WO2002043269A2

WO2002043269A2 - Method and apparatus for improving channel estimates using space-time coding

Info

Publication number: WO2002043269A2
Application number: PCT/EP2001/013723
Authority: WO
Inventors: Bengt Lindoff
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2000-11-27
Filing date: 2001-11-26
Publication date: 2002-05-30
Also published as: WO2002043269A3; AU2002224891A1

Abstract

A method and apparatus for space-time coding transmitted signals is disclosed. The system has a first and second branch in the transmitter. Data is divided between the first and second branch. The data in the second branch is transformed to obtain time diversity. The data is multiplexed with training sequences in both the first and second branch. The signals in the first and second branch are mapped into complex valued symbols. The complex valued symbols in the first and second branch are offset by a first and a second modulation offset, wherein the second modulation offset is different from the first modulation offset. The signals in each branch are then filtered, modulated and transmitted. The use of different modulation offsets in the first and second branches permits even the same training sequence to be used in both branches, while still obtaining acceptable channel estimates for both channels.

Description

METHOD AND APPARATUS FOR IMPROVING CHANNEL ESTIMATES

USING SPACE-TIME CODING

BACKGROUND

The invention relates to data communication systems. In particular, the invention relates_^ to all areas where Space-Time Coding (STC) is used in wireless communication.

The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.

, One way to improve the throughput in a cellular system, such as the EDGE or GSM system, is to introduce space diversity. For example, space diversity can be introduced by using antenna arrays on either the transmitter or receiver or both. An antenna array on both the transmitter and receiver side will provide the best performance. Providing an antenna array on the mobile terminal side is impractical in current cellular systems, because the mobile terminals used are cost and size limited. However, on the base station side, antenna arrays can be implemented because the base stations do not have the same cost and size limitations as the mobile terminals.

One way to introduce both space and time diversity is to use Space-Time Coding. The basic concept of STC using two antennae is shown in Fig. 1. The concept is presented using a Time Division Multiple Access (TDMA) system. The TDMA system transmits information in bursts, however one skilled in the art will appreciated the invention is not limited to this system. Data bits (DB) are supplied to two branches 2 and 3 of the transmitter. In one of the branches, 3, a transformation (BM) 10 of the data bits is performed. For example, the transformation can be a delay of one or several bits. The transformation 10 is performed to obtain time diversity. In order to achieve coherent detection of the transmitted bits in the receiver it is necessary to transmit different and known data sequences, here called training sequences, on both transmitted paths, in each burst. The training sequences are needed in order to be able to estimate the radio channel for each path. Therefore, training sequences (TS A, TS B) are multiplexed with the data bits according to the burst structure in the system. Then, the bit streams are supplied to symbol maps (SM) 20 and 30 in each branch 2 and 3, respectively, that map the bits to complex valued symbols. The complex valued symbols, representing the information (for example, in EDGE, 8-PSK symbols), are offset with certain modulation offsets (er ) 22 and 32, between consecutive symbols. The same modulation offsets are used for both branches 2 and 3 as specified by the system. For example, in an EDGE system, the offset is θ =3π/8. The modulation offset is introduced in order to simplify the transmitter architecture. After the modulation offsets 22 and 32, the complex valued symbols are filtered with pulse shaping filters (p(t)), 24 and 34, and modulated on the carrier frequency using modulators (Mod) 26 and 36, respectively. The information is then transmitted through antennae 28 and 38 synchronously.

Referring to Fig. 2, a typical STC receiver 4 using a single antenna 41 is shown. The combined signal (i.e., the sum of the two transmitted sequences) is received through the antenna 41 and down converted and low pass filtered to a baseband signal in a front end receiver (Fe RX) 40. The baseband signal is then de-rotated (e ), by de-rotator 42, in order to compensate for the modulation offset introduced in the transmitter. The baseband signal is then fed to a synchronization unit (Sync.) 44 that correlates the training sequences A and B (TS A, TS B) with the received signal in order to find the synchronization position (i.e. , the position where each of the training sequences starts). The synchronization position together with the received signal is then fed to a channel estimation unit (Ch. Est.) 46 that estimates the two radio channels.

Mathematically, the baseband signal model used in the channel estimator can be written as:

L u

where H = [h_o,...,h_L]^T, G=[g_g,...,g_u]^τ, are the two radio channels, U =[u_t,...,u_t J^r, V_t=[v_t,...,v_t ^_lf]^τ represent the two training sequences, M is the model order in one channel, and L is the model order in the other channel. Since the transmitted sequences are transmitted synchronously and the two radio paths use different training sequences, the two radio channels can be estimated using standard Least Squares techniques, as is well known in the art, such as:

where θ = [H;G ^τ, φ_k = [U_k;V_k]^T, N_τs is the number of training symbols, and τ _c is the synchronization position.

The estimated radio channels, H and G , together with the received signal are then fed to the equalizer 48. The equalizer 48 uses the estimated radio channels to detect the transmitted symbols. In order to achieve good receiver performance, it is very important to have good channel estimates. Having good channel estimates implies that the training sequences need to have certain properties. The variance of the parameter estimate is proportional to the diagonal elements in the matrix given as:

(see, for example, L. Ljung, "System Identification - Theory for the User", published by Prentice Hall Inc., New Jersey, 1987). From a theoretical point of view, in order to have good parameter estimates, one should choose the input vector, φ_k, such that the matrix A is a diagonal matrix with identical diagonal elements or very close to such. However, in a TDMA system, such as EDGE, the training sequences used are optimized for good channel estimation performance with no STC. Therefore, it is assumed that only a single radio channel has to be estimated. Hence, in EDGE or GSM, the training sequence is chosen such that the A ' matrix can be written as:

where U_k represents one of the predefined training sequences (there are eight different training sequences predefined in the EDGE system). U_k is selected such that A ' is as close to a diagonal matrix as possible. By introducing STC, two channels have to be simultaneously estimated using two different training sequences, see equation (2). This gives rise to two problems. First, the estimation performance is not optimized for simultaneous estimation of two different channels, implying degraded receiver performance. Second, using two training sequences for each carrier may cause ambiguity problems when doing cell planning, since the reuse distance between the training sequences becomes shorter. This can especially be a problem in places where the cells are small and therefore performance limited by the co-channel interference, such as in cities. In such places, if an undesired base station is physically near the desired base station, transmitting on the same carrier frequency, and has the same training sequence as the desired base station, there is a possibility that the mobile terminal could synchronize with an undesired base station.

One approach to solving these problems involves establishing new training sequences that have good estimation properties. However, this may not be an option, at least in the case when STC is included in an existing system. The number of training sequences having the desired performance and fulfilling the constraints of the existing cellular system, such as the burst structure, the number of training symbols used and the like, may be limited or not exist at all.

Therefore, a method and system is needed that makes it possible to improve the channel estimation performance when introducing STC in an existing cellular system, without increasing the number of training sequences used in the system.

SUMMARY

It should be emphasized that the terms "comprises" and "comprising", when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

To facilitate an understanding of the invention, many aspects of the invention are described in terms of sequences of actions to be performed by elements of a computer system. It will be recognized that in each of the embodiments, the various actions could be performed by specialized circuits (e.g. , discrete logic gates interconnected to perform a specialized function), by program instructions being executed by one or more processors, or by a combination of both. Moreover, the invention can additionally be considered to be embodied entirely within any form of computer readable storage medium having stored therein an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein. Thus, the various aspects of the invention may be embodied in many different forms, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form of embodiment may be referred to herein as "logic configured to" perform a described action, or alternatively as "logic that" performs a described action. The current invention overcomes the prior art limitations by providing improved channel estimation performance when introducing STC in an existing system by supplying data comprising data bits and training sequences, into a first and a second branch of a transmitter. The data in the first and second branches is offset with a first and second modulation offset, respectively, wherein the second modulation offset is different from the first modulation offset. The data from each branch having the first and second modulation offset is then transmitted synchronously.

In another embodiment the current invention provides a transmitter using space-time coding comprising a first and a second branch that each have data comprising data bits and training sequences. Each branch has logic that offsets the data in the first and second branches with a first and a second modulation offset, respectively, wherein the second modulation offset is a function of the training sequences.

The above features and advantages of the invention will be more apparent and additional features and advantages of the invention will be appreciated from the following detailed description of the invention made with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be understood by reading the following- detailed description in conjunction with the drawings in which:

Fig. 1 shows a conventional transmitter architecture in an STC system; Fig. 2 shows a receiver in a mobile terminal;

Fig. 3 shows a general radio communication system in which the invention can be implemented;

Fig. 4 shows a transmitter in a STC system according to the invention;

Fig. 5 shows a receiver in a mobile terminal according to the invention; Fig. 6 shows a flowchart illustrating one STC method of the invention; and

Fig. 7 shows a flowchart illustrating another STC method of the invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular circuits, circuit components, techniques, and the like in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, and circuits are omitted so as not to obscure the description of the invention. The exemplary radio communication systems discussed herein are based upon the time division multiple access ("TDMA") protocol, in which communication between the base station and the mobile terminals is performed over a number of allocated time slots. However, those skilled in the art will appreciate that the concepts disclosed herein find use in other protocols, including, but not limited to, frequency division multiple access ("FDMA"), code division multiple access ("CDMA"), or some hybrid of any of the above protocols. Likewise, some of the exemplary embodiments provide illustrative examples relating to the GSM or D-AMPS type of systems; however, the techniques described herein are equally applicable to radio communication systems operating in accordance with any specification.

Prior to discussing exemplary embodiments according to the invention, Fig. 3 will now be described which illustrates a general radio communication system 100 in which the invention can be implemented. The radio communication system 100 includes a plurality of radio base stations 170a-n connected to a plurality of corresponding antennae 130a-n. The radio base stations 170a-n in conjunction with the antennae 130a-n communicate with a plurality of mobile terminals (e.g. terminals 120a, 120b, and 120m) within a plurality of cells UOa-n. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink.

The base stations are connected to a Mobile Switching Center ("MSC") 150. Among other tasks, the MSC coordinates the activities of the base station, such as during the handoff of a mobile terminal from one cell to another. The MSC 150, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a, 180b, and 180c. Both the mobile terminals 120a, 120b, and 120m, and the base stations 170a-n can incorporate Space-Time Coding system structures and techniques according to the invention.

The invention provides a method that enhances the channel estimation procedure in a Space-Time Coding system. By introducing a different modulation offset in each transmitter path, channel estimation performance can be optimized. The offset chosen depends on the training sequences used in the different transmitted paths. Preferably, both training sequences (TS A, TS B) are used to choose the appropriate offset. Therefore, better knowledge of the physical radio channel is achieved and improved receiver performance and throughput are obtained. Further, by an appropriate choice of the modulation offset it is possible to use the same training sequence for both branches and still have good channel estimation performance. Therefore, only the existing set of training sequences are needed in the STC system of the invention. In the following description of the invention, reference numbers will be maintained between drawings where the items referenced are the same. Therefore, reference numbers for a particular figure may not be discussed where the information provided would be redundant.

An embodiment of the invention is shown in Fig. 4. Data bits (DB) are fed into each of two branches 2 and 5. Prior to their being supplied to the second branch 5, a transformation (BM) 10 of the bits is performed. For example, a transformation can be a delay of one or several bits. Time diversity between the branches is achieved by the transformation. In order to have coherent detection of the transmitted bits in the receiver, it is necessary to transmit different and known sequences, training sequences, on both transmitted paths. Thus, training sequences (TS A, TS B) are multiplexed with the data bits according to the burst structure in the system thus creating data (i.e., information data and training sequence data) to be transmitted. The bit streams are fed to symbol maps (SM), 20 and 50, that map the bits to complex valued symbols. The complex valued symbols, representing the information, are then offset with a different modulation offset for each branch. In the first branch 2, a base modulation offset (er ) 22 is used. In the second branch 5, a second modulation offset (er ^ⁿ) 52 is used. The offset Δ that is added to the base modulation is a function of the training sequences used in the two branches 2 and 5. To generate the offset Δ, the training sequences (TS A, TS B) are also fed to a control unit (CU) 12. The control unit (CU) 12 uses a look-up table to find the appropriate offset Δ for given training sequences (TS A, TS B). The value of the offset Δ is chosen to optimize the channel estimation performance in the receiver. An example of a method to choose the offset Δ is provided below. After applying the modulation offsets 22 and 52, the complex valued symbols are filtered with pulse shaping filters (p(t)), 24 and 54, and modulated onto the carrier frequency using modulators (Mod) 26 and 56, respectively. The information is then transmitted through antennae 28 and 58 synchronously.

Referring to Fig. 5, an embodiment of a receiver 6 according to the invention is shown. The structure of the receiver 6 is similar to the one described in Fig. 2. However, the training sequence B (TS B) is further rotated by an amount e to compensate for the additional rotation applied in the transmitter (i.e., the offset Δ that is added to the base modulation offset). Thus, the baseband signal is fed to a synchronization unit (Sync.) 64, that correlates the training sequence A (TS A) and the rotated training sequence B with the baseband signal in order to find the synchronization position (i.e., the position where the training sequences start in the baseband signal). The synchronization unit 64 may correlate the training sequences in parallel or individually. For example, the synchronization unit 64 correlates training sequence A with the baseband signal to find a first synchronization position. Then, the synchronization unit 64 correlates the rotated training sequence B with the baseband signal to find a second synchronization position and determines a third synchronization position for the baseband signal using the first synchronization position and the second synchronization position. Alternatively, the synchronization unit 64 correlates training sequence A and rotated training sequence B with the baseband signal to find a synchronization position for the baseband signal. The synchronized baseband signal is then fed to a channel estimation unit (Ch. Est.) 66 that estimates the two radio channels. Training sequence A (TS A) and the rotated training sequence B are provided to the channel estimation unit 66 to improve the channel estimation. The channel estimation unit 66 also compensates for the additional offset Δ that was transmitted in the second radio channel. In the illustrated embodiment, two rotators 60 and 62 are provided to compensate for the additional offset Δ of the second modulation offset introduced in the second branch in the transmitter. It will be appreciated, however, that a single rotator could alternatively be used to provide the rotated second training sequence to both die synchronization unit 64 and the channel estimation unit.66. The estimated radio channels, H and G , together with the baseband signal are then fed to the equalizer 68. The equalizer 68 uses the estimated radio channels to detect the transmitted symbols using techniques known in the art. Thus, the receiver performance is improved by using the rotated training sequence B to determine characteristics of the received signal that in turn enable one to determine the receiver characteristics such as the synchronization position, the channel estimates, and the like.

Due to the offset Δ, the following channel model is relevant in the channel estimator, given as:

where H = [h_o,...,h_L]^T, G=[g_o,...,g_M]^T are the two radio channels and

U_t =[u_t,...,u_t J^r, V( =[e^iΔtv_t,...,e^{jΔ t 'M)}v_t ^f represent the two training

sequences.

The channel estimation can be performed by using a Least Squares algorithm to estimate the radio channels, such as:

where θ = [H;G]^T, φ_k(A)=[U_k;V_k(Δ)]^τ, N_τs is the number of training symbols,

and τ_sync is the synchronization position.

The value of Δ is selected such that the matrix given as:

is as close to a diagonal matrix as possible, thereby implying optimized estimation performance. The value of Δ can be pre-computed for each pair of training sequences (TS A, TS B). Therefore, the values for Δ can be stored in a look-up table for access by the receiver or transmitter.

Referring to Fig. 6, a flowchart illustrating one STC method of the invention is shown. The method starts by supplying data to both a first and second branch of a transmitter, in step 610. Preferably, the data contains both information data bits and training sequences. The data in the first branch is offset by a first modulation offset, in step 620. In step 630, the data in the second branch is offset by a second modulation offset. The second modulation offset is selected based on the training sequences used. Preferably, the first and second modulation offsets are performed in parallel. The data is then transmitted synchronously, in step 640.

A flowchart illustrating another STC method of the invention is shown in Fig. 7. The method begins by supplying data bits to both a first and second branch of a transmitter, in step 710. The data bits in the second branch are transformed to obtain time diversity, in step 712. The data bits are multiplexed with training sequences in both the first and second branches, in step 714. In step 716, the data bits and the training sequences in each branch are mapped into complex valued symbols. The complex valued symbols in each branch are offset with a first and a second modulation offset, respectively, in steps 720 and 730. The symbols in each branch are then filtered by pulse shaping filters, in step 732, modulated with a carrier frequency, in step 734, and transmitted synchronously, in step 740. .

The foregoing has described the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. For example, the value of Δ could be dynamically calculated based on which training sequences are being used as opposed to storing precalculated values in a look-up table.

In another example, instead of adding the Δ rotation to both the data bits and the training sequence that are transmitted from the second antenna, the Δ rotation could be added to the training sequence only. The synchronization and channel estimator in the receiver will operate essentially the same. However, since the data bits do not have the Δ rotation added, it will not have to be compensated for in the decoding process.

In still another example, additional antennae could be used. In such embodiments , each additional antenna has its own offset (e.g., Θ +Δ_{l 5}...,θ+ Δ_N_ 2). The offsets (Δs) are chosen based on the training sequence used. In yet another example, one skilled in the art will appreciate that multiple antennae can be used in the receiver. The receiver structure would remain substantially as shown in Fig. 5, with each antenna being connected through a front end receiver and related circuits to a channel estimation unit. The signals from the multiple antennae are then combined before the equalizer using well known techniques, such as maximum ratio combining and the like.

Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the invention as defined by the following claims.

Claims

CLAIMSWhat is claimed is:

1. A method of space-time coding comprising: supplying data, comprising data bits and training sequences, into each of a first and a second branch of a transmitter; offsetting the data in the first branch with a first modulation offset; offsetting the data in the second branch with a second modulation offset, wherein the second modulation offset is not equal to the first modulation offset; and transmitting the offset data from each branch synchronously.

2. The method of claim 1, further comprising: transforming the data bits in the second branch to form transformed data;

3. The method of claim 2, wherein transforming the data comprises: delaying the data to obtain time diversity.

4. The method of claim 1, wherein the second modulation offset is a function of the training sequences and is determined by adding a third offset to the first modulation offset.

5. The method of claim 1, wherein the training sequences comprise: a first training sequence associated with the first branch; and a second training sequence associated with a second branch, wherein the first and second training sequences are different.

6. The method of claim 1, wherein the training sequences further comprise: a first training sequence associated with the first branch; and a second training sequence associated with a second branch, wherein the first and second training sequences are the same.

7. The method of claim 1, wherein the second modulation offset is determined by looking up the offset in a lookup table containing pre-computed values for each pair of the training sequences.

8. The method of claim 1, wherein the second modulation offset is determined by calculating the offset dynamically for each pair of the training sequences.

9. A transmitter comprising: a first branch having logic that offsets the data in the first branch with a first modulation offset; and a second branch having logic that offsets the data in the second branch with a second modulation offset, wherein the second modulation is not equal to the first modulation offset .

10. The transmitter of claim 9, further comprising: logic that transforms data bits in the second branch to form transformed data.

11. The transmitter of claim 10, wherein the logic that transforms the data is a delay to obtain time diversity.

12. The transmitter of claim 9, wherein the transmitter is a base station transmitter.

13. • The transmitter of claim 9, wherein the second modulation offset is determined by adding a third offset to the first modulation offset.

14. The transmitter of claim 9, wherein the training sequences further comprise: a first training sequence associated with the first branch; and a second training sequence associated with a second branch, wherein the first and second training sequences are different.

15. The transmitter of claim 9, wherein the training sequences further comprise: a first training sequence associated with the first branch; and a second training sequence associated with a second branch, wherein the first and second training sequences are the same.

16. The transmitter of claim 9, further comprising: a control unit that determines an offset based on the training sequences used and supplies the offset to the logic that offsets by the second modulation offset.

17. The transmitter of claim 16, wherein the control unit comprises: a lookup table containing pre-computed values for each pair of the training sequences.

18. A method of receiving a space-time coded signal comprising: receiving data, comprising data bits and training sequences; and rotating.at least one of the training sequences by an offset thereby- generating at least one rotated training sequence, wherein the at least one rotated training sequence is used to determine characteristics of the received space-time coded signal.

19. The method of claim 18, wherein the offset is a function of the training sequences in the space-time coded signal.

20. The method of claim 18, wherein the training sequences are different from one another.

21. The method of claim 18, wherein the training sequences are the same as one another.

22. The method of claim 18, wherein the offset is determined by looking up the offset in a lookup table containing a pre-computed offset value for each combination of the training sequences.

23. The method of claim 18, wherein the offset is determined by calculating the offset dynamically for a given combination of the training sequences.

24. The method of claim 18, further comprising: down converting the space-time coded signal to a baseband signal; de-rotating the baseband signal by a modulation offset; correlating a first training sequence and a rotated second training sequence with the baseband signal to find a synchronization position for the baseband signal; generating estimated radio channels from the baseband signal using the synchronization position, the first training sequence, and the rotated second training sequence; and providing the estimated radio channels to an equalizer, wherein the equalizer uses the estimated radio channels to detect the data in the baseband signal.

25. The method of claim 18, further comprising: down converting the space-time coded signal to a baseband signal; de-rotating the baseband signal by a modulation offset; correlating a first training sequence with the baseband signal to find a first synchronization position; correlating a rotated second training sequence with the baseband signal to find a second synchronization position; determining a third synchronization position for the baseband signal using the first synchronization position and the second synchronization position; generating estimated radio channels from the baseband signal using the third synchronization position, the first training sequence, -and the rotated second training sequence; and providing the estimated radio channels to an equalizer, wherein the equalizer uses the estimated radio channels to detect the data in the baseband signal.

26. The method of claim 18, wherein the characteristics of the received space-time coded signal space-time coded signal include a synchronization position.

27. The method of claim 18, wherein the characteristics of the received space-time coded signal is used to estimate the channels.

28. A receiver comprising: logic that receives a space-time coded signal containing data, wherein the data comprises data bits and training sequences; and logic that rotates at least one of the training sequences by an offset thereby generating at least one rotated training sequence, wherein the at least one rotated training sequence is used to determine characteristics of the received space-time coded signal.

29. The receiver of claim 28, wherein the offset is a function of the training sequences in the space-time coded signal.

30. The receiver of claim 28, wherein the training sequences are different from one another.

31. The receiver of claim 28, wherein the training sequences are the same, as one another.

32. The receiver of claim 28, further comprising: a lookup table containing a pre-computed offset value for each combination of the training sequences, wherein the offset is determined by looking up the offset in the lookup table.

33. The receiver of claim 28, further comprising: logic that calculates the offset dynamically for a given combination of the training sequences.

34. The receiver of claim 28, further comprising: a front end receiver that down converts the space-time coded signal to a baseband signal; a de-rotator that de-rotates the baseband signal by a modulation offset; a synchronization unit that correlates a first training and a rotated second training sequence with the baseband signal to find a synchronization position for the baseband signal; a channel estimation unit that generates estimated radio channels from the baseband signal using the synchronization position, the first training sequence, and the rotated second training sequence; and an equalizer that receives the estimated radio channels, wherein the equalizer uses the estimated radio channels to detect the data in the baseband signal.

35. The receiver of claim 28, further comprising: a front end receiver that down converts the space-time coded signal to a baseband signal; a de-rotator that de-rotates the baseband signal by a modulation offset; a synchronization unit that correlates a first training sequence with the baseband signal to find a first synchronization position, correlates a rotated second training sequence with the baseband signal to find a second synchronization position and determines a third synchronization position for the baseband signal using the first synchronization position and the second synchronization position; a channel estimation unit that generates estimated radio channels from the baseband signal using the third synchronization position, the first training sequence, and the rotated second training sequence; and an equalizer that receives the estimated radio channels, wherein the equalizer uses the estimated radio channels to detect the data in the baseband signal.

36. The receiver of claim 28, wherein the receiver is a base station receiver.

37. The receiver of claim 28, wherein the receiver is a mobile station receiver.

38. The receiver of claim 28, wherein the characteristics of the received space-time coded signal include a synchronization position.

39. The receiver of claim 28, wherein the characteristics of the received space-time coded signal is used to estimate the channels.