GB2371727A - Synchronisation of a spread spectrum system by the use of timing signals encoded using cyclic extensions of complementary sequences such as Golay sequences - Google Patents

Abstract

In a spread spectrum communications system (1) it is necessary to ensure that base-stations (2-9) are synchronised. Signals are transmitted having timing information to the base-stations from a controlling base-station (2). The signals are encoded using cyclic extensions of Golay complementary sequences. These sequences ensure that the signals have good auto-correlation properties.

Description

A Communication Method and System This invention relates to communication method and system involving spread spectrum coding techniques.

A communication system which uses spread spectrum techniques is UMTS (Universal Mobile Telecommunications System) terrestrial radio access. It is a time division duplex system (TDD) based on a combination of code division multiple access (CDMA), hybrid time division multiple access (TDMA) and TDD.

Such a system makes good use of the frequency spectrum by encoding transmission on the same frequency with different codes. In a receiver operating in such a system a local stored reference code is compared with received signals to obtain a match. This comparison is made in a correlator. When the signal has been correlated it can be decoded.

It is possible for these encoded signals to interfere with each other. This is called cross-correlation. Certain codes are termed orthogonal if they exhibit low crosscorrelation. To ensure that codes remain orthogonal it is important to maintain timing in the system because codes which would otherwise be orthogonal not following the same timing can cause interference.

Timing transmissions are themselves coded. Because of the importance of maintaining timing the codes used for the timing signals have what is called good auto-correlation properties. This means that they are codes selected to cause little or no inference to other auto-correlation codes. It has been proposed for base stations referred to as NodeBs in the art to be synchronised using coded signals transmitted in parallel and remaining orthogonal.

The invention provides a method of transmitting coded signals which may be transmitted in parallel and remain orthogonal.

According to the invention there is provided a method of synchronising communication units in a communication system comprising transmitting timing signals encoded by cyclic shifts of periodically extended complimentary sequences.

Preferably, the complimentary sequences are Golay complementary pairs. Golay complementary codes are error correcting codes known in the art of coding.

A specific embodiment of the invention will now be described by way of example only with reference to the drawings in which: Figure 1 shows a communication system operating in accordance with the invention; Figure 2 shows in block diagram form a base-station including a network management function transmitting a synchronisation signal to another basestation in the communications system shown in figure 1; and Figures 3 to 4 are explanatory diagrams illustrating the generation of sequences used to encode synchronisation signals.

With reference to Figure 1, a communication system 1 operating in accordance with the invention comprises a number of base-stations 2 to 5, mobile units 6 and 7 and a portable unit 8. As will be appreciated, there are in practice more stations and units than illustrated in the system. The base-stations (sometimes referred to as node Bs) are nominally identical with the exception of base-station 2 which includes a controller 9 performing a network management function.

Each of the base-stations 2 to 4 serve a geographical area or cell in a manner well known to those skilled in the art of cellular communication systems. For the system, to function efficiently, it is necessary to synchronise the operation of the base-stations (and also the other units) to a common clock.

This is achieved by the controller 9 using the base-station2 to simultaneously transmit coded synchronisation signals in a manner to be described later.

Each base-station has a transmit path and a receive path. As is shown in figure 2, the receive path includes a receiver section 10 for receiving signals from an antenna 11 and a decoder 12 for decoding signals from the receiver section.

The transmit path includes a coder 13 for encoding data before passing the encoded data to a transmitter section 14. The transmitter section 14 is operably coupled to an antenna 15 for radiating the synchronisation signal over the air interface to the other base-stations.

The code sequences used for generating the synchronisation signal are shown in figure 3. In the first code sequence shown in Figure 3a, the codes s (n) and g (n) are Golay complementary pairs and cyclic extensions of the codes. The use of this sequence provides a window of ideal auto-correlation characteristics of + K about a nominal position.

An alternative sequence is shown in Figure 3b. In this alternative sequence, the pre-extension of each code has been eliminated, that is to say, in the case of code

s (n) the extension the cyclic extension SN-K+l.... SN and in the case of code g (n), the cyclic extension ..... . A yet further alternative sequence would be same but with the post cyclic extension eliminated, that is to say -- and 2-- The sequence provides a single code s (n) for encoding the synchronisation signal transmitted by the base-station 2. The synchronisation signal when received by a base-station is correlated with a rotation of this code stored in memory s' (n).

Considering the way in which this correlation may occur at a nominal code correlation position starting with s, where i = (K-1)/2 + 1 and K is an odd number. Then a correlation of a cyclic rotation of s (n), s' (n) where

t -"-k-i) where is ; N- (K-1)/2 1# Si = S (K-,)/2+,-Nwhere i > N- (K-1) 12

which gives a correlation window of +K/2.

Whilst K in this example is an odd number it need not be so in all cases. Where it is not odd it will render the window of auto-correlation slightly asymmetrical.

More than one code for encoding the synchronisation signals can be generated from the single complementary pair. This is done by generating cyclic shifts S. (n) and Gm (n) of the Golay codes s (n) and g (n) when the controller 9 is programmed when the system 1 is set-up. The cyclic shifts are referred to as

s,. (n), S.,, and are generated by selecting appropriate bits from repetitions of s (n) and g (n) as shown in figure 4a. Considering the repetition of s (n) given by se (n) where its elements are given by se, i = si for i # N .-N for i > N

Then the elements of SM, , are given by ,, (, where wis the offset in number of code elements. Typically w = K and the total available number of offsets M is given by M=N/K. The codes Gm (n) are constructed in the same manner as shown in figure 4b. The mth overall code is then given by the concatenation of S. (n) and Gm (n) as shown in figure 4c.

Claims (7)

1. Claims 1. A method of synchronising communication units in a communication system comprising transmitting timing signals encoded by a code concatenated from cyclic shifts of periodically extended complimentary sequences.
2. 2. A method as claimed in claim 1 wherein the sequences are Golay complimentary sequences.
3. 3. A method as claimed in claim 2 wherein the concatenated code comprises a first and a second complimentary Golay sequence which first and second sequence including a post cyclic extension.
4. 4. A method as claimed in claim 2 wherein the concatenated code comprises a first and a second complimentary sequence which first and second sequence including a pre-cyclic extension.
5. 5. A method as claimed in any preceding claim in which a received transmitted timing signal is correlated against a cyclic rotation of the code
6. 6. A method substantially as hereinbefore described with reference to, and as illustrated by, the drawings.
7. 7. A communications system operating in accordance with a method as claimed in any one of the preceding claims.