signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a base station for cordless TDD telephony, 
and is useful in particular, but not exclusively, in PCS (personal 
communications systems) utilizing cable television plants as signal 
conduits. 
The invention may also be employed, for example, in telephone systems 
utilizing dedicated coaxial cable and/or fiber optic and/or microwave 
signal conduits. 
2. Description of the Related Art 
It is expected that PCS microcells will soon be supporting a rapidly 
increasing number of mobile handsets utilizing time division duplex 
protocols in North America. To support this user base, it is essential 
that the PCS-microcells be both low power to assist frequency re-use, and 
low cost, because the net capital costs of the PCS-microcells will be a 
major factor in the economic viability of PCS. 
It has been suggested by a number of organisations that existing cable 
television distribution plant be used to interconnect microcell equipment. 
Taking advantage of the broadband and the nearly ubiquitous nature of 
cable TV plant, it has been further proposed that the microcell equipment 
consist of simple RF repeaters that simply translate off-air mobile voice 
traffic onto the cable plant and vice versa. It has become apparent in 
tests that this approach to PCS-microcells yields both low capital costs 
and improved user service. 
In summary, the low cost arises from the combination of simple technology 
(an RF repeater), using an existing asset base (i.e. cable plant), in a 
fashion that allows modulation/demodulation and PSTN interface equipment 
to be centrally located. This allows these equipment costs to be amortised 
over a very large net coverage area. 
The improved service arises from better call blocking probability 
associated with the ability to centralise the base station-equipment 
rather than a proper allocation to specific microcells. Additionally, the 
cable plant can act to form distributed antenna arrays that can be shaped 
into "roamer corridors". Within these roamer corridors it is also possible 
to control the off-air dynamic range so as to reduce near user/far user 
interactions and line of sight blocking. 
The cable TV plant supports frequency division duplex (FDD) protocols, but 
not time division duplex (TDD) protocols. Consequently, prior art systems 
employ remote antenna drivers (RADs) for effecting FDD-to-TDD conversions 
and TDD-to-FDD conversions at the microcell serving the TDD handsets, and 
a remote antenna signal processor (RASP) for effecting the TDD-to-FDD and 
FDD-to-TDD conversions at a central location. 
It is, however, a disadvantage of such a system that a significant time 
delay occurs in the transmission of the different signals between the base 
station and the handset, because of propagation delays over the cable TV 
plant. The handset synchronizes with a local remote antenna driver, as 
explained in greater detail below, but the base station perceives the 
receive signals from the handset as arriving late, by approximately twice 
the above-mentioned delay. 
BRIEF SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a novel and 
improved base station which mitigates this time delay problem. 
According to the present invention, receives transmit signals in the form 
of frequency division duplex (FDD) signals from a telephone network, the 
base station including a transmit signal path which processes the transmit 
signals in accordance with signal timing generated by a timing generator 
in the base station. A receive signal path in the base station process FDD 
receive signals transmitted from a cordless time division duplex (TDD) 
handset and is provided with mans for deriving bit timing from the FDD 
receive signals, this bit timing being included in the receive signals by 
the cordless handset. 
The receive signal path thus derives, for the FDD receive signals output 
from the base station, timing which is derived from the FDD receive 
signals which are input into the base station. 
Therefore, delays which occur in the transmission of the transmit signals 
from the base station to the handset and in the receive signals 
transmitted from the handset to the base station, e.g. via a co-axial 
cable or a fiber optic cable, do not affect the timing of the receive 
signals output from the base station. 
As described in greater detail below, when the present base station is in 
use, the transmit and receive signals are exchanged with the handset as 
TDD signals via a remote antenna driver, which also effects FDD-to-TDD to 
conversion and vice versa. 
The handset includes means for synchronizing the operation thereof with the 
timing pulses from the base station. 
The present base station thus has no transmit-receive switch for 
alternately connecting the transmit/receive signal paths to a common 
terminal which is connected to the remote antenna driver. Moreover, the 
transmit and receive signal paths of the present base station may operate 
at different frequencies, or alternatively a signal conduit between the 
base station and the remote antenna driver may have separate paths for the 
transmit signal and the receive signal, thus reducing isolation issues. 
Since the base station derives its receive bit clock timing from the 
incoming receive signal, the interface according to the present invention 
is free of the above-mentioned delay issues.

DESCRIPTION OF PRIOR ART 
FIG. 1 illustrates the principal hardware elements and concepts of a prior 
art cordless telephone system. 
In the system illustrated in FIG. 1, base stations 10 and 11 operate at the 
off-air frequencies and perform demodulation and modulation functions for 
the telephone signals. The base stations 10 and 11 interface directly to 
PSTN lines 13 and use a time division duplex protocol. 
The base stations can be mounted to interface with nearby handsets directly 
(not shown), or as in the case of the base station 11 through microcell 
extenders (MEXs) 19 or can be located at a central site, as is the base 
station 10, where their ability to handle calls can be amortised over a 
larger network of microcells connected by TV cable plant. 
A remote antenna signal processor (RASP) 12 is located at the central site 
and interfaces the base station 10 to a cable plant 14. The RASP 12 serves 
to convert receive signals from the cable plant 14 from a frequency 
division duplex (FDD) protocol to a time division duplex (TDD) protocol 
and also to convert transmit signals, from the base station 10 to the 
cable plant 14, from a TDD protocol to an FDD protocol. 
Typically, signals from the base stations 10 travel over the cable plant 14 
to the handset in the 200-450 MHz band. Signals travelling in the reverse 
direction use the 5-30 Mhz return band on the cable plant 14. 
Bi-directional distribution amplifiers 16 need to be compatible with the 
cable plant 14 and provide return band capability. 
RADs 18 pick-up off-air TDD receive signals and relay them back as FDD 
receive signals to the RASP 12 via the plant's return path, and also 
receive FDD transmit signals from the RASP 12, convert them into TDD 
transmit signals and broadcast them as off-air TDD transmit signals to 
nearby handsets, one of which is indicated by reference numeral 22 in FIG. 
1. 
The remote antenna drivers (RADs) 18 must be compatible with the TV cable 
plant 14 and they may be configured for either coaxial cable or fiber 
optic cable plant. 
The microcell extenders (MEXs) 19 use dedicated coaxial or fiber optic 
cable to enlarge the net coverage zone of one of the RADs 18 or the base 
station 11. The MEXs 19 represent a cost effective way of "filling gaps" 
in coverage zones, particularly when there is no in-situ cable plant, e.g. 
on the different levels of a multi-story car park. 
A distributed antenna pattern can be formed by two or more transmitters, 
that operate synchronously or near synchronously. 
The transmitters may be two or more RADs 18 and/or MEXs 19 operating close 
together, so as to have overlapping coverage zones, and connected to a 
common cable plant. 
The net effect as far as the handset user is concerned is that he can roam 
in the overlapping coverage zones of these transmitters without any call 
interruption or the need for any software or hardware processing. This 
benefit arises directly from the TV cable plant's ability to combine the 
broadband amplitude and phase of signals, and the handsets ability to 
transmit to the RADs. 
The microcell designer can use such distributed antenna effects to simply 
increase a coverage zone, to reduce line-of-sight signal blockage, to 
minimize call hand-off issues, or to minimize dynamic range issues. 
The majority of advanced handsets employ a time division duplex (TDD) 
protocol, in which transmit and receive operations take place on the same 
frequency, but alternate in time synchronism. 
There are numerous examples of TDD handsets, e.g. CT-2, CT-2Plus, CT-3, 
Omnipoint CDMA, and advanced cellular handsets all use TDD. 
However, the use of TDD handsets in systems that impose a sizable time 
delay between a handset and a base station can cause a failure to 
communicate between the handset and the base station. 
Thus, FIGS. 2A and 2B illustrate the relationship of the transmit and 
receive signals in the case of a base station 10A communicating directly, 
through an antenna 20, with cordless handset 22A. From FIG. 2B, it can be 
seen that the transmit and receive signals, at the base station 10A and 
the handset 22A, alternate with one another. The arrangement is such that 
the handset 22 synchronises to the timing of the base station 10A. 
However, when an RF repeater arrangement comprising a RASP 12A and a RAD 
18A are interposed between the base station 10A and the handset 22A, as 
shown in FIG. 3A, in order to pass the transmit and receive signals 
through a TV cable plant or other signal conduit 14A, a time delay t (FIG. 
3B) is introduced by the signal conduit 14A between the base station 10A 
and the handset 22A. The handset 22A synchronises with the local RAD 18A, 
but the base station 10A perceives the receive signals from the handset 
22A as arriving late, by approximately twice the cable delay, i.e. 2t, as 
illustrated in FIG. 3B. 
Prior art base stations employ a switch for alternately connecting transmit 
signals to the RASP and receive signals, from the RASP, to a demodulator 
in the base station, the transmit and receive signals having the same 
frequency. 
However, the delay of 2E in the reply from the handset 22, typically causes 
one or more problems for such a prior art base station: 
1. A portion of the late reply is irretrievably lost by the base station's 
transmit-receive switching action; 
2. The base station is asked to process a received signal at the same time 
as transmitting its own signal at an identical frequency. RF isolation 
issues typically disallow this mode of full duplex operation; 
3. The base station's bit sampling clock for incoming signals is 
misaligned, causing a dramatic increase in errors (i.e. the bit sampling 
clock is generated from the transmit-receive timing by the base station). 
These issues apply for all TDD base stations that support TDD handsets. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the present invention, this time delay problem is avoided by 
omitting the switch from the base station, by separating the transmit and 
receive signal paths of the base station, by deriving bit clock timing 
pulses for the receive signal from the handset communicating with the base 
station and by employing different frequencies of separate signal paths 
for the RF modulated transmit and receive signals. Consequently, the RASP 
12 of the prior art system of FIG. 1 is omitted and the present base 
station operates completely in a FDD mode, although it serves a handset 
operating in a TDD mode, as seen through a RAD. 
FIG. 4 shows a telephone system employing the present invention. 
As shown in FIG. 4, a base station 10B is connected through a cable plant 
14B to a RAD 18B, which exchanges off-air transmit and receive signals 
with handsets, one of which is shown and is indicated by reference numeral 
22B. The handset 22B incorporates AFC for correcting the frequency of the 
off-air signals received by the handset. 
The base station 10B operates in an FDD mode and converts analog transmit 
signals from PSTN lines 13A (FIG. 5) into RF modulated FDD transmit 
signals, which are supplied through the cable plant 14A to the RAD 18B. 
The RAD 18B converts these FDD transmit signals into TDD signals, which are 
broadcast to the handset. Also, the RAD 18B receives the off-air TDD 
receive signals from the handset and converts them into FDD receive 
signals for transmission along the cable plant 14B to the base station 
10B. 
At the base station 10B, the FDD receive signals are demodulated, processed 
and converted into analog receive signals, which are supplied to the PSTN 
lines 13A. 
FIG. 4A shows a modification of the interface of FIG. 4, in which the 
single signal conduit which is provided for both the transmit signals and 
the receive signals between the base station 10B and the RAD 18B in FIG. 4 
is replaced by separate signal paths, in the form of two optical fiber 
signal paths 14B and 14C. In that case, the same frequency can be used for 
the transmit and receive signals. 
As shown in FIG. 5, the base station 10B has an analog-to-digital converter 
24, which receives its input as an analog telephone signal from a public 
switched telephone network (not shown) and provides a digital transmit 
signal to a first or transmit signal processor 26. The transmit signal 
processor 26 serves to store incoming data and to send out the stored data 
in high speed bursts, in a manner which is well known in the art and 
which, therefore, is not described in greater detail herein. 
Timing pulses are provided to the processor 26 from a timing generator 28, 
and the output of the processor 26 is supplied to an RF modulator 30, 
which outputs an RF modulated transmit signal to an output terminal 32, 
e.g. at 400 MHz, for connection to through the cable plant to one or more 
RADs 18B for off-air broadcasting to cordless handset 22A 
The RF modulated receive signal from the handset 22B, through the RAD 18B 
and the cable plant, is applied as input to an input terminal 34, which is 
connected to the input of an RF demodulator 36. 
The output of the RF demodulator, in the form of a demodulated receive 
signal, which includes bit clock timing data provided by the handset 22B, 
is applied to a symbol synchronizer and bit timer 38 which derives the bit 
clock timing pulses from the receive signal and applies them to a second 
or receive signal processor 40. The output of the processor 40 is 
converted by a digital-to-analog converter 42 and applied to the public 
switched telephone network. The processor 40 serves to process received 
data and to process this data so that the voice sound eventually 
reproduced is continuous instead of occurring in bursts. The manner of 
operation of the processor 40 is known in the art and, therefore, is not 
described in greater detail herein. 
The base station 10B also includes a demodulator and data processor 39, 
which is connected to the terminal 34 through a directional tap 41. The 
purpose of the demodulator and data processor 39 is to derive data 
relating, e.g. to the functional condition of the RAD 18B, from a status 
signal transmitted from the RAD 18B. This status signal is present only 
when no receive signal is present, and timing pulses are supplied to the 
demodulator and data processor 39 from the symbol synchronizer and bit 
timer 38 to effect timing of the former for this purpose. Status data thus 
derived in supplied on a status line 43. 
Also, a further RF modulator 45 is connected to the terminal 32 of the 
transmit path through a tap 47 for supplying self test and alignment 
control data to the RAD 18B. 
This self test and alignment data may be used, for example, for correcting 
for plant insertion loss caused by variation in the ambient temperature 
around the cable plant and for causing the RAD 18B to check its own 
functional state and/or to check the state of one or more other RADs (not 
shown) connected to the RAD 18B. 
The data processors 26 and 40, the timing generator 28 and the symbol 
synchronizer and bit timer 38 are implemented as a single microprocessor 
marketed under Part No. VP213070 by VLSI Technology Inc., of San Jose, 
Calif. 
The RF modulator 30 is shown in greater detail in FIG. 6, and includes a 
synthesizer 44, connected to receive the output of the processor 26. The 
output of the synthesizer 44 is connected to an amplifier 46, a bandpass 
filter 48, a further amplifier 50 and a further bandpass filter 52, the 
output of which is connected to the terminal 32. Envelope shaping control 
voltages from the processor 26 are connected to the amplifiers 46 and 50 
through conductors 54 and 56. 
The demodulator 36 is illustrated in greater detail in FIG. 7, and 
comprises an amplifier 58 and a bandpass filter 60 for applying the RF 
receive signal from the terminal 34 to a mixer 62. 
A microprocessor controller 64, connected to the processor 26, controls a 
synthesizer 66, the output of which is connected to the mixer 62. 
From the mixer 62, the receive signal passes through a further bandpass 
filter 68 and a further amplifier 70 to an FM demodulation chip 72, which 
incorporates AFC for automatic frequency control of the receive signal. 
A power detector 74 is connected to a directional tap 76 between the 
amplifier and the FM modulation chip, and provides an output to the 
microprocessor controller 64. 
As will be apparent to those skilled in the art, various modifications may 
be made in the above described base station within the scope and spirit of 
the appended claims. 
For example, the ability of the base station to receive signals from the 
RAD on a continuous basis, when considered with the fact that the handsets 
send signals to the RAD and base station only in predefined bursts, allows 
a "RAD-to-base station" communications epoch. Within this epoch the RAD 
can pass status information to the base station. This status information 
may, for example, be employed for maintenance purposes or may be for 
operational use, for example, for allowing the base station to "tag" the 
location of the handset signal and thus to physically locate the caller. 
Similarly, there is a "base station-to-RAD only" communications epoch, in 
which self-test and set-up signals can be passed. 
There has thus been shown and described a novel base station for TDD 
telephony and methods of operating same which fulfills all the objects and 
advantages sought therefor. Many changes, modifications, variations and 
other uses and applications of the subject invention will, however, become 
apparent to those skilled in the art after considering this specification 
and the accompanying drawings which disclose the preferred embodiments 
thereof. All such changes, modifications, variations and other uses and 
applications which do not depart from the spirit and scope of the 
invention are deemed to be covered by the invention, which is to be 
limited only by the claims which follow.