GB2404534A

GB2404534A - CDMA transmitter which inputs the same signal to at least two parallel spreading elements and applies identical spreading codes at these spreading elements

Info

Publication number: GB2404534A
Application number: GB0317534A
Authority: GB
Inventors: Vagan Shakhgildian; Nicholas Whinnett
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-07-26
Filing date: 2003-07-26
Publication date: 2005-02-02
Anticipated expiration: 2023-07-26
Also published as: GB0317534D0; GB2404534B

Abstract

The invention relates to a CDMA transmitter 200 which generates data signals, CCTrCH1,...,CCTrCHi, and inputs the data signals to processing lines 205, 207, 209, 211, each of which includes a spreading device 221, 223, 225, 227. The transmitter also comprises a combiner 245 which combines signals from the processing lines before transmission. A different spreading code is applied to each of the data signals CCTrCh1,...,CCTrCHi. However a data signal, CCTrCHi, may be allocated two or more processing lines, and in this case identical spreading codes are applied to each of the allocated processing lines 207, 209, 211. The description seems to suggest that identical data is copied to each of the allocated lines 207, 209, 211. The invention makes it possible to apply a higher gain to a data signal than would be possible using a single processing line, without increasing the maximum gain which can be applied to the processing line. In the embodiment of Fig. 3 a first processing line processes a data signal using a first processing parameter (e.g. gain g[(k-1)Ts, i]- W ) and a second processing line processes the data signal using a second processing parameter (e.g. gain g[(k-1)Ts, i]+ W ). One of the two processing lines is then selected to be output, upon receipt of an intimation of the appropriate processing parameter. This embodiment may be used to reduce the loop delay in a power control loop.

Description

A CODE DIVISION MULTIPLE ACCESS TRANSMITTER AND A METHOD

OF OPERATION THEREFOR

Field of the invention

The invention relates to a Code Division Multiple Access (CDMA) transmitter and a method of operation therefor and in particular to a CDMA transmitter for a CDMA communication system, such as a third generation (3G) cellular communication system.

Background of the Invention

In a conventional cellular communication system, a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated.

A typical cellular communication system extends coverage over typically an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as unlink, and communication from a base station to a mobile station is known as downlink.

The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc. Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de- spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed.

Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in 'WCDMA for UMTS', Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876 In a UMTS CDMA communication system, the communication network comprises a Core Network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless user equipment over a radio link being part of the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.

The RNC performs many of the control functions related to the air interface including radio resource management and routing of data to and from appropriate Node Bs. It further provides the interface between the RAN and the ON. An RNC and associated Node Bs are known as a Radio Network System (RNS).

In a typical 3G communication system, each RNC controls a number of base stations and the most numerous network component type is the base station or Node B. Accordingly, the combined cost of the base stations is a significant contribution to the total cost of deploying a communication system.

Furthermore, many characteristics of the performance of a cellular communication system depend directly on the performance of the base stations. For example, the provided quality of service, reliability and capacity achieved in a cellular communication system depends heavily on the performance of the individual base stations. Accordingly, it is important to optimise the performance of base stations while maintaining low cost.

The base stations of a cellular communication system typically support communication over the air interface for a large number of simultaneous mobile stations and services. Especially for 3G communication systems, there is a large variety in characteristics of the voice and data services that may be supported, including for example a large variation in data rates and quality of service parameters such as delays and error rates.

Accordingly, the base stations must be capable of handling a large number of services with very different characteristics and are dimensioned such that the performance requirements can be met for all the allowed combinations and permutations of services.

For example, in UMTS, the spreading factor of a data service generally depends on the required data rate. In particular, spreading codes are allocated from a code tree in response to the required data rate and the spreading codes used by other active services. Thus, a low data rate service will have a long spreading code allowing for a high number of simultaneous low data rate services while a high data rate service has a short spreading code which significantly reduces the spreading codes available to other services. As a specific example, a 6 kbps circuit switched data service may have an allocated spreading code of length 256 whereas a 384 kbps circuit switched data service may have a spreading code of only 4. Higher data rates are mapped on to a plurality of codes, e.g. a 2048 kbps service may be mapped onto four spreading codes each with a spreading ratio of 4.

Hence, depending on the current service distribution, the base station may be required to process e.g. a large number of low data rate services with high spreading factors or a low number of high data rate services with low spreading factors. Although statistical variations will tend to result in the service distribution typically being a mix of low data rate, medium data rate and sometimes high data rate services, it is necessary that the base station is capable of simultaneously processing a large number of low rate services. This number is preferably equal to the total number of low data rate services that can be simultaneously active and may specifically be equal to the total number of spreading codes available at the lowest level of the spreading code tree Conventional base stations are accordingly designed to simultaneously process a large number of services. In a typical CDMA base station, the transmitter comprises a large pool of processing lines each of which may process a channel of a data service. Depending on the service distribution, a smaller or larger number of the available processing lines are used. This ensures that the base station can provide acceptable performance for all possible service distributions but results in a significant part of the base station computational resource being left idle for long intervals. This a significant computational resource does not contribute to the performance of the base station except for in rare situations where the service distribution necessitates it.

Accordingly an improved CDMA transmitter and method of operation therefor would be advantageous and in particular a CDMA transmitter allowing for increased flexibility, improved performance, reduced cost, increased capacity of the communication system and/or improved utilization of the resource of the base station would be advantageous.

Summary of the Invention

Accordingly, the Invention seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided a CDMA transmitter for a CDMA communication system comprising; a data signal generator operable to generate data signals; a plurality of processing lines for processing a plurality of line signals; each processing line comprising at least one means for spreading the line signal of the processing line; combining means for combining line signals from the plurality of processing lines to a combined signal; transmit means operable to transmit the combined signal; and a controller operable to allocate at least two processing lines of the plurality of processing lines having substantially identical spreading codes to a single data signal to be processed as line signals of the at least two processing lines.

The inventors of the current invention have realised that it is possible for a single data service to use a plurality of processing lines and that this may provide improved performance of the CDMA transmitter and accordingly of the CDMA communication system as a whole.

The invention, for example, allows for processing lines which are dimensioned but not currently used to be allocated to support other data signals. The invention may allow for processing to be shared between a plurality of processing lines resulting in possible decreased response times and/or increased performance of the transmitter. Furthermore, sharing of processing between a plurality of processing lines may result in relaxed requirements for each individual processing line thereby allowing for reduced complexity and/or cost of the CDMA transmitter and thus of the CDMA base stations and the CDMA communication system as a whole. For example, the requirement for the dynamic range of each processing line may be reduced thereby reducing the required word length of the data in the processing line and thus resulting in a significantly reduced complexity and computational resource use of the individual processing line.

The data signal generator may for example be a base band processor which receives a number of data streams and arranges these to correspond to data signals which are to be transmitted using the same spreading code. For example, the single data signal may be a coded composite transport channel as! described in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 25.212. The controller is operable to allocate two or more processing lines to the single data signal. However, the controller need not allocate at least two processing lines to all data signals but may allocate only a single processing line in some scenarios and only allocate two or more I processing lines when this is advantageous.

According to a feature of the invention, the controller is operable to allocate a number of processing lines to the single data signal in response to a data rate of the single data signal. This allows for the allocation of processing lines to take into account the requirement of processing lines by other services and/or the benefit of allocating a plurality of processing lines to the service. The invention thus allows for an optimization of the allocation of processing lines to reflect the service distribution for the CDMA transmitter.

According to another feature of the invention, the number of processing lines increases for increasing data rates.

This allows for an appropriate and preferably optimised number of processing lines to be allocated to a given data signal. Typically, the benefit of an additional processing line increases for increasing data rates. Furthermore, the availability of additional processing lines typically increases for increasing data rates as this reduces the air interface resource available to other data seances.

The increase in the number of processing lines is not necessarily monotonous and the number of processing lines may be the same for a given range of data rates.

According to another feature of the invention, the controller is operable to allocate a single processing line to the single data signal if the data rate is less than a threshold and the at least two processing lines if the data rate is above the threshold.

Preferably the number of processing lines is a step function of the data rate where one processing line is allocated for data rates below the first threshold, two processing lines are allocated for data rates over the first threshold but below a second threshold, three processing lines are allocated for data rates over the second threshold but below a third threshold etc. This allows for a simple and low complexity method of allocating processing lines yet results in significantly increased performance.

According to another feature of the invention, the controller is operable to allocate a number of processing lines to the single data signal in response to a spreading factor of the single data signal. This allows for the allocation of processing lines to take into account the requirement for processing lines by other services and/or the benefit of allocating a plurality of processing lines to the service. The invention thus allows for an optimization of the allocation of processing lines to reflect the spreading code allocation and the service distribution for the CDMA transmitter.

According to another feature of the invention, the number of processing lines increases for decreasing spreading factors.

This allows for an appropriate and preferably optimised number of processing lines to be allocated to a given data signal. Typically, the benefit of an additional processing line increases for decreasing spreading factors.

Furthermore, the availability of additional processing lines typically increases for decreasing spreading factors as this reduces the air interface resource available to other data services.

The increase in the number of processing lines is not necessarily monotonous and the number of processing lines may be the same for a given range of spreading factors.

According to another feature of the invention, the controller is operable to allocate a single processing line to the single data signal if the spreading factor is above a second threshold and the at least two processing lines if the spreading factor is below the second threshold.

Preferably the number of processing lines is a step function of the spreading factor where one processing line is allocated for spreading factors above the second threshold. This allows for a simple and low complexity method of allocating processing lines yet results in significantly increased performance.

According to another feature of the invention, each processing line comprises gain means for setting a gain of the processing lines. This allows for the gain of the single data signal to be adjusted and specifically to be adjusted by the modification of gains in a plurality of processing lines. This allows for an increased flexibility in gain setting which may allow for decreased dynamic range of one or more of the processing lines thereby permitting reduced complexity and cost of the CDMA transmitter.

According to another feature of the invention, the CDMA transmitter further comprises means for setting a gain of the gain setting means in response to a power control command. This allows for increased flexibility and performance of power control functionality and may for example allow for faster power control response times and/or for increased dynamic power control range for a given processing line dynamic range.

According to another feature of the invention, the CDMA transmitter comprises means for setting gains of the gain means such that a combined gain of the at least two processing lines corresponds to a desired gain for the single data service.

The gain setting may for example be in response to a power control command.

This allows for increased flexibility as a desired gain can be achieved by individually setting a plurality of processing lines. Specifically, it may allow for a higher dynamic range of the desired gain than of any of the processing lines.

According to another feature of the invention, the gains of the at least two processing lines is set substantially equal.

For example, if a gain of X is required and the processing extends over L processing lines, the gain of each may be set to X/L. This allows for the requirements of the processing lines to be equally divided and allows for simplified processing and control as all processing lines may be set together.

According to another feature of the invention, each processing line comprises a transmit diversity weight means for setting a transmit diversity weight of the processing lines.

This allows for a transmit diversity weight or gain of the single data signal to be adjusted and specifically to be adjusted by the modification of transmit diversity weights in a plurality of processing lines. This allows for an increased flexibility in diversity weight setting which may allow for decreased dynamic range of one or more of the processing lines thereby permitting reduced complexity and cost of the CDMA transmitter. It may also allow for improved and/or faster setting of the transmit diversity weights.

The transmit diversity weight is preferably a complex value comprising both a gain and a phase adjustment. The transmit diversity weight may be set in response to feedback from a receiver receiving the single data signal transmitted from the CDMA transmitter.

According to another feature of the invention, the first processing line is operable to process the line signal in response to a first processing parameter value; the second processing line is operable to process the line signal in response to a second processing parameter value; and the combining means comprises means for selecting between the line signal of the first and second processing line.

This allows for increased performance and flexibility where different processing parameter values may be processed in parallel. Specifically, this may allow for improved dynamic performance as different parameters can be evaluated and/or applied in parallel rather than sequentially.

The parameter values may for example be a transmit gain, a power control parameter value or a transmit diversity weight.

According to another feature of the invention, the first parameter corresponds to a first possible parameter value and the second parameter value corresponds to a second possible parameter value and the combining means is operable to select between the first and second processing line in response to a determined parameter value.

This may in particular provide for fast dynamic performance wherein the single data signal is pre-processed using a plurality of possible parameter values. The signal of the processing line having process the signal with the appropriate parameter may subsequently be selected. Hence, the processing of the data signal may be started before the appropriate value of a processing parameter has been determined. The determined parameter value may specifically be determined in response to a feedback signal from a receiver communicating with the CDMA transmitter. Thus the processing of the data signal may be started before the feedback has been received by processing possible values of the processing parameter corresponding to different feedback messages. Once the feedback message is received, the data signal of the appropriate processing line may instantly be selected. Thus, the loop delay may be substantially reduced.

According to another feature of the invention, the first possible parameter value corresponds to an increased gain step; the second possible parameter value corresponds to a decreased gain step; and the combining means is operable to select between the first and second processing line in response to a power control command. This may allow for reduced time delays and improved dynamic performance for implemented power control loops. This achieves improved power control performance and thus reduced interference and increased capacity of the communication system. Alternatively or additionally, the decreased time delay in the transmitter may be used to keep the same power control loop delay but to increase the time available to the processing of the power control commands in the receiver. This longer processing time may result in a higher reliability of detection for the power control commands for example by allowing longer channel estimation filters to be applied.

According to another feature of the invention, the first possible parameter value corresponds to a first transmit diversity parameter step; the second possible parameter value corresponds to a second transmit diversity parameter step; and the combining means is operable to select between the first and second processing line in response to received transmit diversity information. This may allow for reduced time delays and improved dynamic performance for implemented transmit diversity loops. This achieves improved transmit diversity performance and thus reduced interference and increased capacity of the communication system. Alternatively or additionally, the decreased time delay in the transmitter may be used to keep the same transmit diversity loop delay but to increase the time available to the processing of the transmit diversity commands in the receiver. This longer processing time may result in a higher reliability of detection for the transmit diversity commands for example by allowing longer channel estimation filters to be applied.

According to another feature of the invention, the transmitter furthermore comprises means for storing the possible parameter value of the selected first and second processing line. This may allow for the possible processing parameter values to be based on previous parameter values.

According to another feature of the invention, the combining means comprises summing means for summing the line signals. This allows for a particularly suitable implementation and allows for a simple and efficient way of combining the signals from the processing lines.

According to another feature of the invention, the single data signal corresponds to a data signal for a physical channel of the CDMA communication system. Preferably, the single data signal corresponds to a signal which is spread using the same spreading code. The data signal may comprise a plurality of time multiplexed logical channels which are transmitted using the same spreading code. For example, the single data signal may be a coded composite transport channel as described in the 3rd Generation Partnership Project (3GPP) Technical Specification TS 25.212.

According to another feature of the invention, the single data signal corresponds to a base band data signal. Preferably the processing of the processing lines is performed on base band signals allowing for facilitated implementation of the functionality of the processing lines.

According to another feature of the invention, the at least two processing lines are parallel processing lines. This allows for a suitable implementation which may be particularly useful when fast and/or low power consumption and/or cost and/or size is desired.

According to another feature of the invention, the at least two processing lines are time multiplexed processing lines of a single processor. This allows for a suitable implementation which may be particularly useful when flexibility and/or low cost and/or size is desired.

According to a second aspect of the invention, there is provided a method of operation for a CDMA transmitter of a CDMA communication system, the CDMA transmitter including a data signal generator operable to generate data signals; a plurality of processing lines for processing a plurality of line signals; each processing line comprising at least one means for spreading the line signal of the processing line; combining means for combining line signals from the plurality of processing lines to a combined signal; transmit means operable to transmit the combined signal; the method comprising allocating least two processing lines of the plurality of processing lines to a single data signal to be processed as line signals of the at least two processing lines.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief Description of the Drawings

An embodiment of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 illustrates a block diagram of a CDMA transmitter; FIG. 2 illustrates a block diagram of a CDMA transmitter in accordance preferred embodiment of the invention; and FIG. 3 illustrates a block diagram of a part of a CDMA transmitter accordance with an embodiment of the invention.

Detailed Description of a Preferred Embodiment of the Invention The following description focuses on an embodiment of the invention applicable to a transmitter for base station of a CDMA communication system and in particular to a UMTS communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other CDMA communication systems.

In order to clarify and describe the preferred embodiment of the invention, a description of a CDMA base station transmitter not comprising the invention will be given. This description is provided merely to facilitate and clarify the description of the preferred embodiment of the invention and does not comprise any indication or acknowledgement of the state of the prior art.

FIG. 1 illustrates a block diagram of a UMTS CDMA base station transmitter 100 suitable for but not explicitly comprising the preferred embodiment of the invention. The transmitter 100 comprises a symbol processor 101 which receives a number of data streams from a network interface which connects the base station to the fixed network.

Specifically, the data streams may correspond to a number for user transport channels. The symbol processor 101 performs the operations of interleaving, channel encoding, multiplexing and rate matching and generates a number of composite coded transport channels (CCTrCH) by a map processor 103.

The transmitter 100 comprises a chip processor 104 coupled to the map processor 103. The chip processor 104 is operable to generate a plurality of physical channels by spreading and processing the composite coded transport channels. Specifically, the chip processor 104 comprises a number of processing lines 105, 107, 109 each of which comprise means for processing a physical channel. Each physical channel (PhCH) corresponds to one spreading code with a given spreading factor on the air interface.

For clarity and brevity the current description will focus on dedicated physical data channels and will not describe dedicated control channels and common control channels in detail.

In the shown diagram each processing line comprises a gain element 111, 113, 117 which may set the gain of the processing line and thus the amplitude of the physical channel signal. In addition, each processing line comprise a spreading multiplier 117, 119, 121 which spreads the physical channel signal with the spreading code assigned for the physical channel.

The processing lines furthermore comprise a scrambling code multiplier 123, 125, 127 which multiplies the physical channel signal by a scrambling code (also known as base station colour code or long code) which is a common code for each sector of a base station and which is used to separate between different cells in a CDMA cellular communication system. In some communication systems, there may be multiple scrambling codes per sector with each code being used by one or more user equipment. In the transmitter of FIG. 1, each physical channel is processed in a

dedicated processing line. This processing line performs power control, transmit diversity processing, spreading and scrambling. Each of the processing lines capable of processing any dedicated physical channel with any spreading factor. It will be appreciated that each processing line may perform furthr transmit processing than the operations shown in FIG. 1. For example, transmit diversity processing may also be performed in each individur processing line.

The order of operations in each processing line can be different from that in Figure 1. However, the gain control function is typically placed at the beginning of the processing line because this arrangement avoids true multiplication by the gain factor. Thus, in the arrangement shown, each multiplier has a binary input thereby allowing for a low complexity multiplication operation. Furthermore, the gain multiplication is performed before spreading thus allowing for multiplications to be performed at the symbol rate rather than at the chip rate.

The physical channel signals of the different processing lines 105, 107, 109 are combined in a summer 129 thereby generating a combined signal comprising all the physical channels transmitted by the base station. The summer 129 is coupled to a digital to analogue converter 131 which converts the digital combined transmit signal to an analogue signal. The analogue to digital converter 131 is coupled to an RF transmitter 133 which performs the required amplification and up-conversion of the analogue combined transmit signal.

In the transmitter of FIG. 1, the composite coded transport channels are mapped onto a number of physical channels generated in the chip processor. Each physical channel (PhCH) is essentially one spreading code with a given spreading factor (SF) on the air interface.

The number of processing lines in the chip processor 104 of the transmitter of FIG. 1 is equal the maximum number of supported physical channels. This number may be limited by the maximum number of orthogonal spreading codes with the largest spreading factor supported by the air interface. Thus in the transmitter of FIG. 1 each processing line has a unique associated spreading code which is not used by any other processing line. The associated spreading code may be assignable to a processing line but is not shared by any other processing line. Thus, typically each processing line is configurable in terms of which spreading code is applied.

All coded composite transport channels which have a data rate below a maximum data rate are mapped to a single dedicated processing line. This processing line accordingly must be dimensioned to meet all the requirements for a coded composite transport channel and this imposes strict dimensioning requirements on the processing line. Specifically, the requirements for data word size and processing latencies results in every single processing line being dimensioned such that the full dynamic range of power control per physical channel is met and such that the power control delay is kept sufficiently low.

This results in an suboptimal implementation complexity and thus in complex, power consuming and expensive implementations.

In UMTS, higher data rates are transmitted by use of multiple codes. For example a 2 Mbps data service may be transmitted by use of four parallel spreading codes. In this case, a processing line may be allocated to each of the spreading codes. However, although a plurality of processing lines is used, the spreading code in each processing line is different and the dimensioning and performance requirements of each processing line are unchanged. In effect, multi code operation may be perceived as communication of a high data rate service by use of a plurality of parallel channels.

In the preferred embodiment, the invention provides for a CDMA transmitter which improves on the transmitter of FIG. 1. Similarly to the transmitter of FIG. 1, the CDMA transmitter comprises a data signal generator which generates a plurality of data signals. These data signals may specifically be composite coded transport channels which are generated as in the symbol processor lot and the map processor 103 of the transmitter of FIG. 1. The transmitter of the preferred embodiment further comprises a plurality of processing lines for processing a plurality of line signals where each processing line comprises at least one means for spreading the line signal of the processing line. These processing lines may correspond to the processing lines 105, 107, 109 of the chip processor 104 of FIG. 1. The CDMA transmitter of the preferred embodiment further comprises combining means for combining line signals analogous to the summer 129 of FIG. 1 and also comprises transmit means analogous to the digital to analogue converter 131 and RF transmitter 133 of FIG. 1.

However, in addition to the transmitter of FIG. 1, the CDMA transmitter of the preferred embodiment further comprises a controller which is operable to allocate at least two processing lines of the plurality of processing lines having substantially identical spreading codes to a single data signal to be processed as line signals of the at least two processing lines. Hence, in the preferred embodiment, a data signal such as a composite coded transport channel, which is communicated on a single physical code channel using a single spreading code, may be processed in a plurality of processing lines. This may provide reduced complexity of the CDMA transmitter as well as relaxed dimensioning criteria. Furthermore, improved performance of the base station may be achieved thereby improving the performance of the communication system as a whole.

For example, by processing a composite coded transport channel in two or more processing lines, the dynamic range in each of the processing lines may be reduced. As a specific example, a single processing line may be used to provide a signal having an amplitude ranging from 0 to X. If a higher amplitude than X is required, a second processing line having the same dynamic range may process the same signal in parallel and by adding the two signals together, an amplitude range from O to 2X may be achieved. Thus, the preferred embodiment allows for a dynamic range of the composite coded transport channel which is larger than the dynamic range of the individual processing lines.

In the preferred embodiment, each of the processing lines processing a data signal in parallel applies the same spreading code to the line signal processed by the processing line. The spreading means are synchronized such that when the individual line signals are added together to form the combined transmit signal, the spreading codes will be synchronous resulting in a spreading which is substantially identical to a single spreading operation.

In the preferred embodiment, the allocation of processing lines to each spreading code is in response to the data rate. Alternatively or additionally, the allocation of processing lines to each spreading code may be in response to the spreading rate. In communication systems such as UMTS, the spreading rate is a function of the data rate and the two approaches may in this case be considered equivalent.

In the preferred embodiment, the number of processing lines allocated to a single data signal transmitted with a single spreading code increases with increased data rates and reduced spreading factors. In this embodiment, a single processing line is allocated to the data signal if the data rate is below a first threshold (or the spreading rate is above a corresponding threshold). If the data signal has a data rate between the first threshold and a second higher threshold (or the spreading factor is between two corresponding thresholds), two processing lines are allocated to the data signal. If the data signal has a data rate between the second threshold and a third higher threshold (or the spreading factor is between two corresponding thresholds), three processing lines are allocated to the data signal. The described principle may be extended to as many processing lines as is considered practical and advantageous in the individual embodiment.

Increasing the number of processing lines for higher data rates allows for an increased performance for these data rates. Many characteristics or requirements are particularly critical for higher data rates and therefore improving performance in these conditions is particularly advantageous. It furthermore allows for parameters to be dimensioned for lower data rate services and allowing for the corresponding parameters for higher data rates to be achieved by multiplication of the processing lines. For example, a power control dynamic range may result in a given amplitude requirement for a low data rate. At higher data rates, higher amplitude values may be required but this may be achieved by summing the signals of a plurality of processing lines thereby allowing for the dynamic range of each processing line to be less than the total required dynamic range.

Furthermore, the performance characteristics of higher data rate services are particularly critical for the overall performance of a CDMA communication system, and as a result it is particularly advantageous to improve performance for the higher data rate services.

It should also be noted that the use of a plurality of processing lines for higher data rate services can be achieved without an increase in the total number of processing lines. Specifically, the transmitter must be designed to support a large number of low data rate channels using the maximum available spreading factor. Therefore the number of processing lines must equal the number of low data rate channels supported. However, the air interface capacity in terms of physical channels reduces as the data rates increase and the spreading factor decrease. This is partly due to the number of available orthogonal spreading codes being proportionate to the spreading factor (and for Walsh codes, it is equal to the spreading factor).

Therefore in the transmitter of FIG. 1 wherein a single processing line is used per physical channel, a substantial number of processing lines are left unused when medium and high data rate channels are deployed. For example, a CDMA transmitter may be dimensioned to support 256 physical channels of spreading factor 256 and may therefore have 256 processing lines. When the downlink traffic consists only of channels with a spreading factor of 64, there are 256-64=192 unused processing lines (75%) and it is possible to allocate up to 256/64=4 lines per physical channel. For a spreading factor of 4 and (252-4) =248 unused lines (98%), up to 64 lines can be allocated per channel. In a practical application, the air interface interference limitations may reduce the number of channels supported relative to the number of codes available.

Preferred embodiments of the invention will be described in more detail in the following. The described embodiments are specifically aimed at providing improvements for the power control functionality in a CDMA cellular communication system.

Power control in the downlink (between the base station and the mobile station) is a key feature of any modern CDMA cellular system. Power control is performed by the base station in such a way that transmit power allocated to each individual radio link depends on the signal-tointerference (SIR) ratio at the receive end of the link, i.e. in the mobile receiver. The main characteristics of power control include dynamic range, rate of operation and (for closed loop power control) the loop delay. To achieve the best performance for a given rate of control commands (typically set in the standards), the power control functionality must provide sufficiently large dynamic range per physical channel as well as a loop delay which is as low as possible. However, these requirements are often directly opposed to the desire to reduce cost and complexity of the base station design. The preferred embodiment aims at improving the performance versus complexity trade-off i.e. to achieve better performance with the same complexity or reducing the implementation complexity with the same performance or a combination thereof.

In UMTS, both an inner power control loop and an outer power control loop are implemented. Inner loop power control operates as follows. The receiving entity of a radio link measures the received signal to noise ratio (SIR), and compares it to a locally stored target SIR. A command is sent back to the transmitter to increase transmitted power if the measured SIR is less than the target. Conversely, if the measured SIR is greater than the target, a command is sent to the transmitter to decrease the transmitted power. The target SIR is set by a known feature called outer loop power control. Its function is to maintain the frame error rate (FER) of the radio link at or below a given value or threshold. The frame error rate of the received signal is measured by one of a number of known techniques, and the SIR target is adjusted to try to ensure that the FER is at or below the given value.

In the transmitter of FIG. 1, the multipliers 105, 107, 109 represent the downlink gains g_k,i which are adjusted in response to the power control commands. When closed loop power control is enabled, the gain is varied every power control interval (known as Power Control Group (PCG) in UMTS) or time slot in response to a power control command. In the current description, only binary power control commands with a fixed step size are considered but it will be clear to the person skilled in the art that the invention is not limited thereto. When the power control command received from the mobile station is a power up command, the gain is increased by a step value dB and when the command is a power down command the gain is reduced by the same step value dB.

The required dynamic range of the static power control can be estimated as a sum of the following components The difference between the maximum and minimum spreading factors for a given Quality of Service (QoS): lO*log(512/4)=21 dB.

À QoS (e.g. SIR target) variation per spreading factor: 2 dB typical.

À Maximum difference in gains between channels with soft handover enabled and disabled: 3 dB typical.

Leading to a dynamic range requirement of 26 dB. This dynamic range corresponds to the difference between the gain of the highest data rate physical channel with the lowest spreading factor (4), no soft handover and highest SIR requirement and that of the lowest data rate physical channel with the highest spreading factor (512) and the lowest SIR target and in addition requiring less power because it is in soft handover. Thus the dynamic range of 26 dB corresponds to the largest gain variation required to perform the static power control.

In addition to the static power control, the gain of the processing lines must be adjusted to compensate for the dynamic power control variations (i.e. to match the short term variations in the propagation channel). The range of dynamic power control is typically assumed to be 25 dB, which is the minimum requirement in the UMTS specifications (3GPP Technical Specification TS 25.104 Node B Radio Transmission and Reception (FDD. Thus, the total power control range per dedicated physical channel is (26+25)= 51 dB. With 6 dB of dynamic range per 1 bit of data word length and an extra 1.5 bit (9 dB) headroom, the minimum data path word size for each processing line of the transmitter in FIG. 1 is thus (51+9)/6 = 10 bits. This results in a relative high complexity and it is desirable to reduce the requirements for this word size while retaining the full dynamic range per physical channel.

FIG. 2 illustrates a block diagram of a CDMA transmitter in accordance with a preferred embodiment of the invention. Similarly to the transmitter of FIG. 1, the CDMA transmitter 200 of FIG. 2 comprises a symbol processor 201 which receives a number of data streams from a network interface which connects the base station to the fixed network.

Specifically, the data streams may correspond to a number of user transport channels. The symbol processor 201 performs the operations of interleaving, channel encoding, multiplexing and rate matching. The symbol processor 201 is coupled to a map processor 203 which generates a number of composite coded transport channels (CCTrCH) from the data received from the symbol processor 201.

The transmitter 200 comprises a chip processor 204 coupled to the map processor 203. The chip processor 204 is operable to generate a plurality of physical channels by spreading and processing the composite coded transport channels. Specifically, the chip processor 104 comprises a number of processing lines 205, 207, 209, 211 (only four processing lines are illustrated in FIG. 2).

Each of the processing lines is operable to process a line signal and, in the shown example, each processing line comprises a gain element 213, 215, 217, 219 which may set the gain of the processing line 205, 207, 209, 211 and thus the amplitude of the line signal. In addition, each processing line 205, 207, 209, 211 comprises a spreading multiplier 221, 223, 225, 227 which spreads the line signal with a spreading code. The processing lines 205, 207, 209, 211 furthermore comprise a scrambling code multiplier 229, 231, 233, 235 which multiplies the physical channel signal by a scrambling code (also known as a base station colour code or long code) which is typically a common code for each sector of a base station and which is used to separate between different cells in a CDMA cellular communication system. In the shown embodiment, each processing line 205, 207, 209, 211 further comprises an optional processing line processor 237, 239, 241, 243 which is operable to perform further processing of the line signals.

The processing lines 205, 207, 209, 211 are coupled to combining means which in the shown embodiment is a summer 245 which sums the line signals from all of the processing lines 205, 207, 209, 211 to generate a combined signal.

The transmitter 200 further comprises an allocation controller 247. The allocation controller 247 is operable to allocate at least two of the processing lines 205, 207, 209, 211 having substantially identical spreading codes to a single data signal to be processed by the chip processor 204. In the preferred embodiment, the allocation controller 247 is coupled to the map processor 203 and controls the map processor 203 so that the composite coded transport channels are fed to the appropriate processing lines.

In the example illustrated in FIG. 2, a first composite coded transport channel (CCTrCH1) is fed to a single processing line 205 for processing. The first composite coded transport channel is specifically a low data rate channel having a high spreading factor. The first composite coded transport channel is specifically processed as in the transmitter of FIG. 1.

In the illustrated example, a second composite coded transport channel is a high data rate channel having a low spreading factor. In accordance with the preferred embodiment of the invention, the second composite coded transport channel is not fed to a single processing line but is rather fed to a plurality of processing lines each of which apply the same spreading code. In the example, the second composite coded transport channel (CCTrCH2) is fed to three processing lines 207, 209, 211.

In the preferred embodiment, the gain applied in each of the processing lines 207, 209, 211 is substantially equal. Hence, a third of the desired gain is applied to each processing line and the dynamic range of the line signals is accordingly a third of the total dynamic range of the combined signal. Thus the dynamic range of each processing line may be reduced.

More generally, if L denotes the integer part of the ratio between the maximum number (NMax) of processing lines available at the receiver and the required spreading factor (SF) in i-th physical channel, i.e. L= int [NMax/SF]., the i-th composite coded transport channel can be mapped to L identically configured processing lines without limiting the allocation of processing lines to other channels. The gain applied in each processing line is preferably 1 divided by L of the required gain gk.. The outputs of each of the L processing lines are added together, and the resulting physical channel has the wanted gain of (gk,,/L)*L= gk,i. The advantage of such scheme is that maximum gain (and consequently the gain variation) per processing line is reduced by a factor of L. Using a UMTS example with NMax = 256, where all spreading factors are powers of two, it will be appreciated that 10 log (256/4) = 18 dB of the power control range can be covered by this method, thereby reducing the data path Alternatively, it may be desirable in some cases where the wanted gain falls below some threshold due to power control to temporarily disable (or set the processing line gain to zero) for a subset of the lines. This may be desirable in order that small gains can be provided with required resolution.

It will be appreciated that it is an equally appropriate approach to consider that a given physical channel power gain G can be increased by a factor of K by assigning K processing lines to this channel. Thus if each processing line has a gain between zero and G. a total gain of the combined signal between zero and G K can be achieved by e.g. adjusting the gain of only one processing line and adding the required number of processing lines each having a preset gain of G. Thus, it will be appreciated that by varying the assignment rule for the partial gains and/or the number of processing lines per physical channel, the approach can be easily generalized to provide a very flexible gain variation.

In some embodiments, assignment and de-assignment of processing lines on a time slot or power control update interval basis is impractical and it is therefore preferred to use the gain adjustment by allocation of processing lines to implement the functionality of the static power control rather than of the dynamic power control.

FIG. 3 illustrates a block diagram of a part of a CDMA transmitter in accordance with an embodiment of the invention. The CDMA transmitter corresponds to the transmitter of FIG. 2 but illustrates an alternative or additional way of processing a composite coded transport channel by a plurality of processing lines using the same spreading code. In the example of FIG. 3, the composite coded transport channel has been allocated two processing lines 301, 303. Each processing line 301, 303 comprises a gain element 305, 307 for setting the gain of the processing line; a spreading multiplier 309, 311 spreading the line signal with a spreading code and comprise a scrambling code multiplier 313, 315 multiplying the physical channel signal by a scrambling code. Thus the processing lines of the transmitter of FIG. 3 comprise the same elements as the transmitter of FIG. 2 (for clarity and brevity, the optional processing line processors of FIG. 2 have been ignored).

However, the operation of the transmitter of FIG. 3 differs from that of FIG. 2 in that each of the processing lines 301, 303 process the full composite coded transport channel including the full dynamic range. However, each of the processing lines 301, 303 processes the composite coded transport channel using different values of a processing parameter value. Specifically, each of the processing lines may process the composite coded transport channel in response to a possible or expected processing parameter value, the exact value of which is not yet known.

Thus each processing line generates a line signal corresponding to the desired processed composite coded transport channel assuming that the unknown parameter value is equal to the assumed parameter value. Accordingly, when the unknown parameter value is determined, the appropriate processed composite coded transport channel signal is simply obtained by selecting the processing line which most closely corresponds to the determined value.

Therefore the combining means of the transmitter of FIG. 3 comprises a selection element 317 which selects the appropriate processing line in response to the determined value.

The embodiment of FIG. 3 can result in very fast operation. This may be particularly advantageous for power control loop operation where the loop delay is a very important performance characteristic. The loop delay is a measure of how quickly the power control can respond to variations in the signal-to-interference ratio, and it is essential to minimize it to attain good performance. In the illustrated transmitters, the gain element is at the beginning of the processing line (in order to simplify operation), and therefore all processing latencies between the gain element and the output of the processing line will directly contribute the power control delay. The power control loop delay needs to be less than one time slot to optimize the link performance. It can be shown that in order to maintain a single time slot delay for 384 kbps (SF=4) service in 3GPP downlink with a cell size of 50 km, the total base station processing delay should be less than 34 As. This includes the aforementioned delay in the processing line as well as transport delay between the receiver and the transmitter, which typically equals 20..25 As. The minimization of the delay in the processing line processor is clearly an important factor. Alternatively the decreased time delay in the transmitter may be used to keep the same power control loop delay but to increase the time available to the processing of the power control commands in the receiver.

This longer processing time may result in a higher reliability of detection for the power control commands for example by allowing longer channel estimation filters to be applied.

The embodiment illustrated in FIG. 3 is particularly suited for reducing delays in a power control loop. Typically, a CDMA transmitter waits to receive a power control command before it can set the appropriate gain level and thus start processing the signal in the processing line. However, in the embodiment of FIG. 3, the processing is started simultaneously in multiple processing lines, with each line making an assumption about the value of the power control command which has not yet been received. When the command eventually arrives, it will simply select the output of the processing line that used the correct gain value in its computations. In this way, the latency in each processing line is removed from the power control loop delay and thus improved performance is achieved.

In a specific example, downlink power control commands a[KTs] (having values 0 or 1) are received once every power control interval (Ts). The ith channel gain value g[(K-1)Ts, i] which has been generated for the previous time slot interval is stored in a register. The i-th physical channel is allocated two processing lines 301, 303 which are configured substantially identically with the exception of the gain values of the gain elements. The first processing line has a gain of g[(K-1)Ts, i] - A, where is the power control step, whilst the gain in the second line is g[(K-1)TA, i] + A. Note that the gain addition/subtraction operations imply that the gains and the step in this case are expressed in decibels.

After the new power control command (for the K-th time slot) a[KTs] has been received, it controls the selection element 317 to select the processing line that applied the gain value corresponding to the received power control command.

At the same time it stores the correct value of the gain g[(K-1)Ts, i] + (-1) afKTs] to be used for the following power control time interval.

It will be appreciated that the approach can be extended to cover nonbinary (soft decision) downlink power control commands. Specifically, an N-bit command can take 2N values and therefore may result in 2N processing lines being allocated per physical channel.

It will be appreciated that the approaches of the embodiments of thetransmitter of FIG. 2 and that of FIG. 3 can be advantageously combined in the same transmitter thereby allowing for both reduced loop delay and improved dynamic range performance.

It will be appreciated that the invention may also be applicable to for example other binary and non-binary closed loop applications such as feedback transmit diversity weights and even insertion of commands sent to the mobile over the downlink (such as uplink power control, timing synchronization).

As an example, UMTS allows for transmit diversity techniques wherein two antennas are used to transmit to a mobile station. The mobile station determines preferred weights for each of the two antennas and returns the values to the base station. Typically, the diversity weights are complex values comprising both a gain and phase element. The CDMA transmitter adjusts the weight of the signals for the two antennas in response to the received data whereby the signal received at the mobile station is improved. It will be appreciated that the signals for each of the transmit diversity antennas may be processed in a plurality of processing lines and that the weights can be implemented as combined weights distributed over a plurality of processing lines similarly to the approach used for the gain element for power control.

Also, different processing lines may be used to process the signals in accordance with different possible weight settings before the feedback is received from the mobile station. Hence, the dynamic range and loop delay performance for transmit diversity may be improved similarly to the improvements for the power control implementation.

The CDMA transmitter may be implemented in any suitable way and specifically the plurality of processing lines may for example be imp, by any combination of parallel hardware or software resources or multiplexed processing line of a single processor. Thus, the archj chip processor allows for different implementations including a set of hardware processing chains or a parallel processing engine where e.g. different physical channels are processed in a time-multiplexed fashion.

In general, the invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. However, preferably, the invention is implemented as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term comprising does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality.

Claims

1. A CDMA transmitter for a CDMA communication system comprising; a data signal generator operable to generate data signals; a plurality of processing lines for processing a plurality of line signals; each processing line comprising at least one means for spreading the line signal of the processing line; combining means for combining line signals from the plurality of processing lines to a combined signal; transmit means operable to transmit the combined signal; and a controller operable to allocate at least two processing lines of the plurality of processing lines having substantially identical spreading codes to a single data signal to be processed as line signals of the at least two processing lines.

2. A CDMA transmitter as claimed in claim 1 wherein the controller is operable to allocate a number of processing lines to the single data signal in response to a data rate of the single data signal.

3. A CDMA transmitter as claimed in claim 2 wherein the number of processing lines increase for increasing data rates.

4. A CDMA transmitter as claimed in claim 3 wherein the controller is operable to allocate a single processing line to the single data signal if the data rate is less than a threshold and the at least two processing lines if the data rate is above the threshold.

5. A CDMA transmitter as claimed in any previous claim wherein the controller is operable to allocate a number of processing lines to the single data signal in response to a spreading factor of the single data signal.

6. A CDMA transmitter as claimed in claim 5 wherein the number of processing lines increase for decreasing spreading factors.

7. A CDMA transmitter as claimed in claim 6 wherein the controller is operable to allocate a single processing line to the single data signal if the spreading factor is above a second threshold and to allocate the at least two processing lines if the spreading factor is below the second threshold.

8. A CDMA transmitter as claimed in any previous claim wherein each processing line comprises gain means for setting a gain of the processing lines.

9. A CDMA transmitter as claimed in claim 8 further comprising means for setting a gain of the gain setting means in response to a power control command. 1'

10. A CDMA transmitter as claimed in claim 8 further comprising means for setting a gain of the gain means such that a combined gain of the at least two processing lines corresponds to a desired gain for the single data service.

11. A CDMA transmitter as claimed in claim 10 wherein the gains of the at least two processing lines is set substantially equal.

12. A CDMA transmitter as claimed in any previous claim wherein each processing line comprises a transmit diversity weight means for setting a transmit diversity weight of the processing lines.

13. A CDMA transmitter as claimed in any previous claim wherein the first processing line is operable to process the line signal in response to a first processing parameter value; the second processing line is operable to process the line signal in response to a second processing parameter value; and the combining means comprises means for selecting between the line signal of the first and second processing line.

14. A CDMA transmitter as claimed in claim 13 wherein the first parameter corresponds to a first possible parameter value and the second parameter value corresponds to a second possible parameter value and the combining means is operable to select between the first and second processing line in response to a determined parameter value.

15. A CDMA transmitter as claimed in claim 14 wherein the first possible parameter value corresponds to an increased gain step; the second possible parameter value corresponds to a decreased gain step; and the combining means is operable to select between the first and second processing line in response to a power control command.

16. A CDMA transmitter as claimed in claim 14 wherein the first possible parameter value corresponds to a first transmit diversity parameter step; the second possible parameter value corresponds to a second transmit diversity: parameter step; and the combining means is operable to select between the first and second processing line in response to received transmit diversity information.

17. A CDMA transmitter as claimed in any of the claims 14 to 16 wherein the transmitter furthermore comprises means for storing the possible parameter value of the selected first and second processing line.

18. A CDMA transmitter as claimed in any previous claim wherein the combining means comprises summing means for summing the line signals.

19. A CDMA transmitter as claimed in any previous claim wherein the single data signal corresponds to a data signal for a physical channel of the CDMA communication system.

20. A CDMA transmitter as claimed in any previous claim wherein the single data signal corresponds to a base band data signal.

21. A CDMA transmitter as claimed in any previous claim wherein the at least two processing lines are parallel processing lines.

22. A CDMA transmitter as claimed in any previous claim wherein at least two processing lines are time multiplexed processing lines of a single processor.

23. A method of operation for a CDMA transmitter of a CDMA communication system, the CDMA transmitter including a data signal generator operable to generate data signals; a plurality of processing lines for processing a plurality of line signals; each processing line comprising at least one means for spreading the line signal of the processing line; combining means for combining line signals from the plurality of processing lines to a combined signal; transmit means operable to transmit the combined signal; the method comprising allocating least two processing lines of the plurality of processing lines to a single data signal to be processed as line signals of the at least two processing lines.

24. A computer program enabling the carrying out of a method according to claim 23.

25. A record carrier comprising a computer program as claimed in claim 24.