HK1108231A - Method and apparatus for estimating channelization codes in a wireless transmit/receive unit - Google Patents

Description

Method and apparatus for estimating channelization codes in a wireless transmit/receive unit

Technical Field

The present invention relates to code detection in a wireless communication system. More particularly, the present invention is a method and apparatus for estimating channelization codes using Blind Code Detection (BCD) in a wireless transmit/receive unit (WTRU).

Background

A time division synchronous code division multiple access (TD-SCDMA) (TSM) system for mobile is a narrowband time division duplex/code division multiple access (TDD/CDMA) system. With TSM systems, it is preferable to use a multi-user detector (MUD) as a receiver to overcome small Spreading Factors (SF) and high interference to provide high data rates.

Ideally, the MUD requires information related to the transmitted channelization code, a midamble (midamble) related to the channelization code, and an SF in each slot. This type of information is typically available in the TSM uplink. In contrast, in the TSM downlink, each WTRU knows only a few of its own possible channelization codes, associated midambles, and their SFs in each slot. The WTRUs do not know the specific active transmitted channelization codes, their specific SFs, or their specific associated midambles in each slot. Also, the WTRU is not aware of any information about other WTRUs. The result of this ambiguity is that the performance of the MUD is severely degraded.

Therefore, there is a need for a method and apparatus by which a WTRU may estimate the channelization code transmitted by itself and the channelization codes transmitted by the intended WTRU and other WTRUs.

Disclosure of Invention

The present invention is a method and apparatus for estimating channelization codes in a WTRU. The WTRU receives the communication bursts and detects a midamble in each received communication burst. A candidate code list is generated based on the detected midamble. The candidate code list includes channelization codes intended for the intended WTRU and other WTRUs. These codes, which are considered as candidate codes for detection, are a function of the midamble allocation scheme. Active channelization codes among the codes in the candidate list are identified and forwarded to the MUD.

The present invention addresses the ambiguity in the downlink of the TSM. Orthogonal Variable Spreading Factor (OVSF) codes are used to maintain orthogonality between codes. Thus, even when different SFs are used between codes, the MUD may preserve interference power due to the nature of OVSF codes, and the performance of the channelization codes for a particular WTRU itself is preserved with the SF ambiguity of the channelization codes of other WTRUs. Furthermore, since no data for the channelization codes of other WTRUs are used in the symbol processing after the MUD, the MUD only needs the active codes for the other WTRUs to estimate the intra-cell interference from the other WTRUs, and does not need the data of the other WTRUs.

Drawings

The invention may be understood in more detail by the following description, given by way of example and understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a TDD/CDMA communication system;

FIG. 2 is a diagram of communication pulses;

FIG. 3 is a flow chart of a method for estimating an effective transmitted channelization code in accordance with the present invention; and

fig. 4 is a block diagram of an apparatus for estimating an effective transmitted channelization code in accordance with the present invention.

Detailed Description

Hereinafter, the term "WTRU" includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment.

FIG. 1 is a diagram of a TDD/CDMA communication system. The system 100 includes a plurality of base stations 102. Each base station 102 serves at least one cell 104 associated with each base station 102. The WTRUs 106 communicate with the base station 102 in the cell 104 in which each WTRU 106 is located.

Data is transmitted using one or more communication bursts. Each communication burst carries data in a single time slot (i.e., a single resource unit) that utilizes a single channelization code. As shown in fig. 2, a typical communication burst 200 has a midamble 204, a guard period (guard period)208, and two data bursts 202 and 206. The midamble 204 separates two data pulses 202 and 206. The guard period 208 separates different communication pulses to allow for differences in arrival times of pulses transmitted from different transmitters. The midamble 204 contains a midamble used in estimating the channel response between the receiver and the transmitter.

According to the invention, the receiver utilizes MUD. Since the operation of MUD is well known to those of ordinary skill in the art, the details thereof are not described in detail herein. In general, the MUD processes the baseband signal and recovers all the communication data. To recover the data, the MUD needs to know all of the channelization codes used for the transmit burst, not only the transmitted channelization codes for the intended WTRU, but also the transmitted channelization codes for other WTRUs.

The Blind Code Detection (BCD) algorithm according to the present invention estimates not only the effective transmitted channelization code for the intended WTRU but also the channelization codes of other WTRUs based on the intended WTRU's own possible channelization code and the detected midamble. For the detection of the WTRU's own code, the possibly assigned code for each transmission channel (TrCH) is either retained in the candidate code list or rejected from the candidate code list, depending on the full Discontinuous Transmission (DTX) state. Once the code of the destination WTRU has been detected, the codes of the other WTRUs are detected using a threshold based on the energy of the detected code of the destination WTRU and other rules based on the conditions of the detected code of the destination WTRU.

The present invention addresses the SF ambiguity in the downlink of TSMs. The destination WTRU may obtain its own SF from higher layer information as described in the third generation partnership project (3GPP) standards. However, the coded SF of other WTRUs is not known. The OVSF codes used in TSM can maintain orthogonality between codes due to their own characteristics even when different SFs are used between codes. It allows the intended coding of the intended WTRU to maintain a high signal to interference and noise ratio (SINR) regardless of the ambiguity of the SF. In this way, the MUD is able to maintain the SINR for each expected code of the destination WTRU, which improves the performance of the MUD operating on the code of the destination WTRU. Also, since the data of other WTRUs are not used in the symbol processing after the MUD, the MUD requires only the active coding of the other WTRUs to estimate the intra-cell interference from the other WTRUs. The performance of the MUD will no longer be affected by the ambiguity. Thus, according to the present invention, the coded SF of the other WTRUs is the same as the coded SF of the destination WTRU, and it may be assumed that only one SF is transmitted in the same slot.

Fig. 3 is a flow diagram of a method 300 for estimating the effective transmitted channelization code in accordance with the present invention. The receiver receives the communication bursts and detects a midamble in the received communication bursts (step 302).

The receiver generates a candidate code list based on the detected midamble (step 304). The candidate code list is a list of channelization codes and associated parameters that may have been received in a time slot. The list of candidate codes for the destination WTRU is determined based on the midamble allocation scheme of the time slot, the detected midamble shift, and information on the known number of transmitted codes derived from higher layer information. The candidate code lists for other WTRUs are generated based on the midamble allocation scheme of the slot and the detected midamble shifts.

There are currently three schemes available for midamble allocation: 1) allocating a default intermediate code; 2) distributing common intermediate codes; and 3) WTRU-specific midamble allocation. The code considered as a candidate code for detection is a function of the midamble allocation scheme. In a default midamble allocation scheme, codes associated with the detected midambles are included as candidate codes, including codes for the intended WTRU and other WTRUs. In the common midamble allocation scheme, all codes with the detected SF are included as candidate codes, including codes for the intended WTRU and other WTRUs. In the WTRU-specific midamble allocation scheme, only codes for the intended WTRU are included in the candidate code list.

In the first scheme, the default midamble allocation scheme, each midamble indicates a set of channelization codes that may have been transmitted. Once the midamble is detected, the channelization code or set of channelization codes may be included in the candidate code list. The common channel code is considered to be the same as the dedicated channel code for the intended WTRU, except that if the common channel code is detected to have its own midamble, the common channel code transmitted as a beacon is identified as valid, regardless of its own code energy, as will be described in detail below. In a primary common control physical channel (P-CCPCH) slot, if respective midambles of first and second P-CCPCH code sets are detected, the first and second P-CCPCH code sets are included in a candidate code list and identified as not rejected by a code detection function.

During call setup, the WTRU acquires information about the channelization code and the allocation of timeslots. Thus, the WTRU has a list (i.e., a list of possible allocations) that contains the codes allocated to the WTRU. Thus, the WTRU searches the possible allocation list for each candidate code after all codes have been added to the candidate code list.

Candidate codes found in the WTRU's possible allocation list are retained in the candidate code list by adding their TrCH numbers to the candidate code list. Candidate codes not found in the possible allocation list are removed from the candidate code list. This may be performed by identifying the TrCH as zero, which indicates that the code is assigned to other WTRUs.

In the second scheme, the common midamble allocation scheme, only one midamble shift is transmitted, the common midamble allocation scheme is applied to only one non-P-CCPCH slot. The midamble shift indicates the number of channelization codes transmitted in a time slot. If no common midamble shifts are detected, no codes are inserted into the candidate code list. If a common midamble shift element is detected, an appropriate number of codes are added to the candidate code list based on the detected SF in the current time slot.

In the time slot of the downlink, there are various options for SF. Thus, the SF of one code in the current slot in the intended WTRU allocation list is checked and used to populate the candidate code list with the appropriate number of codes based on the detected SF. For example, in a non-beacon slot, there may be one SF-1 code for high data rate transmissions or eight (8) SF-8 codes and sixteen (16) SF-16 codes for general transmissions in the slot. The SF of one code in the current slot in the WTRU's possible code allocation list is checked and used to populate the candidate code list with one SF-1 code or eight (8) SF-8 codes and sixteen (16) SF-16 codes, each associated to the detected common midamble shift.

After all codes have been added to the candidate code list, the WTRU's possible allocation list is searched for each candidate code. Candidate codes found in the WTRU's possible allocation list are retained in the candidate code list by adding their TrCH numbers to the candidate code list. Candidate codes not found in the WTRU's possible allocation list are removed from the candidate list, for example by identifying TrCH as zero, indicating that the code is allocated to other WTRUs.

In a third scheme, a WTRU-specific midamble allocation scheme, the WTRU has no knowledge of the relation between midambles and codes for codes that may be allocated to other WTRUs. Therefore, it is impractical to detect channelization codes of other WTRUs. Thus, in a WTRU-specific midamble allocation scheme, for each detected midamble shift, the WTRU simply searches its possible allocation list and adds those codes associated with the detected midamble to the candidate code list. Channelization codes for other WTRUs are not added to the candidate code list.

The transmission of P-CCPCH with Space Code Transmit Diversity (SCTD) requires special handling in generating the candidate code list. If only one of the two P-CCPCH midambles is detected, then it may be that only one is transmitted or that both are transmitted but only one is detected. In this case, the detected midamble is added to the candidate code list as codes 0 and 1, or codes 2 and 3, depending on whether the detected midamble code is k-1 or k-2, where k represents the midamble shift. If two P-CCPCH midambles are detected, then the four P-CCPCH codes are essentially split into one another in the MUD, and are therefore treated as two codes. In this case, only two codes, i.e., codes 0 and 1, k being 1, should be added to the candidate code list as common channel codes.

The receiver then performs code power estimation for each code in the candidate code list (step 306). The code power estimation is performed based on matched filtering of the received signal for the data symbols. The power of the kth channelization code is estimated by equation (1):

img id="idf0001" file="A20058003782200111.GIF" wi="257" he="43" img-content="drawing" img-format="GIF"/formula (1)

Odd and even symbols s_o ^(k)(n) and s_e ^(k)(n) estimating from the corresponding odd and even channels, respectivelyh _o ^(k)Andh _e ^(k)the resulting symbols are estimated for the odd and even samples. For fast BCD, these symbols can be estimated by a Whitening Matched Filter (WMF) as shown in equation (2):

img id="idf0002" file="A20058003782200121.GIF" wi="78" he="19" img-content="drawing" img-format="GIF"/formula (2)

Whereins _iRepresenting the estimated odd or even symbols,r _irepresenting either the received odd or even symbols. System matrix A_iEach column of (a) consists of a convolution of the channel response and the spreading code, as shown in equation (3):

img id="idf0003" file="A20058003782200122.GIF" wi="111" he="21" img-content="drawing" img-format="GIF"/formula (3)

WhereinAndis the spreading code and associated channel response. P_kIs thatThe sum of squares of the elements (c). In the case of oversampling, both odd and even powers are added.

For the case that SCTD has been detected, for K_cell＝8(K_cellRepresenting the maximum number of midamble shifts), using two midambles m⁽¹⁾And m⁽²⁾Diversity combining of one data sequence transmitted on two antennas is obtained using the following equation (4) and equation (5), respectively:

img id="idf0007" file="A20058003782200126.GIF" wi="111" he="23" img-content="drawing" img-format="GIF"/and formula (4)

img id="idf0008" file="A20058003782200127.GIF" wi="110" he="17" img-content="drawing" img-format="GIF"/Formula (5)

And then ignored or eliminatedAnd

the receiver then identifies valid channelization codes to be forwarded to the MUD from the codes in the candidate code list (step 308). The identification of valid channelization codes includes identification of valid codes for the intended WTRU and valid codes for other WTRUs. Valid codes for other WTRUs are identified only in the default and common midamble allocation schemes.

The purpose of valid code identification by the intended WTRU is to avoid falsely rejecting valid codes by the intended WTRU (which would result in significant data loss) and falsely declaring that in fact a non-valid code is valid (which has less impact on the block error rate (BLER)). Thus, unless the current slot is in the full DTX state, all candidate codes for the WTRU are accepted and forwarded to the MUD as valid codes. This simplifies the detection procedure.

The purpose of efficient code identification for other WTRUs is to identify the strong codes of other WTRUs and to avoid having the MUD demodulate the erroneous or weak codes of other WTRUs. Strong coding of other WTRUs is a significant source of intra-cell interference. The weak codes of other WTRUs do not interfere significantly or degrade performance. Thus, based on the code power estimate obtained in step 306, the effective channelization codes for other WTRUs are identified.

After identifying the active code of the destination WTRU, the active codes of other WTRUs are identified with a threshold based on the energy of the destination WTRU code and the properties of the TrCH of the destination WTRU. Simply, all codes (in a common midamble allocation scheme) or some codes not allocated to the intended WTRU (in a default midamble allocation scheme) may be identified as valid codes for other WTRUs. Preferably, only codes exceeding the threshold are identified as valid channelization codes. The threshold is determined by the maximum energy among the energies of all codes of the destination WTRU in the candidate code list for the destination WTRU.

Alternatively, the average energy of the destination WTRU codes may also be used as the threshold. By using the maximum energy for the intended WTRU code, the number of possible erroneous codes for other WTRUs is reduced, but with the negative effect of eliminating the codes for other WTRUs.

Weak codes may be rejected if there is more than the maximum number of codes for a time slot in the candidate code list. The number of codes is reduced to the maximum number of codes for the slot by eliminating weak codes of other WTRUs one by one until the number of codes is equal to or below the maximum.

After valid code detection, all relevant information for the detected code (i.e., associated midamble shift bits, SF, and total number of codes) is forwarded to the MUD. The MUD uses this information for demodulation.

Fig. 4 is a block diagram of an apparatus 400 for estimating an effective transmitted channelization code in accordance with the present invention. The apparatus 400 includes a channel estimator 402, a midamble detector 303, a candidate code list generator 406, a code detector 408, and a MUD 410. Although these component devices are shown as separate devices, one or more devices may be incorporated as part of a MUD or other device.

At the receiver, the transmitted signal is received through an antenna (not shown), and the received signal is converted to a baseband signal. The channel estimator 402 estimates a channel response, and the estimated channel response is input to the midamble detector 404 and the MUD 410.

After channel estimation, the midamble detector 404 detects the transmitted midamble. Based on the detected midamble, the candidate code list generator 406 generates a candidate code list based on the detected midamble according to a midamble allocation scheme. The code detector 408 identifies valid codes for the intended WTRU and other WTRUs. Identifying valid codes for the intended WTRU based on the DTX status. The active codes of other WTRUs are preferably identified by scaling the code power of each code in the candidate code list against a reference predetermined code power threshold. The code detector 408 measures the energy of each code in the candidate code list so that the code detector 408 identifies active codes of other WTRUs that exceed a predetermined threshold. The code detector 408 provides the channelization code set, SF, and channel response offset to the MUD 410 for use in the current time slot.

Although the elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.

Claims (26)

1. In a wireless communication system including a plurality of wireless transmit/receive units (WTRUs), a method for estimating channelization codes of a destination WTRU and other WTRUs without knowing Spreading Factor (SF) of the other WTRUs, the method comprising:

detecting a midamble in the received communication burst;

generating a candidate code list based on the detected midamble, the candidate code list including channelization codes for the intended WTRU and other WTRUs;

identifying valid channelization codes from the codes in the candidate list; and

the identified valid channelization codes are forwarded to the multi-user detector MUD.

2. The method of claim 1 wherein the midamble is mapped to WTRUs in accordance with a default midamble allocation scheme, wherein the detected midamble indicates a channelization code used for transmission of the communication burst.

3. The method of claim 2, wherein the detected midamble is used for a primary common control physical channel (P-CCPCH).

4. The method of claim 3 wherein the detected P-CCPCH code is marked as not rejected regardless of the code power of the P-CCPCH code.

5. The method of claim 3 wherein two P-CCPCH midambles are used for spatial code transmit diversity and wherein only the detected P-CCPCH midambles are listed in the candidate list when only one P-CCPCH midamble is detected and the first P-CCPCH is listed in the candidate list when two P-CCPCH midambles are detected.

6. The method of claim 1 wherein the midambles are mapped to WTRUs in accordance with a common midamble allocation scheme, wherein each midamble indicates the number of channelization codes utilized in the time slot.

7. The method of claim 6, wherein the SF for the current slot is determined, and wherein the candidate code list is generated based on the detected SF.

8. The method of claim 1 wherein the midamble is allocated to WTRUs in accordance with a WTRU-specific midamble allocation scheme.

9. The method of claim 1 wherein all codes in the candidate code list for the destination WTRU are forwarded to the MUD unless the current time slot is in a full discontinuous transmission, DTX, state.

10. The method of claim 1 wherein the active channelization codes for other WTRUs are identified based on a maximum code energy among the codes for the intended WTRU.

11. The method of claim 1 wherein the effective channelization codes for other WTRUs are identified based on an average code energy of the code for the intended WTRU.

12. The method of claim 1 wherein if the number of codes in the candidate code list exceeds a predetermined maximum, then eliminating the weakest codes for other WTRUs from the candidate code list until the number of codes in the candidate code list is below the maximum.

13. The method of claim 1, wherein the channelization code is an orthogonal variable spreading factor, OVSF, code.

14. An apparatus for estimating channelization codes of a destination wireless transmit/receive unit (WTRU) and other WTRUs in a wireless communication system, wherein Spreading Factors (SFs) for the other WTRUs are unknown to the destination WTRU, the apparatus comprising:

a channel estimator for estimating a channel response;

a midamble detector for detecting a midamble in the received communication burst;

a multi-user detector, MUD;

a candidate code list generator for generating a candidate code list based on the detected midamble, the candidate code list including channelization codes for the intended WTRU and other WTRUs; and

a code detector to identify an effective channelization code for the destination WTRU from the codes in the candidate list and forward the identified effective channelization code for the destination WTRU to the MUD.

15. The apparatus of claim 14 wherein the midamble is mapped to WTRUs in accordance with a default midamble allocation scheme, wherein the detected midamble indicates a channelization code used for transmission of the communication burst.

16. The apparatus of claim 15, wherein the detected midamble is used for a primary common control physical channel (P-CCPCH).

17. The apparatus of claim 16 wherein the detected P-CCPCH code is marked as not rejected regardless of a code power of the P-CCPCH code.

18. The apparatus of claim 16 wherein two P-CCPCH midambles are used for spatial code transmit diversity and only detected P-CCPCH midambles are listed in a candidate list when only one P-CCPCH midamble is detected

19. The apparatus of claim 14 wherein the midambles are mapped to WTRUs in accordance with a common midamble allocation scheme, wherein each midamble indicates the number of channelization codes transmitted in the time slot.

20. The apparatus of claim 19, wherein an SF of a current time slot is determined, and wherein a candidate code list is generated based on the detected SF.

21. The apparatus of claim 14 wherein the midamble is allocated to WTRUs in accordance with a WTRU-specific midamble allocation scheme.

22. The apparatus of claim 14 wherein all codes in the candidate code list for the destination WTRU are forwarded to the MUD unless the current slot is in a full discontinuous transmission, DTX, state.

23. The apparatus of claim 14 wherein the effective channelization codes for other WTRUs are identified based on a maximum code energy among the codes for the intended WTRU.

24. The apparatus of claim 14 wherein the effective channelization codes for other WTRUs are identified based on an average code energy of the code for the intended WTRU.

25. The apparatus of claim 14 wherein if the number of codes in the candidate code list exceeds a predetermined maximum, the weakest codes for other WTRUs are eliminated from the candidate code list until the number of codes in the candidate code list is below the maximum.

26. The apparatus of claim 14, wherein the channelization code is an orthogonal variable spreading factor, OVSF, code.