Method of establishing a communication link in a digital cordless telephone system

A digital time-division duplex radio communication link is established or re-established between one of a plurality of portable units and one or more base units forming a cordless telephone system, so as to avoid potential interference created by the use of an unsynchronized call-up transmission in the presence of synchronized signals being exchanged by the rest of the system. The call-up transmission is pre-synchronized with the rest of the system so that switchovers between transmission bursts and reception windows occur at the same time as between the call-up signal and the rest of the system. This is achieved by deriving synchronization information from a synchronization signal which may, for example, be an existing signal on the system containing synchronization information (i.e., an existing call on the system) or may be a beacon signal which, in some systems, is transmitted by the fixed units for the purpose of range indication. The method is particularly applicable to the MUX1/MUX2/MUX3 format of signals used in the CT2 common air interface protocol.

The present invention relates to duplex communications systems, and to a 
method of establishing a digital time-division duplex radio communication 
link between one of a plurality of portable units and one or more base 
units forming a cordless telephone system. The invention is directed 
particularly at the interference problem created by the use of an 
unsynchronised transmission in the presence of synchronised systems. 
Such a system is shown, in its simplest form, in FIG. 1 of the accompanying 
drawings to which reference will now be made. The system illustrated 
comprises a fixed part in the form of a base unit 1, and two portable 
parts in the form of respective handsets 2,3. Each handset comprises an 
earpiece, microphone and keypad, this latter being shown diagrammatically 
under reference 4. In addition, each handset contains a respective radio 
transmitter/ receiver (transceiver) and associated aerial 6,7 by which the 
handsets may communicate with the base unit by radio, as represented by 
the dotted lines 8,9. The base unit likewise contains a number of 
transceivers at least equal to the number of handsets, together with an 
aerial 5 for transmission and reception of radio signals from the 
handsets. The handsets may communicate with each other, but only via the 
base unit. The base unit also includes a hard-wired connection 10 to the 
external telephone system, and contains interface circuitry for 
interfacing the base unit transceiver to the external telephone line. 
Although only two handsets are shown, this is to be taken as an example of 
the simplest system and many more handsets, up to the capacity of the 
system, may be provided. 
The present invention is concerned with systems of the type illustrated in 
FIG. 1, in which the speech and other information to be transmitted 
between the base unit and the handsets is digitally encoded before 
transmission, is transmitted as a digital signal, and is decoded after 
reception to reproduce the original. A limited number of radio channels 
are allocated for the radio links 8,9 and it is clearly therefore 
preferable to utilise the same channel for both ends of a radio link--i.e. 
duplex communication. Each transceiver in the system will be able to 
transmit and receive on a number of these channels, if not all. 
In digital second generation (CT2 ) cordless telephone systems, burst mode 
duplex is used to provide full duplex speech on a single channel. This 
essentially means that each transmitter has to compress the encoded speech 
from a particular time interval (called the burst period) down to just 
under half that interval (called the burst duration) in order to transmit 
the encoded speech and have time to receive the returning encoded speech 
in the other half of the burst period. This action is commonly called 
ping-pong transmission mode. it should be noted that the encoded speech 
corresponds to the speech from the entire burst period and on reception is 
expanded to its normal representation as continuous speech. 
There is a need for a common protocol for the exchange of signals, 
primarily control and synchronising signals, between the fixed and 
portable parts of the system. In the case of CT2, such a protocol, known 
as a common air interface (CAI), has been agreed and is described in 
detail in international patent application WO90/09071. A knowledge of the 
contents of this application is desirable for a full understanding of the 
present invention. The present applicant's own air interface, a variant of 
the common air interface is described in European patent application 
0375361. 
In the agreed protocol, exchange of signals is by way of three distinct 
transmitted burst signal patterns or structures exchanged between the 
fixed and portable parts of the system. These different burst structures 
are known respectively as MUX3, MUX2 and 
MUX1, the acronym "MUX " standing for multiplex. In addition, the agreed 
protocol defines three sub-channels to be multiplexed within the available 
data bandwidth: 
i) a signalling channel (D channel); 
ii) a speech channel (B channel; 
iii) a burst synchronisation channel (SYN channel) containing bit and burst 
synchronising information. 
The structure of the various multiplexes is described in detail in the 
above-mentioned patent applications. Briefly the arrangement of sub 
channels within the multiplexes is as follows: 
MUX1 is used hi-directionally over an already-established link between a 
portable part and a fixed part to carry the D and B channels. There are no 
SYN channel bits in MUX1. MUX1 supports both the 66 bit and 88 bit length 
burst structures defined in the protocol: MUX 1.4 is 68 bits long, having 
64 B bits, with 2 D bits at either end; and MUX1.2 is 66 bits long, having 
64 B bits, with 1 D bit at either end. 
MUX2 is used for link establishment, and for re-establishment of a 
previously-broken link. MUX2 comprises 34 bits in the SYN channel 
sandwiched between 32 bits (16 at each end) in the D channel i.e. 66 bits 
long. There are no B channel bits in MUX2. 
MUX3 is used for link establishment and re-establishment in the direction 
portable part to fixed part only. A representation of MUX3 is shown in 
FIG. 2 and will be seen to comprise seven frames, numbered 1 to 7 down the 
right-hand side, each 144 bits long, and of which the first five frames (1 
to 5) are transmitted in sequence. The order of bits within each of frames 
1 to 5 is from left to right in the diagram. The first four frames, 1 to 
4, contain D-channel bits, and comprise 20 bit D-channel words each split 
into two 10-bit sections surrounded by preamble (P) bits. The fifth frame 
comprises 24 SYN channel bits surrounded by 24 preamble (P) bits (12 at 
each end). During frames 6 and 7, the transceiver in the portable part 
listens for a response from a fixed part. The sequence then repeats. 
The MUX3 sequence thus comprises 5 transmission bursts, lasting for a total 
of 10 ms, followed by two transmission-off burst periods lasting for 4 ms 
during which the transceiver is in receive mode. In practice, this 
sequence is repeated for a period of at least 750 ms, or until a link is 
established. 
This use of a multiple repeat transmission to set up a link is known, and 
the MUX3 protocol just described is but one example of this. In the case 
of the CT2 protocol, the fixed part is restricted as to which receive 
window it may use to receive transmissions from the portable part. The 
portable part must therefore transmit in a manner suitable for an 
arbitrary window and previously this has implied an unsynchronised 
transmission. 
In the present invention, the synchronism of the system is deduced, and 
used to adjust the timing of the previously unsynchronised transmission to 
make it effectively synchronous without actually changing its form. The 
technique can be used both for initial establishing of the two-way link, 
and for re-establishment of an existing link which has failed. 
In dense usage environments communications systems are synchronised so that 
the AM (amplitude modulation) splatter caused by the repeated switching on 
and off of transmissions occurs in a "guard band" which is maintained 
between the termination of transmission from one part of the system and 
the start of transmission from the other part of the system. Thus the 
splatter is rendered harmless. However, the use of an unsynchronised call 
up transmission, such as MUX3, at the time of establishing or 
re-establishing a link causes interference because the AM splatter does 
not necessarily occur in the guard band, but at arbitrary times throughout 
the reception phase of the rest of the system. 
The practical effect of the present invention is that it is now possible to 
pre-synchronise the call-up transmission. For example, in the case of 
MUX3, the transmission comprises a 10 ms transmit, followed by a 2 ms 
receive. Since the frame rate of the CAI CT2 system is 2 ms (144 bit frame 
size) then, provided that one of the on/off transitions of MUX3 is aligned 
with the system guard band, the opposite transition will also be aligned 
because the MUX3 transmission can be considered as blocks of the same size 
(2 ms; 144 bits) as the system frame rate. In this way, MUX3 becomes 
synchronised to the system guard band and therefore ceases to be an 
asynchronous interference source with the detrimental effects of splatter. 
In order to enable synchronisation to be effected, the unit, normally the 
portable part, which is to transmit the call-up signal 1s set to receive a 
suitable synchronisation signal which is used to synchronise the call-up 
signal to the rest of the system. This synchonisation signal can be 
derived in various ways. Preferably the synchronisation signal is taken 
from an existing signal which is being transmitted within the system. For 
example an existing call elsewhere on the system can be used to derive 
synchronisation information. Alternatively an agreed off-air 
synchronisation signal could be used, such as a standard time and 
frequency reference signal. As a still further, and currently preferred, 
alternative an existing system beacon signal could be used to supply the 
synchronisation information to the portable part. 
A system beacon signal is sent out by one or all of the fixed units on one 
channel of the band, and is primarily intended as a range indicator to let 
a portable unit know when it is in range, and this enables the portable 
unit to log onto the base unit. In the CT2 CAI system, the beacon signal, 
if present, comprises continuous multiple repeats of MUX2 and thus 
contains the synchronisation information which a portable unit needs to 
establish synchronisation for its call-up signal, be this MUX3 or 
equivalent.

Using as an example the MUX1/2/3 protocol briefly described above, the 
procedure to establish a new link would in detail be as follows: 
1 ) The portable part learns the synchronisation of the target system, for 
example from a system beacon transmitting MUX2 on one of the channels of 
the band. 
2 ) Optionally the portable part may adjust its frequency to that of the 
system. 
3 ) The portable unit selects a channel on which to make its call to the 
fixed part. There are various ways in which this selection can be made, 
but these are known, and will not be described. 
4 ) The portable unit sets the phase of its MUX3 call-up transmission to 
coincide with that of the system, ensuring that the disruption to the 
system is minimised by centering the AM splatter within the system guard 
band. The optimum setting is approximately such that the 5th bit of the 
MUX3 transmission occurs at the same time as the first valid data bit in 
MUX2. 
5 ) The portable part transmits MUX3 in the same way as hitherto. 
6 ) Optionally the portable part checks synchronisation at suitable 
intervals to permit any drift to be corrected. For CT2 CAI the worst case 
drift in the system without step 2 would incur 7.2 bits of drift per 
second. Approximately 1 bit of drift could be tolerated before the onset 
of some degradation. With step 2 included a much longer period would 
elapse before the critical amount of drift had occurred. 
For link re-establishment after failure of an existing link the same 
procedure may be used as described above, but with the additional option 
of using the originally set-up link (i.e. the link as it existed before 
failure) as the source of synchronisation in step 1, rather than the 
system beacon. Synchronisation information can be constantly stored within 
the fixed part for this purpose, and used to synchronise the 
re-established link. 
The procedure described above is fully compatible with non-synchronising 
systems and simply falls back to the non-synchronised method in the 
absence of a suitable system synchronisation reference --i.e. only steps 3 
and 5 above are invoked. 
The attached FIG. 3 is a timing diagram illustrating the above-described 
techniques. The drawing is divided horizontally into 3 parts: FIG. 3A 
illustrates the unsynchronised MUX3 call-up signal from the portable part; 
FIG. 3B illustrates the transmission of MUX1 or MUX2 from a fixed part; 
and FIG. 3C illustrates the MUX3 transmission from the portable part, 
synchronised in the manner discussed above with the base unit 
transmissions. 
In FIG. 3, the taller blocks represent bursts of transmission and the 
shorter blocks represent reception windows. In between transmission and 
reception blocks the graphs fall to zero and rise again to represent 
diagrammatically the changeover from one mode to another, this in between 
period comprising the above-referred to guard band. The top waveform (FIG. 
3A) is unsynchronised with the MUX1 or MUX2 waveform represented in the 
centre part of the drawing (FIG. 3B) and it will be seen that all three 
guard bands 11, 12 and 13 in the change-over from MUX3 transmission 14 to 
reception 15 occur during reception windows 16 of the MUX1/MUX2 waveform 
illustrated in FIG. 3B, thus resulting in am splatter. By contrast, the 
bottom waveform (FIG. 3C) is synchronised with the MUX1/ MUX2 waveform 
illustrated in FIG. 3B, so that any interference occurring during the 
changeover from transmission to reception in MUX3 is rendered harmless. 
It will be noted that the total length of a single transmission burst and 
reception window, together with associated guard bands, amounts to one 
frame, having a period of 2 ms. This is a submultiple of the MUX3 
transmission burst (10 ms) and MUX3 reception window (4 ms) and thus 
enables the call-up signal guard bands and the normal ping-pong 
communications signal guard bands to be brought into synchronism, as 
illustrated with reference to FIG. 3B and FIG. 3C.