WO2007084030A1

WO2007084030A1 - Method and arrangement for despreading in a communication system

Info

Publication number: WO2007084030A1
Application number: PCT/SE2006/000067
Authority: WO
Inventors: Gunnar Peters
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2006-01-17
Filing date: 2006-01-17
Publication date: 2007-07-26

Abstract

In a method for despreading receiving (SO) a spread signal which comprises an original signal which has been spread by a first spreading code and a first spreading factor, despreading (Sl) the spread signal with a second spreading code and a second spreading factor, also despreading (S2) the same spread signal with a third spreading code and a third spreading factor, and finally combining (S3) the two despread signals to provide the original signal.

Description

METHOD AND ARRANGEMENT FOR DESPREADING IN A COMMUNICATION SYSTEM

TECHNICAL FIELD The present invention relates to telecommunications in general, specifically to methods and arrangements for improved despreading of signals in such systems.

BACKGROUND In CDMA communication systems the transmitted data sequence is spread across the spectrum after being encoded by spreading codes, each of which is assigned uniquely to each user at a higher rate than the symbol rate of the information data. The spread high-speed data sequence is referred to as a chip and the rate at which the spread data varies is called the chip rate. The ratio of chip rate to symbol rate is called the spreading factor (SF). The destination receiver (uplink or downlink) uses the same spreading code as the one used for spreading at the transmission point to perform correlation detection (a process called despreading), in order to recover the transmitted data sequence. As signals received by other users carry different spreading codes, the signal power is reduced evenly to 1/SF.

The operation of despreading

In order to exemplify and illustrate the principles of spreading/ despreading a simple example scenario will be described below, with reference to Figure 1. Accordingly, a series of data symbols {t,.},. are spread with a spreading code

{^si}_hΛ t>^ef°^re transmission. The length N of the spreading code is called the spreading factor (SF) of the code. If T₀ is the basic time unit of the system, also called the chip time, and an exemplary chip pulse form p(t) is defined by:

Then the spreading of the data symbols {/,},. with the code {s,}^¹ results in a signal r(t) according to:

r{t) = f_jr_J -p{t-j-T_c) (2)

where the spread symbols rj are given by

r, = *, .*, (3)

and j = i-N + I is the unique decomposition of an integer j for which 0 <l < N .

The original data symbols {/.}. can similarly be reconstructed through the previously mentioned operation of despreading

In other words, the operation of despreading can be described as multiplying the spread signal with the same spreading code once more and accumulating over the symbol time. If the correct spreading code is used the result will be as shown in Figure 2. If the wrong spreading code or a misaligned spreading code is used the result will be as illustrated in Figure 3.

In this way, only the desired user signal will be detected in the receiver.

Another user, with different spreading codes, is suppressed by the operation of despreading. Even if the codes are not perfectly orthogonal, as above, the not desired users will be significantly suppressed by despreading. The process of despreading requires perfect synchronization between the spread symbols (^j. and the spreading code {s,}^¹ . The operations of spreading data before transmission and despreading after reception of the signal are used to suppress any interferer that has not been modulated within the correct spreading code.

If the chip time T_c is small, the operation of despreading will be extremely complex, and must therefore typically be implemented in hardware accelerators.

SUMMARY

An object of the present invention is to provide despreading in a communication system.

Another object is to enable existing hardware utilizing a higher spreading factor to despread signals with lower spreading factors.

A further object is to enable existing systems to implement Enhanced Uplink with SF=2.

Yet a further object is to enable despreading and Rake finger combining of multipath signals.

These and other objects are achieved in accordance with the attached claims.

Briefly, the present invention comprises receiving a spread signal which comprises an original signal which has been spread by a first spreading code and a first spreading factor, the spread signal is despread with a second spreading code and a second spreading factor, the same spread signal is also despread with a third spreading code and a third spreading factor, and finally the two despread signals are combined to provide the original signal. 006/000067

One specific aspect of the present invention enables despreading of signals spread with a lower spreading factor in a receiver that comprises despreaders supporting a higher spreading factor.

More specifically, the second and third spreading factors comprise whole multiples of the first spreading factor.

Another aspect of the present invention enables despreading and Rake finger combining of soft values in a multi path environment where the received signals are spread with a lower spreading factor than supported by the despreaders in the receiver.

Advantages of the present invention include: Enabling despreading signals despread with lower spreading factors than supported by the receiving despreader.

Enabling using existing hardware accelerators for new standards supporting lower spreading factors.

BRIEF DESCRIPTION OF THE DRAWINGS

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:

Fig. 1 is an illustration of the general principle of spreading; Fig. 2 is an illustration of the general principle of despreading with a correct spreading code;

Fig. 3 is an illustration of the general principle of despreading with an incorrect spreading code;

Fig. 4 is a flow diagram of an embodiment of a method of despreading according to the invention;

Fig. 5 is an illustration of despreading according to the invention;

Fig. 6 is an illustration of an arrangement according to the invention. FREQUENTLY USED ABBREVIATIONS

CDMA Code Division Multiple Access

SF Spreading Factor

UMTS Universal Mobile Terrestrial System DCH Dedicated channel

DPDCH Dedicated Physical Data Channel

DPCCH Dedicated Physical Control Channel

E-DPDCH Enhanced Dedicated Physical Data Channel

E-DPCCH Enhanced Dedicated Physical Control Channel DSP Digital Signal Processor

ASIC Application Specific Integrated Circuit.

DETAILED DESCRIPTION

The present invention will be described in but not limited to the context of a WCDMA communication system.

An example of a CDMA system is the 3 G standard UMTS. The UMTS uplink channel DCH consists of two data streams, namely the Dedicated Physical Control CHannel (DPCCH) using SF =256, and the Dedicated Physical Data CHannel (DPDCH) using SF = 4, 8, 16, 32, 64, 128 or 256.

At present, if a new system supporting a lower spreading factor e.g. Enhanced Uplink is implemented, signals received in despreading systems utilizing a higher spreading factor cannot be despread and subsequently decoded. This necessitates expensive upgrades of all receiving units in the system. Consequently, there is a need for methods and arrangements enabling receiving and despreading signals that have been spread with a lower spreading factor than is supported by current despreading systems.

Therefore, the invention provides a despreading unit supporting a certain minimum spreading factor which is adapted to despread received signals originating in a spreading unit utilizing an even lower spreading factor. Basically, as will be shown below, according to the invention, despreading of a data stream or signal with a code {s,}^¹ with SF=N can be reconstructed from the results of despreading the same data stream with two codes of SF = 2*N and combining the respective outputs in an inventive manner.

According to a basic exemplary embodiment of the present invention, despreading with {s,}^¹ can be obtained by allocating two hardware resources for SF = 2*N, transferring the outputs from these hardware resources to a processing unit e.g. a Digital Signal Processor (DSP) and linearly combine the outputs in the DSP to provide the original signal.

A basic embodiment of a method according to the invention will be described with reference to Fig. 4. A signal is received SO at a receiver; the signal comprises an original signal which is spread with a first spreading code SCl utilizing a first spreading factor SFl. The spread signal is provided as an input at a first despreading resource, where it is despread Sl with a second code SC2 utilizing a second spreading factor SF2. Subsequently, or concurrently, the signal is provided as an input at a second despreading resource, where the signal is despread S2 with a third code SC3 utilizing a third spreading factor SF3. Finally, the outputs from the two despreading resources are combined S3 to provide the original signal as despread utilizing the first spreading factor SFl.

According to a specific embodiment, the second and third spreading factors

SF2, SF3 are equal and correspond to a whole multiple of the first spreading factor SFl.

According to a further embodiment the three spreading factors SFl, SF2, SF3 are configured according to SF2=SF3=2*SF1, e.g. SF2=SF3=4, SF1=2. An illustrative example of despreading in a preferred embodiment according to the invention will be described below.

Consequently, it will be shown how to construct the results of despreading with a spreading code -? = {-?, Jj!^ from the results obtained by despreading with two other spreading codes si, s2, both with SF=2*N, where N preferably is a positive integer.

Let si =

be given by:

sl_k=s2_k=s,, for 0≤l<N, (5)

and

sl_k =s2_k = s,__N, for N≤l<2N. (6)

Let {u, }₍ be any sequence of data to be spread and transmitted and write the result of despreading this sequence with s , si and s2 as

N-I

'.=Σ ^SI ^{' U}ι N+l J (7)

/=0

2N-1 tl^∑sl, -u,_2N+l (8)

/=0

and

2W-1 a, = ∑s2, -U_12N+1 (9)

/=0

respectively.

Then, t is obtained from tland t2 according to. Λ, +t2, h, = (10)

where the subscript 2i denotes every even time index, and

^t _^ -Z≠- ^(ID

where the subscript 2i+l denotes every uneven time index.

An example of the signal processing during despreading according to the above embodiment is illustrated in Figure 5.

According to a further specific embodiment, the present invention is also applicable for situations of multi path reception and multi path radio channels. This will be described further below.

If a hardware accelerator combines the despreading with a Rake finger combiner, to produce soft values, then the same operation as described above produces the soft values of {J, }^¹ from the soft values of the two spreading codes with SF = 2* N.

Multi path reception and Rake linger combining

In a wireless communication system, the transmitted signal is scattered, which results in a multi path radio channel. At the receiver, this radio channel is approximated by a finite number of paths described by delays

{d_k I"^"1 and the complex the radio channels {c_k}"_k ^rPa'"^s~l of the paths.

In a so-called Rake receiver, the delays and radio channels have to be estimated, and from them so-called combing weights

are calculated. For Q ≤ k < nrPaths let t_{k i}be despread symbols, calculated as in equation (4) but with a version of the spreading code delayed a time d_k . In general, over sampling of the received signal has to be used to resolve the delays accurately. The soft values corresponding to the i'th data symbol are calculated as

nrPalhs-l so ^", (12)

A=O

Note that, in general, the radio channels {c_k}"_k ^rPa'^hs~l and delays {d_k)"_k ^rPaths'\ and therefore also the combining weights {w_k γ^^Pa"'^s~ι _j win vary with time.

Implementing the invention in a multipath environment can be performed according to the description below.

In a multipath combiner, the despread symbols corresponding to all paths are combined as follows.

nrPalhs-l

SO ≠₁ , = ∑ⁱV'*., (13) k-0

Accordingly, if an original signal is spread with a first spreading code with a first spreading factor that is lower than the spreading factors supported by the despreaders of the receiver, the soft values of the despread symbols need to be combined to provide the soft values of the signal as despread with the original spreading factor.

According to an embodiment of the invention, define soft Ii as the soft value for symbol i for all paths when the symbol is despread with a second spreading code and a second spreading factor. Further, define soft2ι as the soft value for symbol i for all paths when the symbol is despread with a third spreading code and a third spreading factor. In correspondence with the previous embodiments for despreading only the soft values are defined as follows.

nrPalhs-l softl, = ∑w_k -tl_h (14)

A=O

and

nrPaths-l softly ∑w_k -t2_Kι . (15) i=0

Then, assuming that the combining weights {w_k}"_k^"'^s ' do not vary over a data symbol time, the soft values of the despread multipath symbols are obtained according to

and

The assumption that the combining weights are constant over one symbol time is a valid approximation in a CDMA system.

With reference to Figure 6, an arrangement according to the present invention will be described.

The embodiment of the arrangement i.e. despreader comprises an I/O unit, at least two despreading units Dl, D2 and a combiner C. A spread signal is received and supplied as an input signal at the I/O unit. Subsequently, the same signal is supplied to each of the despreading units Dl, D2. The despreading units Dl and D2 are configured for despreading the received spread signal with a respective spreading code S2, S3 and a respective spreading factor SF2, SF3 and supplying the thus despread signals as inputs to the combiner C. The combiner C is adapted to combine the two input despread signals in order to supply the original signal as an output to the I/O unit.

Preferably, the two despreading units Dl, D2 are implemented in hardware accelerators. However, it is implied that the functionality can be implemented in any other suitable manner, such as software implemented and located elsewhere.

According to a preferred embodiment the two despreaders are configured with spreading factors SF2, SF3 that are equal and comprise whole multiples of the spreading factor SFl with which the original signal was spread.

Additionally, the despreading unit is configured for receiving a multi path radio channel and combining despread signals for all paths according to the invention.

The arrangement according to the invention can be implemented in a transceiver station, a receiver in a mobile station or in some other unit suitable for receiving and despreading signals.

Advantages of the present invention comprise:

Enabling despreading signals despread with lower spreading factors than supported by the receiving despreader.

Enabling using existing hardware accelerators for new standards supporting lower spreading factors. In this way despreading with SF =2 for DPDCH and SF =128 for DPCCH can be performed in existing hardware, and Enhanced Uplink can be supported in existing equipment.

The methods and arrangements of the present invention are applicable in any system configured according to any one of CDMA, WCDMA or CDMA2000.

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.

Claims

1. A method of despreading in a communication system, comprising receiving (SO) a spread signal, said spread signal being formed from an original signal spread by a first spreading code (SCl) and a first spreading factor (SFl), despreading (Sl) the signal in a first despreading resource utilizing a second spreading code (SC2) and a second spreading factor (SF2), despreading (S2) the signal in a second despreading resource utilizing a third spreading code (SC3) and a third spreading factor (SF3), and combining (S3) the output from said first resource and the output from said second resource to provide the original signal.

2. The method according to claim 1, wherein said second spreading factor (SF2) is a whole multiple of said first spreading factor (SFl).

3. The method according to claim 1 or claim 2, wherein said third spreading factor (SF3) is a whole multiple of said first spreading factor (SFl).

4. The method according to any of claims 1 to 3, wherein said second spreading factor (SF2) and said third spreading factor (SF3) are equal.

5. The method according to claim 2, wherein said second spreading factor (SF2) is equal to 2N and said first spreading factor (SFl) is equal to N, where N is a positive integer.

6. The method according to claim 5, wherein N is equal to 2.

7. The method according to claim 5, wherein N is larger than 2.

8. The method according to any of the previous claims, wherein said communication system belongs to one of CDMA, WCDMA; CDMA2000.

9. The method according to claim 1, wherein said second spreading code (SC2) is defined according to si = {sl_k f^^'1 , and said third spreading code (SC3) is defined according to s2 = {s2_k ^f₀ ^*1 , wherein said spreading codes are related according to:

sl_k = s2_k = s,, for 0 < / < N sl_k = -s2_k = s,__N , for N < / < 2N

where S₁ is a representation of said first spreading code (SC), and despreading (Sl, S2) a data sequence {w₍ }₍ with the respective second and third spreading code according to:

2N-1 i lN+l

/=0

2JV-1 t2, = ∑s2, - U_{1 2N+1} , axιd

I=O

combining (S3) the respective despread signals according to

_ tl, +t2, ^h' ^~ 2

where the subscript 2i denotes every even time index, and

Λ. -t2, ^l2M j

where the subscript 2i+l denotes every uneven time index.

10. The method according to claim 1, wherein said spread signal is a multi path signal comprising a plurality of signals, and comprising the further steps of combining (SI l) the despread signals for all paths from the first despreading resource into first soft values; and combining (S21) the despread signals for all paths from the second despreading resource into second soft values, and said combining S3 comprising combining said first and second soft values to provide the soft values for the original signal.

11. The method according to claim 10, wherein said second spreading code (SC2) is defined according to si = {sl_k ^₀ ^"1 , and said third spreading code (SC3) is defined according to s2 = {s2_k}™^~1 , wherein said spreading codes are related according to:

sl_k = _s2_k = s,, for 0 < / < N sl_k = s2_k = s,__N , for N ≤ l < 2N

where S₁ is a representation of said first spreading code (SC), and despreading (Sl, S2) a data sequence {«,.}. with the respective second and third spreading code according to:

2N-1

U₁ = ∑sl, - M *i_;-.2₂N+I

/=o

2N-1 t2_t = ∑>2, - κ,,

/=0

and combining (Sl 1) said first soft values according to

and combining (S 12) said second soft values according to nrPalhs-l

Σ A=O'

where Wk are the combining weights {w_k }"^^{ths l}

and combining (S3) said first and second soft values according to

where the subscript 2i denotes every even time index, and

_ soft\, -soft2,

where the subscript 2i+l denotes every uneven time index.

12. An arrangement for despreading a received signal in a communication system wherein: said arrangement is adapted for receiving (SO) a spread signal, said spread signal being formed from an original signal spread by a first spreading code (SCl) and a first spreading factor (SFl), and comprising: first means (Dl), adapted to despread said signal utilizing a second spreading code (SC2) and a second spreading factor (SF2); second means (D2), adapted to despread said signal utilizing a third spreading code (SC3) and a third spreading factor (SF3), and combining means (C) adapted to combine the output of said first and said second despreading means (Dl, D2), to provide the original signal.

13. The arrangement according to claim 12, wherein said second and said third spreading factor SF2, SF3 are whole multiples of said first spreading factor SFl.

14. The arrangement according to claim 12, wherein the spreading factors are represented by SFl=N and SF2=SF3=2*N, where N is a positive integer.

15. The arrangement according to any of claims 12-14, wherein said communication system belongs to one of CDMA, WCDMA; CDMA2000.

16. A receiver in a wireless communication system, wherein said receiver comprises an arrangement according to any of claims 12-15.

17. A communication system comprising an arrangement according to any of claims 12-15.