WO2006106385A1

WO2006106385A1 - Method for avoiding soft handover in a hsdpa system

Info

Publication number: WO2006106385A1
Application number: PCT/IB2005/051162
Authority: WO
Inventors: Per Beming
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2005-04-08
Filing date: 2005-04-08
Publication date: 2006-10-12

Abstract

The present invention relates to a radio communication system supporting macrodiversity. The problem address is the complexity of the macrodiversity links and the difficulties in synchronisation that may occur in the retransmission schemes during macrodiversity mode. The problem is solved by a first step of setting up a single communication link instead of the macrodiversity link. The first step though introduces a new problem of increased uplink interference in the base stations 13b that do not support macrodiversity link and that should have done so if macrodiversity mode was used. The second step of the solution is to cancel interference in the base stations 13b no more supporting the macrodiversity links. For the interference cancelling the no more macrodiversity base stations 13b are informed on the spreading code used by the mobile station transmission which transmissions is to be cancelled.

Description

METHOD FOR AVOIDING SOFT HANDOVER IN A HSDPA SYSTEM

TECHNICAL FIELD OF THE INVENTION

The present invention relates to cellular communications systems supporting macro diversity.

DESCRIPTION OF RELATED ART

In CDMA technology the same frequency channel is shared by many mobile users within the same cell and in adjacent cells. Uplink power control from the multiusers is extremely important since it is closely related to the total traffic capacity of the system and the near-far-problem, i.e. to be able to provide services to mobile stations on the cell boarder.

Figure Ia is a view of some radio base stations 131, 132 of a cellular communications system and that are spread over an a geographical area and each providing communication services within respective area called cell Cl, C2. Figure Ia gives a simplified picture of the cell areas Cl, C2, because in practise the cell boarders are not distinct. When a mobile station is moved along a distance d from a point A in a first cell 131 to a point B in a second cell 132 adjacent the fist cell 131, the signal strength 101 received from the first base station 131 will decrease as the mobile gets closer the second base station 132, and the signal strength 102 from the second base station 132 will increase. The signal strengths 101, 102 from the first and second base station 131, 132 respectively are disclosed in figure Ib as a function of the mobile station distance d relative points A and B. The signal strengths 101, 102 are however not linear proportional to the distance d, and within part Ml of the distance which of one the two base stations 131, 132 providing strongest signal strength will alternate frequently. When the mobile station have a communication link set up with the cellular communications system, it should be set up in macrodiversity mode when the mobile station enters the part Ml of the distance D, were the strongest of base stations may alter.

Macrodiversity means that the mobile station 11, depicted in figure 2 have communication links 21, 22 set up over two or more radio base stations 13, and further up to a connecting Radio Network Controller 14 (RNC) , also depicted in figure 2. The direction from the mobile station 11 to the base station is referred to as the uplink 21 direction, and the opposite direction as the downlink 22. In respond to the uplink transmission from the mobile station 11 the base stations 13 independently send power commands 21c to the mobile station 11. If any of the base stations 13 command the mobile station 11 to decrease its transmission power the mobile station 11 will decrease its transmission power, while it increases its output power only if both the macrodiversity base stations 13 demands a power increase. Similarly, the mobile station 11 controls the power used by the two base stations 13 for downlink 22 transmission, and send commands 22c for power increase if the mobile station 11 can not correctly detect transmission from any of the base stations 13.

For all data packets sent from the mobile station 11, the RNC 14 receives copies on each of the uplinks 21 and selects the best of the copies for delivering to the network. The quality of the radio links varies momentarily, and so does the quality of the data packets received. If at least one of the copies are received with good enough quality the RNC send an acknowledgement back to the mobile station 11. As long as the RNC 14 positively acknowledges receipts of packets the mobile station 11 continues transmission of new data packets, otherwise it retransmits the packets. Figure 2 is simplified for the understanding of the present invention. In a real communications system the base station

13 serves many mobile stations. In addition to mitigating interference in the radio base stations by firm control of the mobile stations transmission power, there have been proposal in articles on cancelling intracell interference, i.e. cancelling the interference the wanted signals from the different mobile stations within the cell causes on each other.

HSDPA (High Speed Downlink Packet Access) is a bearer service, within the 3GPP standard WCDMA, see e.g. TS 25.308, optimised for transmission of downlink data with very short delays. Downloading video clips is one example of a user service that benefit from the HSDPA. Due to the tough requirement on short delay, the base station 13 buffers and retransmits downlink packets when the mobile station have not positively acknowledged receipt of packets. The base station retransmissions must be handled by one base station only, and accordingly the HSDPA link must be a single link and not a diversity link. Figure 3a illustrates the HSDPA bearer service with a single data downlink 23 from the RNC

14 over a first base station 13a' further down to the mobile station 11, and an acknowledgment/non-acknowledgemnt 23A sent from the mobile station 11 to the first base station 13a' is also depicted.

In WCDMA when a mobile station downloads a videoclip using HSDPA, the mobile station 11 need to send acknowledgments 23A also to the RNC 14 and or to the internet using the uplink 21. The uplink 21 may be set up in macro diversity mode in parallel to the single HSDPA downlink 23, as is depicted in figure 3b. Acknowledgments 23A sent from the mobile stations on receipt of the HSDPA downlink transmissions, must be received by the first base station 13a in charge of HSDPA retransmissions. There is a problem of the HSDPA acknowledgements 23A not being correctly received by the first base station 13a. The problems owes to both the first and a second base stations 13a, 13b supporting the macrodiversity uplink 21, are in control of the mobile station transmit power and if any of them commands power decrease, the mobile station transmit power is decreased. For a period the second base station 13b may have better reception from the mobile station 11 than the first base station 13a, and then control the mobile station 11 transmit power to be under the first base stations 13a detection level. Synchronisation of the HSDPA downlink 23 transmissions are then severely effected or even lost.

In order to increase the probability of the HSDPA acknowledgements being correctly detected by the first base station 13a, the mobile station 11 shall use up to 6 dB higher transmission power for the HSDPA acknowledgements compared to other transmissions, according to the 3GPP standard. A drawback of this solution is the increased power adds to the overall power level of the system and negatively effects its ability to detect transmissions from far away mobile stations, while still not ensuring that the acknowledgments may be detected by the first base station 13a.

An alternative solution for increasing the probability of the acknowledgements being detected by the base station supporting the HSDPA link, could be to let two radio base stations alternately support the HSDPA downlink, and make the alteration very rapid. The base station momentarily in charge of the downlink should then be the one that momentarily also has the best uplink. This solution have been discussed on a very general level in 3GPP standardisation groups. It expected to be a very technical demanding and difficult solution to implement, if at all being possible. SUMMARY OF THE INVENTION

The object of the present invention is to present an alternative solution to the problem of lost synchronisation for a HSDPA link that may occur when there is a second uplink channel in macrodiversity mode.

The first step of the solution to the problem is to abandon macro diversity on the parallel radio uplink and just let one base station support the uplink, the same base station that supports the HSDPA downlink. Thereby, the mobile station transmission power will be controlled to be over the detection level of the base station, its acknowledgments on the HSDPA reception be detected by the base station and the HSDPA downlink communication be continued.

However, the first step to the solution of the problem also introduces a new problem to be solved, namely that of increased interference in some of the neighbouring radio base stations, because the neighbouring radio base stations lack the possibility to decrease the mobile station transmission power.

The present invention solves the second problem by providing information on spreading code used by the interfering mobile station to one or more of the neighbouring base stations that are most exposed the interference from the mobile station. The spreading code is used by the one or more of the neighbouring radio base stations to cancel the interference caused bythe mobile station tranmission on signals from other mobile stations served by the one or more of the neighbouring base station/s.

In addition to providing a solution for synchronisation of the WCDMA HSDPA bearer service the solution of the present invention may also be used in other situations when macro diversity causes synchronisation problems. Such situations will be described with embodiments further down in the description. The solution of the present invention may also be used when macrodiversity is to be avoided for other reasons. Such reasons can for example be, that macro diversity is complicated to perform when different RNCs control the base stations needed to establish a macro diversity link. Another example is the cost of setting up several macro diversity links within the fixed part of the cellular network and that motivate a single link to the mobile station to be set up.

DESCRIPTION OF THE DRAWINGS

Figure Ia is a view over an abstract model of cells served by bases stations.

Figure Ib is a diagram disclosing power versus distance to two base stations.

Figure 2 is a prior art block diagram of a mobile station, radio base station and an RNC in macro diversity mode.

Figures 3a -3c are block diagrams of the mobile station, base stations and RNC, with different type of channels setup.

Figure 4 is a flow diagram over the steps of a method.

Figure 5a is a block diagram of a PIC circuit.

Figure 5b is a block diagram of a ICU circuit.

Figure 6 is a block diagram of a radio base station.

Figures 7a, 7b are block diagrams similar to those of 3a-3c.

Figure 8 is a sequence diagram of messages sent between a mobile station and macrodiversity base stations. DESCRIPTION OF PREFERRED EMBODIMENTS

The problem underlaying the present invention can be understood from figure 3b. When there is a macrodiversity link 21 set up over two or more base stations 13a' , 13b' in parallel to the HSDPA downlink 23 set up over just one of the base stations 13a', both the base stations 13a', 13b' sends power commands 21c to the mobile station 11 for regulating its transmission power. When temporarily the second base station 13b' that serves the macrodiversity link 21 but not the HSDPA link 23, receives a higher signal strength that the first base station 13a' serving both links 21, 23, the second base station 13b' may control the mobile station 11 transmission power to be under the detection level of the first base station 13a' . Acknowledgments or possible non-acknowledgments sent from the mobile station 11 in respond to the HSDPA reception, are then not detected by the first base station 13a' and cannot be further process neither in the base station 13a' itself nor in the RNC (Radio Network Controller) 14. The HSDPA transmission will then loose its synchronisation, and the communication may eventually get lost.

The mobile station 11 need have at least one uplink 21 in addition to the HSDPA link 23, for transmission of acknowledgments/non-acknowledgements to the RNC.

The first basic step of the solution of the present invention, and that is illustrated in figure 3c, is to abandon the macrodiversity uplinks 21 and set up a single uplink 24 from the mobile station 11 over the single first base station 13a that also supports the HSDPA link 23 to the mobile station 11. This is disclosed as the first step 41, in a method flowchart disclosed in figure 4.

The second base station 13b, see figure 3c, that normally should have been supporting a macrodiversity link is exposed to higher radiated power from the mobile station 11 than it should have been in macrodiversity mode. This owes to the second base station 13b looses the possibility to decrease the mobile station 11 transmission power. The mobile station 11 transmit power negatively effect the second base stations 13b ability to detect uplink transmissions from other mobile stations that it serves. Some of these other mobile stations 10 are depicted in figure 3c, with their radio links 31 supported by the second base station 13b and further links 32 to the RNC 14.

According to the second step of the present invention, see

42 of figure 4, the second base station 13b is informed of the spreading code used by the mobile station 11 for its uplink 24 transmissions.

In the last step 43 of the method, the second base station 13b cancels interference caused by the transmission from the mobile station 11 on signals from the other mobile stations 10. Figure 3c, discloses the same nodes as figures 2 and 3a- 3b, with exception of the RNC 34 that have some modified functions and that is given an other reference number for that reason, and the base stations 13a, 13b that have some new functionality compared to the prior art base stations 13, 13a', 13b'.

In the example above there is one second base station 13b that is informed of the mobile station 11 spreading code and that cancels interference caused by the mobile station 11 transmission. There may be more then one second base station 13b being neighbour to the first base station and selected for cancelling interference. The one or more second base stations 13b to be informed of the spreading code in step 42, are selected similar to or in the same way as base stations 13 are selected for supporting macrodiversity links 21,22. In WCDMA terminology the base stations 13 supporting macrodiversity links 21,22 with the mobile station 11 are referred to as the active set. The RNC 34 selects which base stations 13 to be selected as the first and second base stations 13a, 13b based upon measurements made by the mobile station 11 and transmitted to the RNC 34. Typically it is the neighbour base stations 13b measured to have the highest signal strengths that are selected. The mobile station 11 repeatedly measures the signals strengths of first and second base stations 13a, 13b and of other base stations having neighbour relations to any of the first or second base stations 13a, 13b and reports to the RNC 34.

The RNC 34 informs the second base station/s 13b of the spreading code used by mobile station 11 on the radio uplink 21, in a modified Radio Link Setup message. The message is standardized in WCDMA for ordering a base station 13 when being included in the active set to set up a new radio link. The original message includes information on the uplink spreading code to be detected and of Iub information. When used for informing the second base station/s 13b the Radio Link Setup message is slichtly modified to be recognised as a command of cancelling interference from a radio link 23, 26 having the specified spreading code instead of being an instruction of setting up a new link as in the prior art. Some information normally included in the Radio Link Setup message such as Iub information can also be excluded.

In WCDMA the uplink spreading code is separated into a channelisation code used for spreading user data before transmission, a channelistation code used for spreading control signalling including pilot sequencies and a scrambling code used on top of both spread control signalling and spread user data. The spreading code information to the second base station/s 13b comprises specific information on the scrambling code, channelisation code and pilot sequence used by the mobile station 11. The interference cancellation in the third step 43 of the method will now be described with reference to figure 5a, that is a box diagram of a parallel interference cancellation unit 51, denoted PIC 51. The PIC 51 consists mainly of parallel chains of cascaded interference cancellation units 52, in the further text denoted as ICUs 52, one chain of ICUs 52 for each signal to be detected. Detection of a signal is here a prerequisite for cancelling it from other wanted signals. Thus, each ICU 52 also functions as a detection unit and regenerates a replica of the signal based on the detected symbols, estimated channels responses and the user's spreading codes. The ICU 52 receives as input signal replicas from the previous stage of all other parallel chains, i.e. signal replicas of all interfering signals detected. The original total received signal is also an input to the ICU 52.

Figure 5b is a box diagram of the ICU 52 layout. An adder 61 subtracts the signal replicas received from previous stage of the other chains from the total received signal also inputted, and feeds the subtracted signal to a channel estimator 62 and a rake receiver 63. Both the rake receiver 63 and channel estimator 62 have the spreading code of the signal as an input, though it is not depicted in figure 5b. More specifically for the WCDMA embodiment, the channel estimator uses the pilot code for estimating the channel. The Rake receiver uses the scrambling code and channelisation code for detecting the user data and the pilot sequence and channelisation code for detecting control signaling. The rake receiver 63 also has an input from the channel estimator 62 and produces a despread bit stream that is fed to a tentative decision unit 64 that makes tentative decision on what symbols are detected. The tentative symbol decision is produced at the output of the ICU 52 together with a weighted form of the signal replica, i.e. the tentative decided symbol stream respread and exposed to the estimated channel. The weight factor used reflects the confidence in the tentative decision.

The last stage ICU 52 of the PIC 51 need not produce a replica, neither is a weighting factor needed. Typically, three cascaded ICU 52 stages are to be used in the PIC 51.

The received signal fed to the ICUs 52 is the total energy within the common frequency channel. By use of the spreading codes the radio links 31 supported by the second base station 13b as well as the links 23,26 it does not support will be detected and their mutual interference be cancelled. The symbol streams at the PIC output will be transmitted to the RNC for the links 31 the second base station 13b support while the signals on the radio links 23, 26 it does not support are wasted.

Figure 6 is a box diagram of the base station 13b elements, however, for simplifying the understanding of the present invention only base station elements relevant for the present invention are included. A transceiver 601 connects to the antenna, for transmitting and receiving radio signals. A receiving chain 604 within the transceiver 601 comprises the PIC 51, however the PIC 51 is not disclosed in figure 6. An interface 602 controls the communication with the RNC both with respect to streams of user data on the links 32 and for control signalling. A control unit 603 controls the operation of the base station 13b including the transceiver 601 and interface 602. The control unit 603 receives the modified Radio Link Setup message via the interface 602, and commands the PIC 51 to decode and cancel the interference for a specified spreading code, while it refrains from connecting the signal of the unsupported link 23, 26 from the transceiver 601 further to the RNC interface 602. There are also other methods and constructions for cancelling an interfering signal from a wanted signal than the PIC 51 solution described above, and that can be used for cancelling the interference from the mobile station 11 in the second base station 13b. In the prior art they have all, inclusive the PIC 51 solution, been used only for intracell interference cancellation.

One advantage with the PIC 51 solution is that when a hard handover is made from the first base station 13a to one of the second base station/s 13b, the handover is made rapidly, because the target base station has already synchronized to the uplink radio channel 21 from the mobile station 11.

Figure 7a is a box diagram over the same nodes as in figure 3b, namely an RNC 14, the first base station 13a' and the second base station 13b' and the mobile station 11. Both the first and second base stations 13a' , 13b' supports an enhanced uplink 26' , as is being standardised in 3GPP, see e.g. TS25.309. The enhanced uplink 26' is thus in macrodiversity mode. As usual in macrodiversity mode, both base stations 13a' , 13b' control the mobile station 11 transmission power with the result of power decrease if any of the two base stations 13a^', 13b^' orders it, and power increase only if ordered by both the base stations 13a', 13b'.

As defined by 3GPP TS25.309 the enhanced uplink 26' have tough requirements on low delays. For the reason of decreasing delays an acknowledgment protocol is located in the base stations 13a', 13b' supporting the enhanced macrodiversity uplink 26' . The two or more base stations, 13a', 13b' independently acknowledges 26A the mobile station 11 on the receipt of packets. If at least one of the base stations 13a', 13b' positively acknowledges receipt of a packet, no retransmission is performed because the RNC 34 should then receive at least one correct copy of the packet. If, however, both base stations negatively acknowledge a packet transmission, the mobile station 11 retransmits the packet. The acknowledgement and retransmission process involves the RNC 34 rescheduling the stream of packets into its original sequence.

The acknowledge and retransmission process functions well as long as the mobile station 11 receives positive or negative acknowledgements on its transmission from both base stations 13a', 13b'. The inventor have, however, identified that there is a problem of the mobile station 11 ability to detect acknowledgment from more than one of the base stations 13a', 13b'. Similar to the base stations 13a', 13b^' regulating the mobile station 11 transmission power, the mobile station 11 controls the power transmitted from the base stations 13a', 13b'. In macrodiversity mode, the mobile station 11 cannot control the transmission from each of the base stations 13a', 13b' independently. As long as it receives downlink transmissions from one of the base stations 13a' , 13b' above the threshold level for detection, it will not request increased power transmission from any of the base stations 13a' , 13b' . This result in that at least temporary the acknowledgements from one of the base stationsl3 a' , 13b' may be lost. If the uplink and downlink radio links were repriocally exposed to radio disturbances, the uplink would fade away for the same time as the downlink would, and there be not much of a problem. Unfortunately, the uplink conditions may by good when the downlink is not and vice versa, and the acknowledgement and retransmission process discussed above being seriously effected, or not at all working.

The problem of retransmission also result in the original sequence of packets being difficult to re-establish in the RNC 14 as will now be further elaborated with reference to figure 8. Figure 8 discloses packets 801-803 sent from the mobile station 11 as arrows in direction from the lower to the upper horizontal line, and acknowledgements from the two base stations 13a' , 13b' as arrows 820a-821b in the direction from the upper horizontal line to the lower horizontal line. These messages 801-821b are transmitted sequential in time starting from left going to right in the figure .

Initially the mobile station 11 transmits a packet 801 for the first time to the two base stations 13a, 13b. The packet 801 includes a flag N indicating that the packet is new, i.e. transmitted for the first time. None of the two base stations 13a', 13b' correctly receives the packet and, thus, both negatively acknowledge 820a, 820b the packet. The base stations 13a' , 13b' also saves a copy of the incorrectly received first packet 801.

In response to the negative acknowledgements 820a, 820b from both base stations 13a', 13b', the mobile station 11 retransmits the first packet 801, however in the retransmitted packet 802 the N flag is replaced with a flag Re indicating retransmission.

The first base station 13a' correctly receives the packet 802. For detection of the packet 802 information, the first base station may softly combine the symbols received with the first transmitted packet 801 with those of the retransmitted packet 802. In either case the first base station 13a' forwards the softly combined packet to the RNC 34, deletes the first packet 801 and the retransmitted packet 802 from its buffer and positively acknowledges 821a the mobile station 11 on the receipt of the retransmitted packet 802.

The second base station 13b' incorrectly receives the retransmitted packet 802, saves it in its buffer and negatively acknowledges 821b the mobile station 11 on the receipt. In response to having received one positive acknowledge 821a and one negative acknowledge 821b, the mobile station is assured that at least one correct copy of the retransmitted packet 802 will be received by the RNC and then transmits a second new packet 803 with a flag N indicating that the new packet 803 is transmitted for the first time.

At this instance the problem will arise in the second base stations 13b when the second new packet 803 is received with such a bad quality that the second base station 13b cannot detect the flag N indicating that the packet is new. The second base station 13b' may interpret the second packet 803 as a retransmission of the first packet 801, and make a soft combination of the symbols received in the first packet 801, in the retransmitted packet 802 and in the second packet 803. Neither the first packet 801 nor the second packet 803 may thereby be correctly detected, and even if many retransmissions of the second packet 802 are added for the detection, the copies of the first packet 801, 802 will corrupt the detection of the second packet 803. Alternatively upon receipt of the second packet 803 without correct detection of the flag N, the second base station negatively acknowledges the mobile station 11, buffers the second packet 803 and incorrectly combines it with a later incorrectly received new packet.

The present invention solves the problems of synchronising the macrodiversity enhanced uplink 26', by replacing it with a single enhanced uplink 26 set up over a first base station 13a only, and that is depicted in figure 7b. The interference from the mobile station 11 transmission caused on the reception in the second base station 13b, is cancelled as is disclosed above in connection with the second and third steps 42, 43 of the method of figure 4.

In common with the procedure for setting up macrodiversity links 21, 22 in the prior art, selection of one or more second base station/s 13b is made only when the mobile station 11 is geographically located in places were its transmitted power is received over a threshold level in two or more base stations 13. Figures Ia and Ib depicts a distance Ml between two base stations were a macrodiversity mode should be implemented in the prior art, and for the same distance a first and second base stations 13a, 13b should be selected according to the present invention.

The RNC 34 of figure 3c and 7b differs from the prior art RNC 14, in that it is arranged for selecting the first base station 13a and the one or more second base stations 13b, among the base stations having neighbour relations with the first and second base stations 13a, 13b already serving the mobile station 11. The selection of a first base station 13a is the same process as prior art deciding on a hard handover. The RNC 34 also differs from the prior art RNC 14 by being arranged for informing the second base stations 13b in a Radio Link Setup message on the spreading code used by the mobile station 11.

Claims

1. Method for a cellular radio system, comprising the steps of,

setting up an uplink (21,26) from a first mobile station (11) over a first radio base station (13a) within the system;

c h a r a c t e r i s e d by the further steps of :

informing at least one second base station (13b), of the spreading code used by the first mobile station (11) when transmitting on said uplink (21, 26);

cancelling the interference caused by the transmission from the first mobile station (11) in the second base station/s (13b) when detecting signals from mobile stations (10) other than the first mobile station (11).

2. The method of claim 1 wherein in addition to the uplink there is a downlink (23) set up over the first base (13a) station only.

3. The method of claim 2 wherein the first base station (13a) at least partly controls retransmission on the downlink (23) .

4. The method of claim 1 wherein acknowledgements (26A) on the receipt of any packet transmission from the first mobile station (11) are generated by the first radio base station (13a) .

5. The method of claim 1 wherein the one or more second base station/s (13b) is/are selected in the same way as base stations are selected for supporting a macrodiversity link (21,22) .

6. The method of claim 1 or 5 wherein the one or more second base station/s (13b) is/are selected because their signal strengths measured by the mobile station (11) exceed/s a threshold value.

7. A radio base station (13b) for use in a cellular radio communication network comprising,

a radio transceiver (601) for providing radio links (31) to a first group of mobile stations (10),

an interface (6C2) towards the fixed part of the radio communications network for receiving information on a spreading code of a second radio uplink (24,26),

characterised by;

cancelling means (51,52) being arranged for cancelling interference caused by transmissions from a second mobile station 11 in a second radio uplink (21, 26) using a specific spreading code, said cancelling means being arranged for cancelling said interference by use of said spreading code from signals from the first group of mobile stations (10) although the second mobile station (11) is not included in the first group of mobile stations (10).

8. A Radio Network Controller (34) having connections to a number of radio base stations (13a, 13b) under its control wherein a first of said base stations (13a) supports a first radio uplink (23,26) from a first mobile station (11) transmitting a signal stream spread by a spreading code characterised by:

means for selecting one or more second base station/s (13b) based on measurements on their signals strengths by the mobile station (11); means for informing the second base station/s (13b) of the spreading code, for the purpose of cancelling interference from the mobile station transmission, while not setting up a radio link from the first mobile station.

9. The method of claim 1, the radio base station (13b) of claim 7 or the Radio Network Controller (34) of claim 8 wherein the spreading code comprises a scrambling code and a channelisation code.