GB2431074A

GB2431074A - Method of reducing interference between orthognal signals.

Info

Publication number: GB2431074A
Application number: GB0525611A
Authority: GB
Inventors: Russell John Coy; Anthony Peter Hulbert; David Thomas
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2005-07-04
Filing date: 2005-12-16
Publication date: 2007-04-11
Anticipated expiration: 2025-12-16
Also published as: GB2431074B; GB0525611D0; GB0513570D0

Abstract

A method of reducing interference between orthogonal signals in contention on a channel of an unsynchronised mobile communications system, the source of each signal being not known comprises for each signal received at a receiver, setting a delay for a subsequent transmission from a transmitter of that signal based on data in the signal itself, such that different delays are applied to different signals, thereby avoiding subsequent overlap of the signals.

Description

A METHOD OF REDUCING INTERFERENCE BETWEEN ORTHOGONAL

SIGNALS

This invention relates to a method of reducing interference between orthogonal signals, in particular for downlmk broadcast control channel transmissions.

Base stations of a cellular radio system may transmit a short duration down- link broadcast control channel, or beacon, with a fixed repeat period. The beacons are typically short duration, high power signals, sent from a base station with information about a cell, which can be heard by other cells. Beacons tend to be periodic, for example if a beacon is sent every 92 ms, two may be received together, but a receiver in a mobile terminal cannot tell which base station sent which beacon and the beacons interfere. If the base stations are synchronised to a common timing reference, then the broadcast channel transmissions may be arranged in time such that they do not interfere at mobile stations. If the base stations are not synchronised to a common time reference, then the timing of the broadcast channel transmissions for each base station will be randomly selected in time and due to timing drift between the timing references of each base station, may become aligned in time and remain aligned for some time. In this case, significant inter-cell interference can occur at the receiving terminal.

Conventionally, it has been assumed that the time difference between each beacon means that they cannot collide, but this assumption fails because the base stations are not perfectly synchronised. The timing at each base station drifts and eventually the beacons overlap. The periodicity of the beacons means that they will then continue to overlap. Current systems do not employ a specific inter-cell * * interference reduction technique, but tolerate the additional interference caused. S...

In accordance with the present invention, a method of reducing interference * between orthogonal signals in contention on a channel of an unsynchronised mobile communications system, the source of each signal being not known, comprises for S..

each signal received at a receiver, setting a delay for a subsequent transmission from * a transmitter of that signal based on data in the signal itself, such that different * delays are applied to different signals, thereby avoiding subsequent overlap of the signals.

Preferably, a timing chart is stored and accessed by each receiver and transmitter to determine the time delay before a subsequent transmission.

Preferably, maximum and minimum ranges of timing adjustment are set.

This prevents the periods between any two transmissions being too long or too short.

Preferably, an additional pseudo random information field is inserted into each transmission.

This improves timing randomisation.

In this case, the orthogonal signals maybe sine waves of different frequencies, as in orthogonal frequency division multiplex (OFDM).

Preferably, the data comprises signal frequency.

An example of a method of reducing interference between orthogonal signals in accordance with the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a block diagram showing apparatus for carrying out the method of the present invention; Figure 2 illustrates an example of a conventional situation in which broadcast channel transmission collisions occur due to unsynchronised base stations and drift; Figure 3 shows an example of the method of the present invention, showing a reduction in collisions due to applying a transmission timing randomisation process.

Figure 4 is a timing diagram illustrating in more detail the principles of the method of the present invention; and, Figure 5 is an alternative illustration of how the present invention deals with potential collisions of transmissions from different base stations.

S

Fig.! illustrates a typical system in which a plurality of base stations BSI, * :: 5 BS2 transmit to one or more mobile stations, MS 1, MS2, MS3. In a conventional S..

system as illustrated by Fig. 2, each base station transmits at fixed regular intervals, * . but these intervals are different for different base stations. For example, BS I * S..

transmits at nominal transmission times la to le all separated by a time period a.

Similarly, BS2 transmits at nominal transmission times 3a to 3e each separated by a time period b. On the assumption that a transmission 2, 4 respectively takes place at the nominal time, it can be seen that due to uncontrolled initial timing and drift of BS2 relative to BS I there is a partial or complete overlap for the transmissions from the base stations at times lb to I d and 3b to 3d causing collisions at the mobile.

In Fig. 3, an example of the present invention is illustrated, showing the nominal transmission times for BS 1 iDa to IDe separated by a time period a, together with the corresponding actual transmissions 11, 12, 13, 14, 15 where the transmit timing of BS 1 and BS2 has been randomised using the information content of the transmissions. En Fig. 3, the same information is shown for BS2 with nominal times 16a to 16e and actual transmissions 17, 18, 19, 20, 21. It can be seen that although the nominal times 1 6a to 1 6e reach a point at which they occur together 1 Oc, 1 6c the actual transmissions 13, 19 for that nominal time are separated and the actual transmissions for the cases b to d which previously overlapped are also separated leaving only one case where the actual transmissions 15, 21 coincide, instead of the three instances of collisions in Fig. 2. Thus, the probability of collision has been significantly reduced due to the randomised transit timing position.

This invention provides a method of avoiding inter-cell interference due to time coincident downlink broadcast channels, at the mobile terminal, within an unsynchroniscd cellular radio system using a common transmit carrier frequency.

This is made possible by the basestation downlink broadcast control channel transmission timing being randomised from a nominal timing position. The transmissions from each basestation in the system have a different transmission timing pattern and hence the probability that transmissions from more than one basestation will become time coincident, even for a small period of time, is greatly reduced.

Fig. 4 shows that for a transmission (n) 25 at a nominal transmission time 22 with infornmtion content 1(n), the timing of the next transmission (n+l) is a function (f of the information content 1(n) of the current transmission (n). The transmission S..

* 30 timing delay D(n +1) f(l(n)). The timing delay D(n+1) 28 may be relative to the * current transmission time 22, or the delay 30 may be relative to the nominal timing *e..

position 23 of the next transmission 26. For the next transmission timing delay D(n+2) 29, this is a function of the information content 1(n+1) of the transmission (n+1) 26, i.e. D(n+2) = f(1 (n+l). Transmission (n+2) has an information content of I(n±2), so the same principles apply for each subsequent transmission to (n+m).

In the transmitter and receiver there can he look up tables (LUT) or other forms of storage which set when the next transmission should be, based on what number of transmission it is. For example, the first frequency makes its next transmission N seconds later; the second frequency makes its next transmission P seconds later and so on.

To avoid the requirement for the mobile terminals to know the randomisation sequence of each basestation in advance and to search each possibility, the scheme of the present invention uses the information carried on each broadcast channel transmission to define the timing of the next transmission. The range of adjustment of timing position may be limited to avoid excessively short or long periods between any two transmissions. In general, the information carried on each successive broadcast channel transmission, and on each basestation, will be different and hence the system will be self randomising.

Fig. 5 shows an alternative presentation of the effect of the method of the present invention. Before any randomisation is applied to the timing transmissions from two base stations are received, 1-1 and 2-1. However, the next transmissions are separated, so the second burst from BS 1 1-2 is in transmission 12 and the second burst from BS2 2-2 is in transmission 16. Similarly, the 3rd burst 2-3 from BS2 is in transmission 17 and the 3rd burst from BS 1 is in transmission 2-3.

In the present invention, the penodicity of the beacons is removed, so that non-periodic beacons are sent out and the information sent is used to determine which beacon has been received. Where the possibility exists that the information carried on successive broadcast channel transmissions, and between basestations, is not sufficiently random, then an additional pseudo randomly generated information *: : : : field can he inserted into each broadcast channel transmission to achieve a greater transmission timing randornisation. It is not necessary for the mobile terminal to know this sequence, as it simply uses the information decoded from the current transmission to derive the expected timing of the next transmission. For example, a sine wave, which is unique, can represent a certain combination of five bits sent at one of 32 frequencies. In a Pseudo random code, such as a Walsh code, if the number of possibilities is known, then these are separated out and an appropriate time delay added to each.

On the basis that each beacon has about 5 bits of information, it is possible to work out when the next burst will occur, so that when more than one beacon is received, it is possible to tell that there are two beacons and which beacons they are, but not where they have come from. 3 columns of 5 bits are necessary to get the lull infbrrnation about the received beacon.

Although, the general problem of sending data on the same channel with contention has been addressed using Aloha protocols, these use straight randomness to resend if a collision occurs. In the present invention, the information sent provides the definition of how long it is necessary to wait before resending a burst and so avoid interference. A specific example of this is OFDM, where the frequency of the signal determines when it is resent, whereas for other mechanisms, it depends upon data contained in the burst, rather than frequency.

The method uses a set of orthogonal waveforms, that is, any one waveform (which is a member of a set) cannot be made up of the sum of any sub-sets. The use of the information carried by the current transmission to indicate the timing of the next transmission means that the mobile terminal does not need to know in advance the random sequences used, nor to search all possibilities (as would be the case if an independent random, or pseudo random timing position sequence was used).

Embedding a random, or pseudo random, information field within the broadcast channel information to be transmitted can be used to improve the characteristics of the resulting timing sequence. 4* * f as, , * *5 *5** S * 514$ a a -I.e $5 as a. ,

Claims

I. A method of reducing interference between orthogonal signals in contention on a channel of an unsynchronised mobile communications system, the source of each signal being not known; the method comprising for each signal received at a receiver, settmg a delay for a subsequent transmission from a transmitter of that signal based on data in the signal itself, such that different delays are applied to different signals, thereby avoiding subsequent overlap of the signals.
2. A method according to claim 1, wherein a timing chart is stored and accessed by each receiver and transmitter to determine the time delay before a subsequent transmission.
3. A method according to claim 1 or claim 2, wherein maximum and minimum ranges of timing adjustment are set.
4. A method according to claim 1 or claim 2, wherein an additional pseudo random information field is inserted into each transmission.
5. A method of reducing interference between orthogonal signals in contention on a channel of a mobile communications system, as hereinbefore described with reference to the accompanying drawings. ( ,

5. A method according to claim 4, wherein the data comprises signal frequency.

6. A method of reducing interference between orthogonal signals in contention on a channel of a mobile communications system, as hereinbefore described with reference to the accompanying drawings. . * *. * I'.. a * S.. & I 4 s.. *

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S 1 S I, 4.

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Amendments to the claims have been filed as follows

-I

1. A method of reducing interference between orthogonal signals in contention on a chaimel of an unsynchronised mobile communications system, the source of each signal being not known; the method comprising for each signal received at a receiver, setting a delay for a subsequent transmission from a transmitter of that signal based on data in the signal itself, such that different delays are applied to different signals, thereby avoiding subsequent overlap of the signals, wherein an additional pseudo random information field is inserted into each transmission..

2. A method according to claim 1, wherein a timing chart is stored and accessed by each receiver and transmitter to determine the time delay before a subsequent transmission.

3. A method according to claim I or claim 2, wherein maximum and minimum ranges of timing adjustment are set.

4. A method according to any preceding claim, wherein the data comprises signal frequency.