Telecommunications systems

Details Telecommunications systems Telecommunications systems

1997-12-22

2001-09-25

Ton, Dang (Department: 2661)

Multiplex communications

Communication over free space

Combining or distributing information via time channels

Reexamination Certificate

active

06295290

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of controlling synchronisation of a radio transmitter compared with other receivers in a network, in order to minimise the effect of interference from other transmitters. In particular, the invention relates to the synchronisation of a network of base stations in a mobile communications system.
DESCRIPTION OF RELATED ART
In a digital cellular radio system which operates in accordance with the TDMA-principle, such as GSM, D-AMPS or PDC, radio messages are transmitted in frames from base stations, each frame including a given number of time slots. Transmissions in the different time slots are generally intended to be received by different mobile radio receivers, and so it is necessary to ensure that the receiver is synchronised with the transmitter. The mobile transceivers are synchronised to their respective base stations by a signal from the base station.
For example, in full-rate D-AMPS, three mobile receivers share the same frequency channel, and therefore each channel is divided into three time slots, each lasting 6.7ms, three time slots forming a frame. The frames are repeated 50 times per second.
Each time slot in the frame is assigned to a particular mobile receiver, until either the call is released, or the mobile is handed over to another channel, for example in another cell.
In each time slot, 324 bits are transmitted, most of which are data bits, but 28 of which form a synchronisation word.
The standard published by the Electronics Industries Association as EIA/TIA Publication IS136, which specifies the D-AMPS system, defines six different synchronisation words, but only three of those are used for a channel running at full-rate. Thus, a different synchronisation word is allocated to each time slot in a frame, and the base station transmits the relevant synchronisation word once during each time slot. The mobile receiver is able to recognise transmissions intended for it by identifying the synchronisation word, and similarly includes the same synchronisation word in its own transmissions to the base station.
The synchronisation words in IS136 are chosen such that there is minimal correlation between them. Thus, there is only a very small chance that a receiver will mis-identify a transmitted synchronisation word with a different synchronisation word. A danger, however, is that a receiver will receive the expected synchronisation word from an interfering transmitter operating on the same frequency, and will falsely interpret it as its expected synchronisation word.
Attempts have been made in the prior art to overcome these problems.
One known possibility is simply to allow each base station transceiver to select its own timing, independently of other base stations, meaning that there is no synchronisation between the base stations. In this situation, it is possible that an interfering transmitter will be transmitting the same synchronisation word with a signal level sufficient to cause interference, and at a point in time sufficiently close to the expected time to give the possibility of false synchronisation.
An alternative known possibility is to synchronise the entire network, such that every base station is transmitting the same synchronisation word at the same time. This in fact increases the probability that the base station will falsely receive the synchronisation word from an interfering transmitter, and interpret it as its own expected synchronisation word.
The probability of a false identification in this way depends on the carrier/interference (C/I) ratio, which relates to the relative signal levels of the transmissions from a desired transmitter, and from an interfering transmitter operating on the same frequency. Because there are only a limited number of frequencies available for use in a system, it is necessary to re-use the frequencies. Frequency planning can maximise the frequency re-use distance, and hence maximise the C/I ratio, but in general cannot guarantee that the C/I ratio will be high enough to avoid any possibility of interference by falsely detecting the synchronisation word from an interfering transmitter.
SUMMARY OF THE INVENTION
The invention relates to a method of operation of a base station transceiver, in which the transceiver detects signals from interfering transmitters, and selects its own air frame position, namely the time at which sync word
1
is transmitted, such that the possibility of falsely detecting a synchronisation word from a cochannel interferer is minimised.
The invention also relates to the transceiver itself, and to a base station incorporating a transceiver.


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patent: 5528597 (1996-06-01), Gerszberg et al.
patent: 5666366 (1997-09-01), Malek et al.
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patent: 5802076 (1998-09-01), Weigand et al.
patent: 5822313 (1998-10-01), Malek et al.
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British Search Report Under Section 17 re GB 9626681.2 Date of Search: Mar. 14, 1997.

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