WO1997026720A1

WO1997026720A1 - Delay insensitive basestation-to-handset interface for radio telephone systems

Info

Publication number: WO1997026720A1
Application number: PCT/CA1996/000038
Authority: WO
Inventors: Andrew Beasley
Original assignee: Pcs Wireless (Ip) Inc.
Priority date: 1996-01-19
Filing date: 1996-01-19
Publication date: 1997-07-24
Also published as: AU4476896A

Abstract

A basestation-to-handset interface for a telephone system has a cordless handset (22B) operating in a TDD mode, a remote antenna driver (18B) for converting off-air TDD receive signals from the handset (22B) to FDD receive signals and for converting FDD transmit signals into off-air TDD transmit signals and broadcasting the off-air TDD signals to the handset (22B), a basestation (10B) operating in an FDD mode for processing the transmit and receive signals, and a signal conduit (14A or 14B, 14C) connecting the basestation (10B) to the remote antenna driver (22B). The basestation (10B) has a transmit signal path for supplying the FDD transmit signals to the remote antenna driver (22B) and a receive signal path separate from the transmit signal path for processing the TDD receive signals. The transmit signal path includes a timing pulse generator (28) and an RF modulator (30) and the receive signal path includes a demodulator (36) and derives bit clock timing pulses from the FDD receive signals for controlling the timing of the receive signal path, so that such timing is derived from the handset (22B) instead of from the timing pulse generator (28) in the transmit signal path.

Description

Delay Insensitive Basestation -To - Handset Interface For Radio Telephone Systems.

Technical Field

The present invention relates to basestation - to - handset interfaces for radio telephone systems, 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.

Background 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 basestation 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 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 basestation 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 basestation perceives the receive signals from the handset as arriving late, by approximately twice the above-mentioned delay.

Disclosure of the Invention

According to the present invention, there is provided a basestation - to - handset interface for a telephone system which has a cordless handset operating in a TDD (time division duplex) mode, a remote antenna driver for converting off-air TDD receive signals from the handset to FDD (frequency division duplex) receive signals and for converting FDD transmit signals into off-air TDD transmit signals and broadcasting the off-air TDD signals to the handset, a basestation operating in an FDD mode for processing the transmit and receive signals, and a signal conduit connecting the basestation to the remote antenna driver.

The basestation has a transmit signal path for supplying the FDD transmit signals for transmission to the remote antenna driver and a receive signal path separate from the transmit signal path for processing the TDD receive signals.

The transmit signal path including means for generating timing pulses for the FDD transmit signals and means for modulating the FDD transmit signals, and the receive signal path including a demodulator for demodulating the FDD receive signals and means for deriving bit clock timing pulses from the FDD receive signals for controlling the timing of the receive signal path, whereby the timing of the receive signal path is controlled by the handset.

The handset includes means for synchronizing the operation thereof with the timing pulses from the basestation.

The present basestation thus has no transmit-receive switch for alternately connecting the transmit/receive signal paths to a common teπninal which is connected to the remote antenna driver. Moreover, the transmit and receive signal paths of the present basestation may operate at different frequencies, or alternatively the signal conduit between the basestation and the remote antenna driver may have separate paths for the transmit signal and the receive signal, thus reducing isolation issues.

Since the basestation 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. Brief Description of The Drawings

The present invention will be more readily apparent to those skilled in the art from the following description of embodiments of the present invention when taken in conjunction with the accompanying drawings, in which:

Figure 1 shows a block diagram of a prior art time division duplex cordless telephone system;

Figure 2A diagrammatically illustrates a prior art basestation communicating directly with a cordless handset, and Figure 2B illustrates the relationship of the transmit and receive signals of the arrangement of Figure 2A;

Figure 3A shows a prior art basestation communicating with a handset through a remote antenna signal processor, a dedicated coaxial cable and a remote antenna driver, and Figure 3B illustrates a time delay occurring in the arrangement of Figure 3A;

Figure 4 shows a block diagram of a basestation - to - handset interface embodying the present invention;

Figure 4A shows a block diagram similar to that of Figure 4 but illustrating a modification of the basestation of Figure 4;

Figure 5 shows a block diagram of a basestation for an interface embodying the present invention;

Figure 6 shows a block diagram of an RF modulator forming part of the base station of Figure 5; and

Figure^" 7 shows a block diagram of an RF demodulator forming part of the basestation of Figure 5. Description of the Rest Module

Figure 1 illustrates the principal hardware elements and concepts of a prior art cordless telephone system.

In the system illustrated in Figure 1, basestations 10 and 11 operate at the off-air frequencies and perform demodulation and modulation functions for the telephone signals. The basestations 10 and 11 interface directly to PSTN lines 13 and use a time division duplex protocol.

The basestations can be mounted to interface with nearby handsets directly (not shown), or as in the case of the basestation 11 through microcell extenders (MEXs) 19 or can be located at a central site, as is the basestation 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 basestation 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 basestation 10 to the cable plant 14, from a TDD protocol to an FDD protocol.

Typically, signals from the basestations 10 travel over the cable plant 14 to the handset in the 200 - 450 MHz band which is the downstream band. Signals travelling in the reverse direction use the 5 - 30 Mhz return band on the cable plant 14 which is the upstream band. This cable plant is known as a bidirectional plant.

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 Figure 1. More particularly, the RADs 18 obtain frequency division duplex transmit signals from the cable TV plant and radiate them as time division duplex signals, and also obtain time division duplex signals from the handsets and send them over the plant as frequency division duplex signals.

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.

However, the use of TDD handsets in systems that impose a sizable time delay between a handset and a basestation can cause a failure to communicate between the handset and the basestation.

Thus, Figures 2 A and 2B illustrate the relationship of the transmit and receive signals in the case of a basestation 10A communicating directly, through an antenna 20, with cordless handset 22A. From Figure 2B, it can be seen that the transmit and receive signals, at the basestation 10A and the handset 22 A, alternate with one another. The arrangement is such that the handset 22 synchronises to the timing of the basestation 10A.

However, when an RF repeater arrangement comprising a RASP 12A and a RAD 18A are interposed between the basestation 10A and the handset 22 A, as shown in Figure 3 A, in order to pass the transmit and receive signals through a TV cable plant or other signal conduit 14A, a time delay t (Figure 3B) is introduced by the signal conduit 14A between the basestation 10A and the handset 22 A. The handset 22 A synchronises with the local RAD 18A, but the basestation 10A perceives the receive signals from the handset 22A as arriving late, by approximately twice the cable delay, i.e. 2t, as illustrated in Figure 3B.

Prior art basestations employ a switch for alternately connecting transmit signals to the RASP and receive signals, from the RASP, to a demodulator in the basestation, the transmit and receive signals having the same frequency.

However, the delay of 2r in the reply from the handset 22A typically causes one or more problems for such a prior art basestation:

1. A portion of the late reply is irretrievably lost by the basestation' 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 basestation '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 basestation).

These issues apply for all TDD base stations that support TDD handsets.

Description of the Best Mode

According to the present invention, this time delay problem is avoided by omitting the switch from the basestation, by separating the transmit and receive signal paths of the basestation, by deriving bit clock timing pulses for the receive signal from the handset communicating with the basestation and by employing different frequencies or separate signal paths for the RF modulated transmit and receive signals. Consequently, the RASP 12 of the prior art system of Figure 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.

Figure 4 shows a telephone system according to the present invention. As shown in Figure 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 basestation 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 basestation 10B, the FDD receive signals are demodulated, processed and converted into analog receive signals, which are supplied to the PSTN lines 13A.

Figure 4A shows a modification of the interface of Figure 4, in which the single signal conduit which is provided for both the transmit signals and the receive signals between the basestation 10B and the RAD 18B in Figure 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 Figure 5, me basestation 10B has a transmit signal path provided with an analog-to-digital converter 24, which receives its input from the public switched telephone network (not shown) and provides a digital transmit signal to a data processor 26. The data 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 data processor 26 from a timing generator 28, and the output of the data 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 from the transmit signal path to through the downstream band of 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 downstream band of the cable plant, is applied as input to an input terminal 34, which is connected to the input of an RF demodulator 36 in a receive signal path in the basestation 10B.

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 data processor 40. The output of the data processor 40 is converted by a digital-to-analog converter 42 and applied to the public switched telephone network. The data processor 40 serves to process received data so that the voice sound eventually reproduced is continuous instead of occurring in bursts. The manner of operation of the data processor 40 is known in the art and, therefore, is not described in greater detail herein.

The basestation 10B also includes a demodulator and data processor 40, 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 40 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.

VP263070 by VLSI Technology Inc. , of San Jose, California.

The RF modulator 30 is shown in greater detail in Figure 6, and includes a synthesizer 44, connected to receive the output of the data processing converter 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 data processor 26 are connected to the amplifiers 46 and 50 through conductors 54 and 56.

The demodulator 36 is illustrated in greater detail in Figure 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 data processing component 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 basestation within the scope and spirit of the appended claims.

For example, the ability of the basestation to receive signals from the RAD on a continuous basis, when considered with the fact that the handsets send signals to the RAD and basestation only in predefined bursts, allows a "RAD-to-basestation" 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, allowing the basestation to "tag" the location of the handset signal and thus to physically locate the caller.

Similarly, there is a "basestation-to-RAD only" communications epoch, in which self- test and set-up signals can be passed.

Claims

1. A method of exchanging telephone signals between a basestation and a mobile cordless time division duplex (TDD) handset, comprising the steps of passing transmit signals from the basestation to a remote antenna driver; broadcasting the transmit signals from the remote antenna driver to the mobile handset; receiving at the remote antenna driver receive signals broadcast by the handset, the receive signals including bit clock timing: and passing the receive signals from the remote antenna driver to the basestation (10B); characterized by deriving bit clock timing from the receive signals at the basestation (10B); and employing the bit clock timing to process the receive signals at the basestation.

2. A method as claimed in claim 1 , characterized in that the transmit and receive signals are exchanged between the basestation (10B) and the remote antenna driver (18B) as frequency division duplex (FDD) signals, the transmit signals are converted to time division duplex (TDD) signals at the remote antenna driver (18B) and the receive signals are transmitted from the handset (22B) to the remote antenna driver (18B) as TDD signals which include the bit clock timing.

3. A method as claimed in claim 1, characterized in that the transmit and receive signals are exchanged between the basestation (10B) and the remote antenna driver (18B) through separate signal paths at the same frequency.

4. A time division duplex (TDD) cordless telephone system, for carrying out the method according to claim 1 , comprising a cordless handset (22B) operating in a TDD mode; a remote antenna driver (18B) for converting TDD receive signals into broadcast from the handset (22B) frequency division duplex (FDD) receive signals and for converting FDD transmit signals into TDD transmit ^* signals and broadcasting the TDD signals to the handset; a basestation (10B) operating in an FDD mode; and a signal conduit (14A or 14B, 14C) connecting the basestation (10B) to the remote antenna driver(18B); the basestation (18B) comprising an FDD transmit signal path for transmission of the FDD transmit signals to the remote antenna driver (18B) and an FDD receive signal path for the FDD receive signals from the remote antenna driver (18B); the transmit signal path including an FDD transmit signal timer (28); and the receiver signal path including an FDD receive signal demodulator (36), characterized in that the receive signal path also includes a receive signal timer (38) responsive to bit clock timing pulses in the FDD receive signals and a receive signal processor (40) responsive to the demodulator (36) and the receive signal timer (38).

5. A time division duplex (TDD) cordless telephone system as claimed in claim 4, characterized in that the signal conduit comprises separate first and second signals paths (14B, 14C) connected to the transmit and receive signal paths, respectively.

6. A time division duplex (TDD) cordless telephone system, for carrying out the method according to claim 1, comprising a remote antenna driver (18B) responsive to frequency division duplex (FDD) transmit signals from a basestation (10B) and time division duplex (TDD) receive signals from a handset (22B); and a signal conduit (14A) connecting the basestation (10B) to the remote antenna driver (18B); the basestation (10B) including a transmit signal path, a transmit signal processor (26,28) in the transmit signal path, and a receive signal path, characterized in that the receive signal path includes a receive signal processing apparatus (38,40) with timing capability independent of the transmit signal processor (26,28) in the receive signal path.

7. A time division duplex cordless telephone system as claimed in claim 6, characterized in that the receive signal processing apparatus includes a bit clock timer (38) responsive to bit clock timing pulses in the receive signals.

8. A time division duplex cordless telephone system according to claim 6, wherein the basestation (10B) includes an analog-to-digital converter (24) for providing digital transmit signals in response to analog transmit signals from a public switched telephone network; an FDD transmit signal timing generator (28); a first data processor (26) for providing the FDD transmit signals in response to the digital transmit signals from said analog-to-digital converter and in response to said timing generator (28); and an RF modulator (30) for modulating the FDD transmit signals; the basestation (10B) further including an RF demodulator (36) for demodulating the FDD receive signals from the remote antenna driver; characterized a symbol synchronizer and bit timer (38) in that the receive signal processing apparatus comprises for deriving receive bit clock timing and symbol clock timing from the demodulated FDD receive signals; and a second data processor (40) for processing the output of said symbol synchronizer and bit timer (38) in accordance with the receive bit clock timing.

9. A basestation for a time division duplex (TDD) cordless telephone system, comprising a frequency division duplex (FDD) transmit signal path and an FDD receive signal path; the transmit signal path including a transmit signal timing generator (28); and the receive signal path including a demodulator for demodulating FDD receive signals, characterized in that the receive signal path also has a bit timer (38) responsive to the demodulator and independent of the transmit signal timing generator for controlling the timing of the receive signal in accordance with bit timing provided by a handset (22B) broadcasting the receive signal.