WO2009034221A1

WO2009034221A1 - Enhanced channel estimation for fast moving terminals

Info

Publication number: WO2009034221A1
Application number: PCT/FI2007/050490
Authority: WO
Inventors: Juha Ylitalo; Jukka-Pekka Nuutinen; Aaron Byman
Original assignee: Elektrobit Corporation
Priority date: 2007-09-14
Filing date: 2007-09-14
Publication date: 2009-03-19

Abstract

The invention relates to a channel estimation method utilized in a radio access system, for example WiMAX system, for fast moving subscriber station. The invention relates also to a radio access subscriber station and a computer program utilized in the radio access subscriber station.

Description

Enhanced channel estimation for fast moving terminals

Technical field of the invention

The invention relates to a method for estimating and predicting a wireless trans- mission channel in a radio access system. More specifically the invention relates to a method for estimating and predicting a wireless transmission channel in a WiMAX network (Worldwide interoperability of microwave access). The invention also relates to a WiMAX subscriber station and a computer program fulfilling the estimation method.

Background of the invention

The emerging mobile WiMAX standard IEEE 802.16e is aimed to support IP traffic (Internet Protocol) over the air to the mobile devices. To support the mobility in changing radio channel conditions, one essential feature is to perform reliable channel estimation at the receiver. In principle we have to have at least two sam- pies per coherence time of the channel for making a reliable and continuous estimate for the channel. In fast moving conditions, the coherence time is very short, and therefore the conventional pilot-based channel estimation may not be adequate in all cases.

A channel response from a base station antenna to a mobile station antenna can be described with a single delay tap if the bandwidth of the desired burst is sufficiently small. This criterion can be arranged by the base station when allocating sub-channels in frequency domain to a burst.

Fig. 1 a depicts a simplified example depicting downlink (DL) and uplink (UL) sub- frames of a WiMAX system using TDD (Time Division Duplex). It shows also an exemplary fading channel response 2a for a fast moving terminal and a channel response 1a, 3a for a slowly moving terminal. In Fig. 1 a one TDD downlink frame is depicted. The downlink subframe starts with a preamble symbol. This is often also called the frame preamble or the frame start preamble. The preamble is followed by PUSC zones (Partial Usage of Subchannels). In the depicted example PUSC includes FCH (Frame Control Header) and DL-MAP (DownLink MAP) which are mandatory in a WiMAX frame for carrying control information and which occupy at least 2 consecutive symbols. They can be followed by several user bursts #1 , #2,..., #n. The exemplary frame comprises three downlink bursts, references 1 , 2 and 3.

The depicted downlink subframe ends to a TTG period (Transmit/receive Transition Gap). After the TTG period an uplink subframe 4 starts. The uplink subframe 4 ends to an RTG period (Receive/transmit Transition Gap) which is not shown in Fig. 1 a. After the RTG period a new downlink subframe begins.

In the example of Fig. 1a subscriber stations which receive downlink bursts 1 and 3 have a long enough coherence time (references 1a and 3a) so that a channel estimate can be made from the received preamble for the whole downlink sub- frame.

Let us suppose that a subscriber station for receiving downlink burst 2 is moving fast. Then the behavior of the radio channel during a downlink subframe can be for example as depicted by reference 2a. In that case the coherence time of the channel is short compared to the downlink subframe time. It is obvious that a channel estimation based only on the preamble does not apply for the desired downlink burst #2 of Fig. 1 a.

In a WiMAX system the shortest delay T between the preamble and a burst to the fast moving mobile can be in the range of 2-3 symbol durations due to the fact that the FCH and DL/UL-MAP information requires two symbols after the preamble. This corresponds roughly to 300-500 μs.

On the other hand the coherence time of the radio channel is proportional to the subscriber terminal velocity. In the frequency domain the terminal velocity transfers to a Doppler shift. The channel coherence time is defined approximately as:

tc ~ 1 / (Doppler spread) = 1 / (2^* v/λ) (1 ),

where v is the subscriber terminal speed and λ is the wavelength of the carrier frequency. If the carrier frequency is 2.6 GHz (λ = 0.115m) the coherence times for mobile speeds of 300 km/h (= 83.3 m/s) and 400 km/h (= 111.1 m/s) are:

- 300 km/h: t_c = 1 / (2^* v / λ) = 1/ (2^* 83.3 / 0.115) s = 692 μs - 400 km/h: t_c = 1 / (2^* v / λ) = 1/ (2^* 111.1 / 0.115) s = 519 μs.

The calculated results refer to the fact that after 500-700 ms the radio channel has totally changed. In this case the radio channel is changing so fast that the channel estimation achieved from the frame preamble does not apply as such to the whole burst. As a rule of thumb the estimation period should be at least 5-10 shorter than the coherence time of the channel in order to get a reliable enough estimate. Therefore, the channel estimation from the preamble may be already quite different from that of the two following symbols carrying the FCH and the DL/UL-MAP info in the example of Fig. 1 a.

According to the mentioned IEEE 802.16 and IEEE 802.16e standards WiMAX systems can utilize also multi-antenna techniques.

In WiMAX specification IEEE 802.16 in chapter 8.4.8 "Space-Time Coding" it is said that in the MIMO case (Multiple Input Multiple Output) a preamble and other non-MIMO zones are transmitted with single antenna only (antenna 0). The same applies also to a case where an adaptive antenna system (AAS) or adaptive modulation and coding (AMC) are utilized.

An exemplary frame structure of a prior art WiMAX MIMO and STC case (Space Time Coding) is shown in Fig. 1 b. It should be noted that in transmit diversity (STC) or MIMO modes the preamble and the FCH/DL-UL-MAP info is transmitted only from antenna 0.

A downlink subframe of Fig. 1 b can comprise following exemplary zones. The downlink subframe starts with a preamble which is transmitted using only antenna 0. The preamble is followed by PUSC zones. In the example PUSC includes FCH, DL-MAP and UL-MAP. They can be followed by several PUSC zones #1 , #2,..., #n. In the depicted example also several user zones are transmitted with one antenna only (antenna 0).

After the last PUSC zone #n an STC DL zone begins where STC can be utilized. In the STC DL zone antenna 0 and antenna 1 are transmitting different symbols which are obtained from a single data stream by a well-known space-time coding scheme (the so-called Alamouti scheme). After the STC zone the depicted downlink subframe ends to a TTG period. After the TTG period an uplink subframe starts. The uplink frame ends to an RTG period. After the RTG period a new downlink subframe begins.

The first OFDMA symbols including the frame start preamble and MAP info are transmitted only from antenna 0 in non-STC zones. If the coherence time is not clearly longer than the downlink subframe time a channel estimate which is based on the preamble is in practice unusable. In WiMAX it is possible to utilize also FDD (Frequency Division Duplex) where the downlink and uplink use different frequencies. When utilizing FDD a new downlink subframe begins immediately when the preceding downlink subframe has ended. In FDD basically the same structure is utilized in the beginning of a subframe. It comprises also a preamble and FCH, DL-MAP and UL-MAP zones. If a channel estimate is based on the preamble it is unreliable for a fast moving subscriber station also in an FDD case.

A proposal concerning IEEE P802.22 Wireless RANs teaches that the frame preamble can be utilized for channel estimation in a fixed subscriber station case. It could be adequate for an estimate of the channel for the whole downlink frame. However, the paper states that in a mobile case more frequent channel estimation is needed.

Therefore there exists a problem in a WiMAX system about how to estimate transmission channel if one wants to utilize the strong preamble in channel estimation for the fast moving terminal. The prior art pilot-based channel estimation may be too slow for the high speed subscriber stations. In addition, four pilot subcarriers in a cluster may not be adequate for reliable enough channel estimation especially at a low SNR region (i.e. cell edge). There is always room for improving the channel estimation which can improve the system performance in various ways. Summary of the invention

The aspect of the present invention is to provide a channel estimation method suitable for a fast moving WiMAX subscriber station having a short channel coherence time. Another aspect of the invention is to reduce utilized pilot overhead (boosting). Also an aspect of the invention is to extend the coverage area of the WiMAX signal.

The aspects of the invention are achieved by a channel estimation method where initial channel estimation is based on a frame preamble and the initial estimate is refined by the dedicated pilots in a specific burst allocated to a fast moving subscriber station.

The invention has the advantage that a reliable channel estimate can be created for a fast moving WiMAX subscriber station. Another advantage of the invention is that it allows to reduce preamble and pilot overhead in a certain coverage area when compared to a prior art estimation method.

Yet another advantage of the invention is that the initial estimate based on the preamble is reliable because the transmitted preamble utilizes low modulation (BPSK) and has boosted power compared to transmitted symbols on the subchannel pilots.

Yet another advantage of the invention is that the channel can be tracked starting from the preamble symbol until the end of a desired burst. The tracked channel can be used also to improve the channel estimation for the FCH and the DL/UL- MAP symbols.

Yet another advantage of the invention is that it improves WiMAX coverage in a slow moving case if normal boosting is utilized in the preamble and subchannel pilots.

Furthermore, the invention has the advantage that it improves MIMO performance.

Some advantageous embodiments of the invention are presented in the dependent claims.

The basic idea of the invention is the following: Initial channel estimation is made based on a frame preamble. The initial channel estimate is refined by the pilot symbols in the following symbols, e.g. those in the specific burst allocated to the desired user. The prediction can be guided gradually from the first estimate to the final estimate. The predicted estimate is improved symbol by symbol by applying the additional information from the normal pilot symbol grid in each new OFDMA symbol. In addition to the pilot symbols also payload data symbols may be utilized for improving the channel estimate. This can advantageously be accomplished by utilizing a proper decision feedback algorithm with payload data.

The frame preamble covers a whole symbol and it has high power. Therefore the signal to noise ratio is good enough for initial synchronization. In the method according to the invention also the preamble is used to improve the channel estima- tion for the desired user burst which typically employs for channel estimation only dedicated pilot symbols within the user burst. The total time of the frame preamble is short (<150 μs) which allows a quick initial channel estimation from the frame preamble. Then the following pilots, which are embedded in the subchannels, are used only for refining the channel estimate provided by the preamble. The estimation method according to the invention makes a sophisticated initial guess of the complex channel response which estimate is then updated using the additional information provided by the dedicated pilots about the channel.

In one advantageous embodiment a trend based on the pilots is utilized to dynamically adjust existing channel estimate. In another advantageous embodiment the pilot information is utilized as an initial estimate for the consecutive channel estimate, thus giving faster converge to the next channel estimate.

In one advantageous embodiment the tracking can be accomplished advantageously by a Kalman or Wiener filter or Lagrange interpolation or similar tech- nique.

In specific WiMAX cases, e.g. in the AAS zone or in the STC zone, the same principle can be followed by taking into account the WiMAX specified pilot structures in those zones. For example, the optional MIMO midamble can be utilized for improved channel estimation. Further scope of applicability of the present invention will become apparent from the detailed description given hereafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Brief description of the drawings

Fig. 1 a shows a prior art WiMAX downlink subframe;

Fig. 1 b shows a prior art WiMAX downlink subframe where STC is utilized;

Fig. 2 shows an example of a WiMAX downlink subframe where an estimate according to the invention is utilized; Fig. 3 shows an example of a WiMAX MIMO downlink subframe where an estimate according to the invention is utilized;

Fig. 4 shows an example of a WiMAX AAS permutation downlink subframe where an estimate according to the invention is utilized; Fig. 5 shows an example of a WiMAX AMC permutation downlink subframe where an estimate according to the invention is utilized;

Fig. 6 shows an example of an AAS diversity map zone; and

Fig. 7 shows, as an exemplary flow chart, the main steps of estimating a transmission channel in a fast moving WiMAX subscriber station case. Detailed description

Figures 1 a and 1 b were explained above in connection with the description of the prior art.

The inventive idea is explained in the following by using a WiMAX downlink frame as an example. It is obvious to a man skilled in the art that the inventive idea can be applied also to other radio access systems where information is transmitted using a frame structure of some kind.

Fig. 2 shows an example of a basic WiMAX TDD downlink frame structure. It corresponds to Fig. 218 disclosed in IEEE 802.16e standard. The downlink subframe is started with a frame preamble having an exemplary OFDMA symbol number k. The OFDMA symbol duration can be in the range of 100-150 μs and it is transmitted in each applied subcarher s to s+L except the null subcarhers. The preamble can be boosted up to 9 dB. The preamble is a known BPSK modulated pseudorandom sequence. Therefore it allows straightforward channel estimation over the whole utilized bandwidth.

The preamble is followed by FCH info and DL-MAP which takes typically two OF- DMA symbol periods.

Next six different downlink bursts #1 - #6 are transmitted starting from an exemplary OFDMA symbol number k+3. The user bursts can share the subcarhers and OFDMA symbol periods as depicted in Fig. 2. In the depicted example downlink bursts #1 , #3, #4, #5 and #6 can be transmitted according to prior art. The receiving subscriber stations of these bursts are either slow moving or stationary. There- fore subchannel pilots in each burst can be utilized when estimating the channel at each subscriber station.

However, for example the subscriber station for receiving burst #2 can be a fast moving terminal. Also the burst #2 may comprise several subchannels each of which has eight pilot subcarriers. In the channel estimation method according to the invention the channel is first estimated utilizing the frame preamble for the same bandwidth as that of the depicted burst #2. Then the initial channel estimate is refined within the burst #2 using advantageously the pilot subcarriers in its subchannels.

The tracking of the channel can be performed over the whole bandwidth for the FCH and the DL-MAP symbols but only at the bandwidth of the desired burst #2 for the payload data. If the burst bandwidth is adequately small the channel can be described by a single tap (complex number) in the delay domain. In WiMAX this is possible when e.g. only one subchannel is allocated to a burst.

In one advantageous embodiment of the invention the tracking of the channel can be performed by filtering for example by using a Wiener filter or Kalman filter. Tracking is also possible to accomplish by interpolation for example by using a Lagrange type of interpolator. Third possibility for tracking is to utilize gradient- based methods such as LMS (Least Mean Square) or RLS (Recursive Least Squares) techniques.

The depicted exemplary downlink frame ends in an exemplary OFDMA symbol number k+M. The downlink frame is separated from an exemplary uplink frame by a TTG period. The uplink frame ends in an exemplary OFDMA symbol number k+N. An RTG period separates a new downlink frame from a new uplink frame.

Fig. 3 shows an example of a MIMO WiMAX case. It is noteworthy that in transmit diversity (STC) or MIMO modes the preamble 31 and the FCH/DL-UL-MAP info is transmitted only from one antenna (antenna O). Thus the method according to the invention can be applied to the channel estimation from antenna 0 to the subscriber terminal in a tracking zone 35a. In STC mode this is not a big problem since all the payload data symbols are sent from both of the antennas; antenna 0 and antenna 1.

The tracking using the preamble 31 is not employed to the channel from the antenna 1 and the estimation is performed solely on the dedicated symbols in the desired burst in the tracking zone 35b. In STC/MIMO modes it is possible also to utilise the optional midamble which covers one symbol; references 32a, 32b, 34a and 34b. The midamble can be boosted by 3 dB. The desired user bursts 33a and 33b are transmitted immediately after the midambles 32a and 32b. According to IEEE 802.16e, 8.4.8.5 in multi-antenna case the midamble is allocated on subcarrier-by-subcarrier basis to utilized antennas.

Fig. 4 shows an example of AAS on FUSC/PUSC permutation. The exemplary downlink subframe comprises a preamble, FCH, DL-MAP, UL-MAP and five user bursts #1 - #5 and an optional AAS diversity map zone. The AAS burst can be sent also in an AMC permutation zone. In the conventional user bursts the channel estimation method described above in connection with Fig. 2 can be applied for fast moving subscriber stations taking advantage also of the specific AAS preamble. If the subscriber station is slowly moving or stationary, estimation method according to prior art can be applied in these bursts.

In AAS mode each user burst comprises an AAS preamble according to IEEE 802.16e, 8.4.5.3.3. In the AAS mode the AAS preamble symbols are transmitted with the same beamforming weights as the payload data symbols of the burst. Thus the AAS preamble can be used for initial channel estimation according to the invention. The subsequent pilot symbols in the burst can be utilized for refining the initial channel estimate.

However, special care has to be taken if the frame preamble is applied for tracking the AAS channel since the frame preamble is not beamformed. The frame preamble can be utilised for example in a case in which the angular spread of the radio channel is relatively small. This is often the case in rural and suburban environ- ments if the base station antenna is well above the surrounding buildings or e.g. trees.

Fig. 5 shows an example of AAS on AMC permutation. The exemplary downlink subframe comprises a preamble, FCH, DL-MAP, UL-MAP and four user bursts #1 - #4 and also two AAS diversity map zones. In the conventional user bursts the channel estimation method described above in connection with Fig. 4 can be applied for fast moving subscriber stations. If the subscriber station is slowly moving or stationary, estimation method according to prior art can be applied in these bursts. Fig. 6 shows an example of a diversity map zone structure according to IEEE 802.16e, 8.4.4.6.1. The depicted AAS diversity map zone comprises four AAS downlink preambles. One AAS-DLFP block (Down Link Frame Prefix) follows each AAS downlink preamble. The DLFP block is utilized to transmit base station pa- rameters to a subscriber station. When utilizing the method according to the invention the first AAS downlink preamble can be utilized for an initial channel estimate and the consequent pilot symbols and preambles can be utilized for refining the initial channel estimate.

Fig. 7 shows the main steps of the tracking method according to the invention as an exemplary flow chart. In the following description it is assumed that the WiMAX subscriber station comprises prior art means for receiving and transmitting messages, means for processing instructions and means for saving data in the subscriber station.

In step 70 a subscriber station receives a frame preamble. The receiver is syn- chronized and the following DL-MAP info is decoded. Next, in step 71 the subscriber station makes a decision if a channel prediction according to the invention is needed.

One possible reason for utilizing the estimation method according to the invention is that the subscriber is moving so fast that the coherence time of the receiver is too short for giving a reliable estimate when a prior art estimation method is utilized. Advantageously the subscriber station informs the serving base station about the circumstance. Alternatively, the mobile velocity is estimated by known methods using the frame preamble and the consecutive two control info symbols.

Another reason for utilizing the method according to the invention may be to re- duce preamble overhead.

Yet another reason for utilizing the method according to the invention may be to improve WiMAX coverage area.

Yet another reason for utilizing the method according to the invention may be to improve WiMAX MIMO performance.

If the channel conditions are such that a reliable estimate for the channel can be based on subframe pilots then the process moves to step 72. In step 72 the subscriber station makes a channel estimate for the subscriber burst to be received. If needed the receiver of the subscriber station is adapted using the channel esti- mate. In step 73 the subscriber station decodes the payload data of the received user burst. When the burst is over, the receiver moves to a state where it is waiting a new frame preamble.

If the subscriber station decides that a channel prediction according to the inven- tion is needed then the process moves from step 71 to step 74. In step 74 the subscriber station makes an initial estimate of the channel utilizing the received preamble.

Step 75 is optional. This is depicted in Fig. 7 by a rectangle with a dashed line. In the optional step 75 received OFDMA payload data symbols can be utilized for fur- ther improving the initial channel estimate by utilizing decision feedback with pay- load data. In one advantageous embodiment step 75 is bypassed. In another advantageous embodiment step 75 is utilized in the channel estimation.

In step 76 the subscriber station receives the subchannel pilot symbols of the user burst. One WiMAX subchannel comprises two OFDMA symbols which both com- prise forty-eight user data symbols and eight pilot symbols.

In step 77 the received subchannel pilots are utilized for refining the channel estimate.

Step 78 is optional. This is depicted in Fig. 7 by a rectangle with a dashed line. In the optional step 78 the channel estimate is advantageously further refined utiliz- ing decision feedback with payload data symbols. In one advantageous embodiment step 78 is bypassed. In another advantageous embodiment step 78 is utilized in the channel estimation.

After refining the channel estimate the receiver of the subscriber station decodes the payload data symbols using the refined channel estimate in step 79.

In step 80 is decided if any OFDMA symbols are left in the data burst to be decoded. If the whole user burst has been decoded, the process returns to step 70. The receiver is then waiting for a new frame preamble.

The optional steps 75 and 78 can be combined in four different ways. In the simplest embodiment both optional steps 75 and 78 are bypassed. In the most com- plex embodiment both optional steps are included in the channel estimation. Two remaining embodiments include one or the other of the optional steps. If it is decided in step 79 that there is more payload data to be decoded the process returns to step 76 and new OFDMA symbols are received. Using the new pilot a new estimate can be formed. The estimation loop runs so long that in step 79 is noticed that all the data of the user burst has been decoded.

The method described above can advantageously be accomplished by an application program which fulfills the depicted method steps. The application program is advantageously saved in a memory of the subscriber station. The program can be advantageously executed in processing unit of the subscriber station.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A method for estimating a downlink channel of a radio access system, the method comprising:

- receiving a preamble (70), characterized in that the method further comprises:

- making a decision about a need for a channel prediction (71 ), and if needed;

- estimating the channel by utilizing the received preamble (74); and

- predicting a new channel estimate on symbol-by-symbol basis until the end of the downlink subframe (76-80).

2. The method according to claim 1 , characterized in that the radio access system is a WiMAX system.

3. The method according to claim 2, characterized in that the preamble is a WiMAX frame preamble.

4. The method according to claim 3, characterized in that additional channel estimates obtained from symbol specific pilot subcarriers are utilized in the channel prediction.

5. The method according to claim 3, characterized in that additional channel estimates obtained from control data symbols are utilized in the channel prediction.

6. The method according to claim 3, characterized in that additional channel estimates obtained from payload data symbols are utilized in the channel prediction.

7. The method according to claim 3, characterized in that MIMO midambles are utilized in the channel prediction.

8. The method according to claim 1 , characterized in that AAS preambles are utilized in the channel prediction.

9. The method according to claim 2, characterized in that the decision about the need for the channel prediction is based on speed of the WiMAX subscriber station.

10. The method according to claim 2, characterized in that the decision about the need for the channel prediction is based on a measured coherence time of the WiMAX subscriber station.

11. The method according to claim 2, characterized in that the decision about the need for the channel prediction is made for improving WiMAX coverage area.

12. The method according to claim 2, characterized in that the decision about the need for the channel prediction is made for improving WiMAX MIMO performance.

13. The method according to claim 1 , characterized in that the decision about the need for the channel prediction is made for reducing preamble overhead.

14. A wireless subscriber station comprising:

- processor means;

- receiver means;

- transmitter means; and - memory, characterized in that it further comprises:

- a means for making a decision about a need for a channel prediction;

- a means for estimating the channel by utilizing the received preamble; and

- a means for predicting a new channel estimate on symbol-by-symbol basis until the end of the downlink subframe.

15. The subscriber station according to claim 14, characterized in that the utilized preamble is a frame preamble.

16. The subscriber station according to claim 15, characterized in that the subscriber station is configured to utilize in the channel prediction additional channel estimates obtained from symbol specific pilot subcarriers.

17. The subscriber station according to claim 15, characterized in that the subscriber station is configured to utilize in the channel prediction additional channel estimates obtained from control data symbols.

18. The subscriber station according to claim 15, characterized in that the sub- scriber station is configured to utilize in the channel prediction additional channel estimates obtained from payload data symbols.

19. The subscriber station according to claim 15, characterized in that the subscriber station is configured to utilize in the channel prediction additional channel estimates obtained from MIMO midambles.

20. The subscriber station according to claim 14, characterized in that the sub- scriber station is configured to utilize in the channel prediction additional channel estimates obtained from AAS preambles.

21. The subscriber station according to claim 14, characterized in that the subscriber station is configured to make a decision about the need for the channel prediction based on speed of the subscriber station.

22. The subscriber station according to claim 14, characterized in that the subscriber station is configured to make a decision about the need for the channel prediction based on a measured coherence time of the subscriber station.

23. The subscriber station according to claims 14 to 22, characterized in that the subscriber station is a WiMAX subscriber station.

24. A computer readable medium encoded with software for estimating downlink channel of a radio access system, characterized in that it comprises:

- computer readable code for making a decision about a need for a channel prediction (71 );

- computer readable code for estimating the channel by utilizing the received pre- amble (74); and

- predicting a new channel estimate on symbol-by-symbol basis until the end of the downlink subframe (76-79).

25. The computer readable medium according to claim 24, characterized in that it comprises computer readable code for utilizing a subframe preamble in the channel prediction.

26. The computer readable medium according to claim 25, characterized in that it comprises computer readable code for utilizing additional channel estimates obtained from symbol specific pilot subcarhers in the channel prediction.

27. The computer readable medium according to claim 25, characterized in that it comprises computer readable code for utilizing additional channel estimates obtained from control data symbols.

28. The computer readable medium according to claim 25, characterized in that it comprises computer readable code for utilizing additional channel estimates obtained from payload data symbols.

29. The computer readable medium according to claim 25, characterized in that it comprises computer readable code for utilizing MIMO midambles in the channel prediction.

30. The computer readable medium according to claim 24, characterized in that it comprises computer readable code for utilizing AAS preambles in the channel prediction.

31. The computer readable medium according to claims 24-30, characterized in that the computer readable code is configured to be utilized in a WiMAX subscriber station.