Patent Application
20100275107
The present invention relates to telecommunication system in general, specifically to method and arrangements enabling improved turbo decoding in such systems.
A turbo decoder improves soft values through an iterative procedure. Either it can be run with a fixed number of iterations, or adaptive stop criteria can be used to decide when the process has converged. In order to decode a frame e.g. a block of data, the turbo decoder performs several iterations. For the first iteration, it assumes that the probabilities of 0 and 1 are equal and based on the channel information bits it produces a soft decision output for each data bit. For other iterations, the decoder will use the soft output of the other decoder as a priori probability to perform the next iteration.
Even if a turbo decoder algorithm predicts an average number of iterations N for a frame or job many more may be needed in the worst case, e.g. during a fading dip. If a fixed number of iterations is used, this fixed number has to correspond to the worst case, but even if an adaptive stop algorithm is used the hardware has to be dimensioned for the number of iterations needed in the worst case.
Due to a finite number of resources, the allowed number of iterations must be limited. This number will in the following disclosure be denoted the maximum number of iterations.
Many criteria for adaptive stop are known in prior art. Either the quality of the soft values, or the convergence of hard decisions based on the soft values can be used.
In a WCDMA system with power control, the system tries to control the transmit power in such a way that a certain block error rate (BLER) target is obtained. If the turbo decoder is allowed to always converge, the BLER will be very small and the power control loop will not work. Using e.g. a maximum number of iterations ensures that, depending on the C/I, the BLER is not always equal to zero. If, however, for an adaptive stop algorithm the maximum number of iterations is set per user, it will be equal to the number of iterations needed in the worst case.
The average number of iterations needed for a turbo decoder job to converge depends on the coding scheme used and the requirement on the carrier over interference power ratio (C/I) expected by the receiver. In general, this can be expressed by saying that the average number of iterations depends on the service type.
Even with an adaptive stop criterion with an average number of iterations equal to N is used, many more iterations might be needed in the worst case. If a maximum number of iterations per user is used, then this maximum number will be equal to the number of iterations used in a fixed number of iterations algorithms, and the adaptive stop cannot be used to save resources.
To avoid momentary overload, on a digital signal processor (DSP) or any other type of platform implementing a receiver, more resources than corresponds to the average number of iteration per user need to be allocated. These resources will not be fully utilized. In certain cases, more than 100% more resources than is needed on average, have to be allocated. This has a severe impact on the capacity of the receiver.
Therefore, there is a need for methods and arrangements for improved Turbo decoding.
An object of the present invention is to provide an improved turbo decoder and turbo decoding arrangement.
A further object is to provide an improved turbo decoder and turbo decoding arrangement utilizing an adaptive stop algorithm.
This object is achieved in accordance with the attached claims.
Briefly, in a method for improved turbo decoding in a wireless communication system jointly allocating (S10) a predetermined maximum number of decoding iterations to a batch of received decoding jobs; and consecutively performing decoding iterations (S20) adaptively for each job in said batch until a convergence criteria is reached for each job in the batch, or until said predetermined maximum number of iterations for the batch is reached.
Advantages of the present invention comprise:
The invention, together with further objects and advantages thereof, may best be understood by referring to the following description taken together with the accompanying drawings, in which:
3GPP 3'd Generation Partnership Project
BLER BLock Error Rate
C/I Carrier over Interference power ratio
DSP Digital Signal Processor
SIR target Signal over Interference Ratio target
UMTS Universal Mobile Terrestrial System
WCDMA Wideband Code Division Multiple Access
The present invention will be discussed in the context of a general wireless communication network where a plurality of decoding jobs from multiple users are received consecutively at a decoder arrangement. The users and or jobs may have various associated service type requirements.
With reference to
In its most general aspect, the present invention enables that for a given service type a number of available turbo iterations are shared between several decoder jobs. This enables a significant reduction in the total maximum number of iterations needed, and less Turbo decoder resources need to be allocated.
Basically, a subgroup or batch of received turbo decoder jobs are allocated S10 a predetermined maximum number of decoding iterations. Subsequently, in consecutive reception order or some other predetermined order, the jobs in the subgroup or batch are subjected to an adaptive number of decoding iterations S20, in which a first job in the batch is allocated the predetermined maximum number of decoding iterations. The iterations are continued either until the job fulfills a convergence criterion or until the predetermined number of iterations is used up.
If the convergence criterion is fulfilled, and there are still remaining allocated iterations, the next consecutive job in the subgroup or batch is processed. If a job has not converged before the allocated number of decoding iterations have been consumed, the job is declared as having a block error. Similarly, any remaining job in the subgroup is also declared as having a block error. In other words, the remaining jobs are not allowed access to the decoding resource. Instead, the method according to the disclosure moves on to a next subgroup of decoding jobs and repeats the above described process.
Below, a further embodiment and description of the invention is disclosed. Consider a turbo decoder resource, and let
Q=(J1, J2, J3, J4, J5; . . . )
where Q is a master queue of turbo decoding jobs J for this resource. The queued jobs will be subjected to the decoding process in consecutive order, e.g. job J1 will be served first, job J2 next and so on. The order of the jobs within the master queue is determined by e.g. the order in which they arrive to the resource, timing constraints, and any associated priorities of the respective jobs. Also, other factors can be taken into consideration, which will be further described below.
Next, for a given service type Si form a sub-queue Qi in step S0
Q
i=(Jk
consisting of all jobs in the previously described master queue Q with the specific service type Si. Note that the service type can be distinguished by coding scheme, BLER target, required C/I for the BLER target, and the average number of iterations Ni needed for convergence at the given C/I. The skilled person realizes that also other factors determined the service type of each specific job.
The service queue or sub queue Qi is divided, in step S1, into a plurality of consecutive batches of Di number of jobs. A predetermined number Mi of decoding iterations per job is determined.
Note that a specific user might have zero, one or several jobs in the same batch. In the extreme case of only one user with service type Si, all jobs in Qi are from this user. Note that the different sub queues, corresponding to different service types, are preferably interleaved in time according to the ordering given by the master queue Q.
Subsequently, consider only one of the sub-queues Qi. All the jobs in one batch of length Di will share Mi·Di iterations, in a sense that no job in this batch will be stopped before it has converged, or a total of Mi·Di have been used for all jobs in the batch.
If all the Mi·Di iterations have been used before the end of the batch, the remaining turbo jobs are thrown away and the data is lost. If the end of the batch is reached before all the iterations are used, then the next batch is considered.
Note that at one specific time there might not be enough jobs to fill one batch of length Di. In this case the Mi·Di iterations will be shared with future jobs until Di jobs have been processed.
It can be shown, from simulations, that Mi≈Ni or even Mi≦Ni is sufficient, where Ni is an average number of iterations per job for a certain service type Si. In this way, only resources corresponding to at most Ni iterations per job need to be allocated.
The present invention can be implemented in an algorithm according to the following:
Consider only one of the sub queues Qi, the queue is divide into a plurality of batches each comprising Di number of jobs. Let Ki denote the remaining number of iterations for the jobs in a batch.
1. When the first job in a batch arrives to the Turbo decoding resource, then set Ki=Mi·Di.
2. When a job arrives to the resource, then set the maximum number of iterations for this job equal to Ki, and run the job. Let Li be the actual number of iterations used for the job.
3. After a job has run on the resource set Ki=Ki−Li. If Ki becomes equal to zero, then all the following jobs in the batch is declared to have a block error, and are not allowed access to the resource.
4. If a turbo decoder job does not converge in the allowed maximum number of iterations, then it is declared to have a block error.
One aspect that needs further attention is the fact that by decoding the jobs within a batch consecutively and potentially discarding the remaining jobs in a batch, some users may always end up in the end of a batch and thereby be discarded frequently.
According to a specific embodiment of the present invention, the jobs associated with different users are permutated at the decoding resource, thereby preventing the same user from always ending up at the end of a batch. A further potential remedy is to ensure that the length of the sub-queue is shorter than the number of users, or at least does not have a common factor with the number of users.
With reference to
The decoder arrangement 1 includes, in addition to any conventional and known units for reception and decoding received jobs, a unit 10 adapted for allocating a predetermined maximum number of decoder iterations to a batch of received decoder jobs, a unit 20 adapted for performing consecutive and adaptive decoding iterations to the jobs within each batch based on the allocated predetermined maximum number of decoder iterations. The unit 20 is further adapted to perform the decoder iterations for each job until a convergence criterion is met, or until the allocated number of decoder iterations for the batch have been consumed.
According to a further embodiment, also referring to
The arrangement according to the invention can be implemented as hardware in a hardware accelerator HWA which is in communication with a digital signal processor DSP. It is equally applicable that the arrangement is implemented as software in a DSP. Both embodiments are indicated by the dotted boxes in
Advantages of the present invention comprise:
It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE07/50979 | 12/12/2007 | WO | 00 | 6/11/2010 |
