Direct Sequence Spread Spectrum (DSSS) techniques are used in many different applications of radio communication, for example mobile telephone systems and in wireless local area networks. In DSSS communication, the bandwidth of a signal is increased prior to transmission by multiplying the signal by a spreading code. Many such spread signals may be transmitted simultaneously by different transmitters, or by a single transmitter, using different spreading codes, and the individual signals may be detected in a receiver by correlation with their particular spreading code. In order to ensure a high reliability of detection of the signals, the spreading codes must have a high auto-correiation and a low cross-correlation properties. Many sets of codes are known which have good correlation properties under ideal conditions, but after transmission through a radio channel the correlation properties may be degraded such that the probability of successful detection is reduced and the probability of erroneous detection is increased.

An object of the present invention is to improve the reliability of detection of a DSSS signal.

According to a first aspect of the invention there is provided a method of radio communication comprising selecting from available spectrum spreading codes a spreading code according to a first selection criterion and transmitting a signal modulated with the selected spreading code, wherein according to the first selection criterion first data representative of the performance of the selected spreading code in the presence of at least one spreading code in

current use in the radio environment complies with a first predetermined criterion.

According to a second aspect of the invention there is provided a radio apparatus comprising means for storing first data representative of the performance of each available spectrum spreading code in the presence of at least one spreading code in current use in the radio environment, means for selecting from the available spectrum spreading codes a spreading code for which the first data is in accordance with a first predetermined criterion, means for modulating a signal with the selected code, and means for transmitting the modulated signal.

According to a third aspect of the invention there is provided a radio communication system comprising a plurality of radio apparatuses at least one of which is in accordance with the second aspect of the invention.

The invention is based on the realisation that, even within one set of spreading codes, the cross-correlation can be different between different codes, that the reliability of detection depends on which other codes are transmitted simultaneously, and that the reliability of detection can be improved by selecting a code taking account of which other codes are likely to be transmitted simultaneously.

In one embodiment of the invention the selection takes account of the radio propagation environment. Such an embodiment is based on the realisation that the cross-correlation between two codes can be changed by radio propagation, and that the reliability of detection can be improved by selecting a code taking account of the radio propagation environment and of which other codes are likely to be transmitted simultaneously. In particular, the delays often seen in a radio channel can cause the cross-correlation to be degraded and can affect different codes to different extents.

In a further embodiment of the invention the selection takes account of both the impact of other codes that are likely to be transmitted simultaneously on reliability of detection of the selected code, and the impact of the selected code on reliability of detection of the other codes that are likely to be transmitted simultaneously. Such an embodiment is based on the realisation

that the cross-correlation between a first code that has been subject to radio propagation and a second code that is generated locally in a receiver may be different from the cross-correlation between the first code when generated locally in a receiver and the second code when subject to radio propagation.

Data representative of the performance of a code in the presence of other codes may be pre-stored in a radio apparatus for use during code selection, or may be generated by the radio apparatus from information about which codes are in use and optionally information about the prevailing radio environment.

The other codes that are in current use and therefore likely to be transmitted simultaneously may be already known prior to the selection process. For example a base station in a mobile telephone system would be aware of which mobiles are currently active in the same geographical area and which codes are in use for communication with those mobiles.

The other codes that are in current use may be determined remotely from, and transmitted to, the radio apparatus selecting a code in preparation for transmitting. For example, the intended recipient apparatus or the base station may report to the radio apparatus making the selection those codes in current use locally. Similarly, the radio propagation environment may be determined remotely, for example by the intended recipient apparatus, and reported to the radio apparatus selecting a code in preparation for transmitting.

The other codes that are in current use may be determined by monitoring and analysing radio signals in the local environment. Such a scenario can be applicable to a radio apparatus in wireless local area network where spectrum is shared by independent networks.

The predetermined criterion for selecting a code may be, for example, minimisation of the cross-correlation value, or ensuring the cross-correlation value is below a maximum acceptable value, or minimisation of bit error rate, or ensuring the bit error rate is below a maximum acceptable value.

The invention will now be described, by way of example only, with reference to the accompanying drawings wherein: Figure 1, divided in parts 1 a and 1 b, is a table of correlation values for a set of seven codes subject to a range of delays representative of radio propagation; and Figure 2 is a block schematic diagram of a radio communication system.

First the underlying technical basis for the invention is explained, and then example embodiments are described.

A set of seven spreading codes is considered as an example. The codes are Orthogonal Variable Spreading Factor codes of length 8 chips and having a spreading factor of 8, and the codes are transmitted at a chip rate of 3. 84 Mchip. s~1. Figure 1 is a table of magnitude and normalised values of correlation between each possible code pair (x, y) when the y code is subject to a propagation delay of 0,1, 2,... 7 chips with respect to the x code. The duration of one chip is 260ns. With zero delay, each code has a auto- correlation value of 1.0, and zero, or very small value, cross-correlation with each of the other codes in the set.

It can be seen from Figure 1 that some auto-correlation values are degraded considerably by a delay in the propagation channel. For example, the auto-correlation value of code 1 is reduced to 0.5 by a delay of 1,3, 5 or 7 chips, and is reduced even further for a delay of 2 or 6 chips. As another example, the auto-correlation value of code 2 is reduced to 3.9E-06 for a delay of 1,3, 5 or 7 chips. In contrast, a auto-correlation value of 1.0 is maintained for code 4 for all of the values of delay. Therefore, if a code is to be selected for use where the prevailing channel includes a propagation path having a delay of 1,3, 5 or 7 chips, and if the selection is based solely on a consideration of auto-correlation values and hence probability of successful detection, code 4 would be preferable to code 1, and code 1 would be preferable to code 2.

It can be seen also from Figure 1 that some cross-correlation values are degraded considerably by a delay in the propagation channel. For example, the cross-correlation value between codes 2 and 6 is increased to 1.0 by a delay of 1,3, 5 or 7 chips. As another example, the cross-correlation value between codes 1 and 5 is increased to 0.5 for these same values of delay. In contrast, the cross-correlation value between codes 1 and 2 is maintained at a very small value for all of the tabulated values of delay. Therefore, if a code is to be selected for use where the prevailing channel includes a propagation path having a delay of 1,3, 5 or 7 chips, and code 2 is already in use, code 6 should not be selected, and code 1 would be a good selection.

For these example codes, the increase in cross-correlation value to 1.0 at certain values of delay arises because some codes are equal to a cyclic shift of another code. For code 4, the cross correlation value of 1.0 for all delays arises because code 4 comprises alternating binary chip values.

In a practical scenario the code set may comprise more than seven codes, and cyclic shifts of codes might not be used, but the above example illustrates how the auto-correlation and cross-correlation values can depend on delays in the propagation channel, and how performance can be optimised by consideration of auto-correlation and/or cross-correlation values. In a typical propagation channel, there may be a direct propagation path and several delayed propagation paths having different delays with respect to the direct path. The actual auto-correlation and cross-correlation values will depend on the combined effect of the propagation paths.

Referring to Figure 2, there is a DSSS radio communication system 100 comprising first, second and third radio apparatuses 10,20, 30 respectively.

For example, the first radio apparatus 10 may be a base station and the second and third radio apparatuses 20,30 may be portable terminals. There is an active link between the first radio apparatus 10 and the second radio apparatus 20. Spreading code 1 is in use for transmissions from the first radio apparatus 10 to the second radio apparatus 20, which for convenience of description will be referred to as the downlink. Spreading code 4 is in use for transmissions from the second radio apparatus 20 to the first radio apparatus

10, which for convenience of description will be referred to as the uplink. The uplink and downlink transmissions occur in different frequency bands, so their spreading codes may be selected independently and reused for the uplink and downlink. The first radio apparatus 10 has a transceiver 12, a processing means 14, a first storage means 16 which contains data indicating the spreading codes in current use and the spreading codes available for use, and a second storage means 18 which contains the correlation data of Figure 1.

The processing means 14 performs code selection for the uplink and downlink, performs spreading prior to transmission of signals by the transceiver 12, and performs de-spreading of signals received by the transceiver 12. When the first radio apparatus 10 activates a new link with the third radio apparatus 30, the processing means 14 receives from the transceiver 12 information about the prevailing propagation conditions, it accesses the first storage means 16 to determine which spreading codes are available, not being in current use, and it accesses the second storage means 18 to determine the cross-correlation values under the prevailing propagation conditions of the available spreading codes with the spreading codes in current use. The first radio apparatus 10 then selects for the link with the third radio apparatus 30 the available spreading codes which have the low cross-correlation values with the codes in current use in the prevailing propagation conditions. Considering the example that the transceiver 12 determines that there is a strong multipath component having a delay of 3 chips, in Figure 2, code 2 has been selected for the downlink to the third radio apparatus 30 as it has a low cross-correlation value with code 1 for a delay of 3 chips, and for the uplink from the third radio apparatus 30, code 3 has been selected as it has a low peak cross-correlation value with code 4 for a delay of 3 chips.

In order to supply information about the prevailing propagation conditions, the transceiver 12 includes an equaliser or RAKE receiver. In determining the cross-correlation values under the prevailing propagation conditions, best results are obtained where most information about the propagation conditions is available. Ideally information should be available for

the delay spread of the new and existing links, for the amplitude spread of the multipath components, and the differential delays for the links.

Optionally, the selection criterion may take account of both the impact on the performance of the selected code of the other codes in current use, and the impact of the selected code on the performance of the other codes in current use.

Optionally, alternative criterion may be used for selecting a code. For example, preference may be given to codes having a cross-correlation value below a maximum acceptable value, the auto-correlation value may also be taken into account, and a combination of the correlation values for a code may be used.

Optionally, information about the prevailing channel conditions need not be used, with code selection being based on correlation values under predetermined propagation conditions. Furthermore, code selection may be based solely on correlation values, without consideration of prevailing propagation conditions. For example, a code may be selected that has a good correlation performance across a range of delays or a range of propagation conditions. In this way the apparatus processing or storage requirements may be reduced while still reaping some of the performance benefit of optimum code selection.

Optionally the second storage means 18 contains correlation data for one or more radio channels of different characteristics and the processing means uses the correlation data most applicable to the prevailing channel conditions.

The correlation data contained in the second storage means 18 can be determined in advance of code selection, for example during manufacture of the first radio apparatus 10, for one or more typical radio channels.

Alternatively, the correlation data can be calculated by the processing means 14 for a propagation channel having the characteristics measured by the transceiver 12. Such adaptive determination of correlation data for the prevailing radio channel can result in a more precise determination of the optimum spreading code.

Optionally, performance data for each code, other than correlation values, may be stored in the second storage means 18 and be used for code selection. Some examples of the type of performance data are: bit error rate for a specific signal to noise (S/N) or signal to interference (S/l) ratio; required S/N or S/I for a target bit error rate; a relative grading value ; or a binary value (acceptable/unacceptable).

Optionally, performance data for each code in the presence of combinations of other codes may be stored in the second storage means 18 and used for code selection.

In the example radio communication system described above in relation to Figure 2, the first radio apparatus is a base station having responsibility for allocating the spreading codes for uplink and downlink communication with both the second and third radio apparatuses 20, 30. In some radio communication systems, the code selection may be performed by the second and third radio apparatuses 20,30 themselves. These apparatuses do not inherently have any knowledge of codes being used by each other because they are not involved in the selection of codes for links other than their own.

For example, the third radio apparatus 30 is not involved in selecting the codes used for communication between the first radio apparatus 10 and the second radio apparatus 20 so does not inherently know which codes are in use for that link. In this case, information about which spreading codes are in current use may be transmitted to the radio apparatus making the code selection. Such information may be transmitted, for example, during establishment of a link or when a change of spreading code takes place.

Optionally, a radio apparatus 10,20, 30 may discover which codes are in current use by monitoring and analysing, in the processing means 14, the signals in the radio environment. In this way a radio apparatus 10,20, 30 can discover which codes are in use by independent radio communication systems and these spreading codes can be included in the data stored in the first storage means 16.

Optionally, spreading code selection can take place when a new communication link is established and the selected code maintained for the

duration of the link, or re-selection of the spreading code may take place during the link in order to re-optimise the code selection.

In the present specification and claims the word"a"or"an"preceding an element does not exclude the presence of a plurality of such elements.

Further, the word"comprising"does not exclude the presence of other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of radio communication and the art of spread spectrum signalling and which may be used instead of or in addition to features already described herein.