Imported: 17 Feb '17 | Published: 28 Feb '12

USPTO - Utility Patents

A radio channel estimation technique is described for use in a OFDM-based radio communications system. A block of OFDM symbols is transmitted from multiple antennas over multiple sub-carrier frequencies. The block of OFDM symbols includes known pilot symbols as well as data symbols to be determined by a receiver. The pilot symbols are transmitted in a predetermined pattern at periodic times on periodic sub-carriers. A pilot channel estimate is determined for each pilot symbol in the received block of OFDM symbols. An

This application claims priority from U.S. provisional patent application No. 60/688,721, filed on Jun. 9, 2005, the contents of which are incorporated here by reference.

The technical field relates to communications, and more particularly, to estimating a channel in time and in frequency.

Receivers often employ equalizers to compensate for signal distortion suffered during signal propagation over a channel. Most equalization methods include estimating the channel characteristics to determine how the channel is distorting a signal. One way to determine that distortion is to send over the channel a signal known by the receiver. The receiver compares the received signal with the known signal, and an estimate of the channel can be computed. One example of a known signal is a simple pulse. In that case, the received signal is called the channel impulse response and corresponds to the transfer function h of the channel. A more sophisticated known signal is a pilot signal that includes for example a known sequence of bits or symbols. The known pilot sequence is compared with the received sequence to determine how much and where the received signal differs from the known sequence. An equalizer can be viewed as filter of sorts that tries to remove the channel distortion from the received signal.

Orthogonal Frequency Domain Multiplexing (OFDM) pertains to a technology that transmits multiple signals simultaneously over a wired or wireless communication medium. Specifically, the data is distributed over a large number of sub-carriers spaced apart at precise frequencies. That spacing provides the orthogonality needed to facilitate demodulation of each frequency.

Wireless-based OFDM receivers may be employed to transmit multiple data streams over a number of parallel flat fading channels. Equalization may be performed in the frequency domain using one-tap digital filters. Channel estimation is performed using known pilot sequences. Pilot signals are transmitted at specific time slots and frequency sub-carriers known to the transmitter and the receiver. The channel at these pilot time slots and frequency sub-carriers may be estimated using pilot-assisted channel estimation techniques such as zero-forcing, minimum mean square error (MMSE), etc. The channel must also be estimated for the data transmitted at time slots and sub-carriers that are different from those on which pilots are sent. Channel estimation for the data may be determined using prediction methods like linear interpolation and MMSE interpolation.

Even though it is relatively easy to implement, linear interpolation often gives poor results in a frequency selective time-varying environment. In other words, between two points where the channel is estimated based on received pilot signals, the channel may change significantly (not uncommon in mobile radio environments). As a result, the interpolated channel estimates between the two pilots can be quite different from the actual channel at those points. Another drawback is a large mismatch in OFDM between the estimated channels and the true channels at the borders of the time-frequency grid.

Linear MMSE interpolation is based on a model that determines the variation of the channel in time and frequency. For example, the time variation in many cases follows the Jakes model (a model for Rayleigh fading based on summing sinusoids), and the frequency response can be determined using the power delay profile. The linear MMSE interpolation method may be quite satisfactory if the correct model is selected. But if there is a model mismatch, performance suffers. Another drawback of this linear MMSE interpolation is large memory requirements and computational complexity.

A radio channel estimation technique is described for use in a OFDM-based radio communications system. A block of OFDM symbols is transmitted from multiple antennas. The block of OFDM symbols includes known pilot symbols as well as data symbols to be determined by a receiver. The pilot symbols are transmitted in a predetermined pattern at periodic times on periodic sub-carriers. A pilot channel estimate is determined for each pilot symbol in the received block of OFDM symbols. An

To facilitate the radio channel estimation, the multiple antenna transmitter determines one or more known pilot symbols and a periodic pilot transmission pattern where the pilot symbols are placed at periodic times on periodic sub-carriers. A block of OFDM symbols is formed that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern. Certain ones of the OFDM symbols are assigned to corresponding ones the multiple antennas so that each antenna has an associated OFDM symbol stream. Each OFDM symbol stream is then transmitted from its corresponding antenna. The periodic pilot transmission pattern is preferably a symmetric pattern. But if not, the pilot transmission pattern may be transformed into a symmetric pattern where the pilot symbols are interspersed symmetrically with the data symbols in the OFDM symbol block.

In the following description, for purposes of explanation and non-limitation, specific details are set forth, such as particular nodes, functional entities, techniques, protocols, standards, etc. in order to provide an understanding of the described technology. It will be apparent to one skilled in the art that other embodiments may be practiced apart from the specific details disclosed below. In other instances, detailed descriptions of well-known methods, devices, techniques, etc. are omitted so as not to obscure the description with unnecessary detail. Individual function blocks are shown in the figures. Those skilled in the art will appreciate that the functions of those blocks may be implemented using individual hardware circuits, using software programs and data in conjunction with a suitably programmed microprocessor or general purpose computer, using applications specific integrated circuitry (ASIC), field programmable gate arrays, one or more digital signal processors (DSPs), etc.

FIG. 1 shows an example of radio communications system **10** which can be based upon Orthogonal Frequency Domain Multiplexing (OFDM) an OFDM transmitter **12** includes a pilot processor **18** for providing one or more known pilot sequences to a combiner **22**. The transmitter **12** includes multiple antennas A_{1}, A_{2 }. . . A_{x}. Data processor **20** provides OFDM data symbols to the combiner **12** which are combined with OFDM pilot symbols in a predetermined pattern (described in more detail below). The combined signal is transformed and then processed in the radio frequency (RF) block **24** before transmission over a corresponding one of the antennas.

The antenna transmissions propagate over a radio channel **16** and are received at one or more antennas A_{1}, A_{2 }. . . A_{y }at an OFDM receiver **14**. The antenna signals are processed in an RF block **26** and converted to baseband. The baseband signal is provided both to a channel equalizer **28** for equalization and a channel estimator **30** for estimating the radio channel **16** for a received OFDM block of symbols. The channel equalizer **28** compensates for distortion caused by the radio channel **16** using the estimated channel from the channel estimator **30**. If the receiver **14** employs multiple receiving antennas, each antenna branch is separately processed to provide an equalized symbol stream, e.g., to a maximum ratio combiner (MRC) **32**. The maximum ratio combiner **32** combines the symbol streams from each antenna typically providing a higher weighting to antenna symbol streams that have a better signal-to-interference ratio or some other quality measure. The combined symbol stream **34**, which demodulates the information on each OFDM sub-carrier to provide demodulated bits for further processing.

As described in the background, channel estimation may be performed for known pilot symbols using one or more known pilot signals and any one of well-known pilot-assisted channel estimation techniques. But the channel must also be estimated for the data symbols, which unlike the pilot symbols, are not known in advance by the receiver. Performing channel estimation for the unknown data symbols in the OFDM block using an interpolation technique is less than optimal for the reasons explained in the background. The inventors conceived of a better, more accurate, and simpler technique for estimating the channel for the data symbols based on the transmission of an OFDM block using a predetermined, periodic pilot transmission pattern.

FIG. 2 illustrates example, non-limiting procedures in a “transmit” flowchart for setting up and transmitting such a periodic pilot transmission pattern. Initially, one or more known pilot symbols are determined in step S**1**. A periodic pilot transmission pattern is determined in step S**2**. FIG. 4 shows a non-limiting example predetermined pilot pattern for a block of OFDM symbols. Each vertical column associated with a time slot corresponds to a single OFDM symbol having **26** sub-carriers. Those skilled in the art will appreciate that the number of time slots and the number of sub-carriers can be any suitable number. The pilot symbols are positioned in the OFDM block so that they occur periodically both with respect to sub-carrier and with respect to time slot. In FIG. 4, a pilot is inserted every fifth sub-carrier and every fourth time slot. The number of pilots in the frequency and time direction depends on the maximum delay of the channel and the maximum Doppler frequency. The periodicity of the pilot in the time domain should at be at least twice faster than the Doppler frequency in order to satisfy the Nyquist criteria. In the frequency domain, the frequency separation between two pilots can be at most equal to the coherence bandwidth of the radio channel.

Returning to FIG. 2, the pilot symbols are inserted into a block of OFDM data symbols at periodic times on periodic sub-carriers in accordance with the periodic pilot pattern as just explained (step S**3**). Then, time-space coding is performed on the block of OFDM symbols so that different symbols are transmitted from different transmitter antennas at different time intervals (step S**4**). A cyclic prefix is added to each antenna data stream to reduce intersymbol interference at the receiver (step S**5**). The antenna data streams are frequency up-converted and transmitted over the radio channel via its corresponding antenna (step S**6**).

FIG. 3 illustrates a “receive” flowchart outlining example, non-limiting steps that may be performed by the receiver **14**. As explained earlier, the receiver may have one or several antennas. For each receive antenna, the received signal is down converted to baseband frequency and the cyclic prefix removed from the symbol stream (step S**10**). A two-dimensional Fourier transform (preferably a Fast Fourier Transform (FFT)) is performed on each antenna symbol stream, and the known pilot symbols are detected (step S**11**). A first channel estimate is calculated for each detected pilot symbol (step S**12**).

Processing circuitry in the receiver then forms an **13**). Zeros are inserted in the remaining matrix positions which correspond to the as of yet unknown data symbols (step S**14**). A two dimensional inverse Fourier transform (preferably a two dimensional Inverse Fast Fourier Transform (IFFT)) of the **15**). This produces multiple copies of the channel estimate. The number of copies corresponds to the number of zeros inserted between the pilot symbols in one time slots. In the FIG. 4 example, there are four zeros between each pilot in each OFDM symbol. Therefore, four copies of the channel estimate would be generated. One of the copies of this channel estimate is selected (step S**16**). The copy may be selected, for example, using a low pass filter. A two-dimensional FFT is then calculated for the selected channel estimate to obtain an actual channel estimate for each position in the **17**). The received OFDM data symbols are then equalized based on the obtained channel estimates (step S**18**).

As explained above, the predetermined pilot pattern for the OFDM block of symbols should be symmetric. A non-limiting example of a symmetric pilot pattern is shown in FIG. 4. But if the pilots are interspersed in a block of OFDM data non-symmetrically, that OFDM block of data should be transformed so that the pilot symbols are symmetrically and periodically positioned in the block. FIG. 5 illustrates on the right hand side an example of a block of OFDM symbols in which the pilots are interspersed non-symmetrically. That block of non-symmetric OFDM symbols is then transformed so that the pilot symbols are symmetrically and periodically spaced as shown on the left-side of the Figure. In order to obtain the channel estimates, the pilots should be located periodically in the time/frequency domain to perform the two-dimensional FFT/IFFT operations.

FIG. 6 shows one non-limiting example implementations for the transmitter **12**. As explained above, for OFDM transmission, one OFDM symbol consists of **50** performs an OFDM symbol-to-sub-carrier assignment to generate **52**. When a complete OFDM block has been stored in buffer **52**, the OFDM block is then provided to a two dimensional inverse Fast Fourier Transformer **54**. Transformer **54** generates a serial symbol stream S which is provided to space-time coder **56**.

The space-time coder **56** determines which OFDM symbols are transmitted over a particular transmit antenna during a particular time slot. Consider a simplified example where one time slot corresponds to transmitting three OFDM symbols one, two, three. During a time slot, OFDM symbols one, two, and three are transmitted via the first antenna A**1** in that order. During the same time slot, OFDM symbols are transmitted over antenna A**2** in a different order two, three, and one. The sequence transmitted over antenna A**2** is a cyclically shifted version of the sequence transmitted over antenna A**1**, where the shift is equal to one OFDM symbol. Similar to cyclic delay diversity, the cyclic shift applied on the transmitted data artificially modifies the radio channel while making the data sequence transparent at the receiver. Each antenna symbol stream S**1** and S**2** is provided to a respective add cyclic prefix block **58** which outputs a modified symbol of stream S**1**′ and S**2**′ to a frequency up converter block **60** for converting the baseband signal to radio frequency and transmission over the associated antenna A**1** or A**2**.

FIG. 7 shows a non-limiting example receiver **14** that includes one antenna A**1**. The received signal is down-converted from RF to baseband in down-converter **70**. The cyclic prefix is removed in block **72**. Then an OFDM block of symbols of size **74** is applied to the OFDM block that is provided to a channel estimator **78**. The channel estimator **78** performs the channel estimation procedures outlined, for example, in FIG. 3. The received OFDM data symbols are then equalized by channel equalizer **76** in accordance with the actual channel estimate. The equalized signal is then provided to baseband processor **80** for further baseband processing.

An example channel estimation for a two antenna transmitter and a one antenna receiver is now explained in conjunction with the FIGS. 8-11. FIG. 8 illustrates the actual channel impulse response h for each of the two transmit antennas A**1** and A**2**. For purposes of explanation and simplification, the channel impulse response can be viewed as corresponding to the channel estimate. The channel impulse response for antenna A**1** is indicated with a circle, and the channel impulse response for antenna A**2** is indicated with a square.

FIG. 9 illustrates the channel response in both the frequency and time domains for both antennas. The channel response must be shown in three dimensions to illustrate the multiple sub-carriers for each OFDM symbols as well as the multiple OFDM symbols in an OFDM block. FIG. 10 illustrates the channel response at the pilot symbol positions.

Recall from the example predetermined pilot pattern shown in FIG. 4 that certain symbol samples in each OFDM column symbol correspond to known pilot symbols. For example, FIG. 10 illustrates the channel response for individual pilot symbols—OFDM symbol No. 1 and OFDM symbol No. 9. The channel response at each pilot symbol can then be compared with the known pilot symbol to determine an estimate of the channel at that pilot position. Only the channel response values for the known pilot positions are maintained in the OFDM block because the channel responses are not known for the data symbol positions. As a result, zeros are inserted in these data positions by taking a two dimensional IFFT of this modified OFDM block.

The 2D-IFFT results are illustrated in FIGS. 11A-11C. FIG. 11A shows that the channel impulse response is periodic producing a number of copies of the channel impulse response corresponding to the number of zeroes inserted between two adjacent pilots. Only one copy of the channel is required to obtain the final channel estimate. The copy may be selected using a low pass filter which selects the first portion of the data and ignore the sub-sequent repeated versions of it. FIG. 11B illustrates a “slice” of the first of the OFDM symbols (corresponding to Antenna A**1**) taken from the three dimensional channel impulse response shown in FIG. 11A. The channel impulse response for each antenna is periodic with the periodicity corresponding to the number of zeros inserted between the sub-carrier frequencies (15 for this example). FIG. 11C is another slice of the first sub-carrier frequency dimension which corresponds to all the OFDM symbols. The channel impulse response for all antennas (A**2** and A**1**) is periodic, with the period determined by the number of zeros inserted between the OFDM symbols (7 for this example).

A two-dimensional FFT is then performed on the selected copy and results in the channel impulse response for both antennas in the time domain as illustrated in FIG. 12. Comparison between FIG. 12 and FIG. 8 reveals that the two graphs are the same. Thus, the above technique estimates the channel for all points of the OFDM symbol block without performing the linear interpolation between the pilot symbols. Because FIGS. 8 and 12 match identically, the accuracy of this technique is readily apparent.

The above description may be supported mathematically. Let _{x }as explained earlier, i.e., the columns of S are selected in a cyclic fashion and transmitted onto the physical antennas. Let _{1 }s_{2 }. . . s_{M}], where s_{k }corresponds to the kth OFDM symbol of S. Let _{1 }y_{2 }. . . y_{M}] be the received signal at the receiver, e.g., a mobile station, where y_{k }corresponds to the kth received OFDM symbol. As demonstrated below, a 2D-FFT of

*{tilde over (Y)}=√{square root over (NM)}{tilde over (H)}·{tilde over (S)}=√{square root over (NM)}{tilde over (H)}·B* (1)

where {tilde over (H)} and {tilde over (S)}, denote the 2-Dimensional Fast Fourier Transforms (FFTs) of the channel matrix H and the block of data S, respectively, and B is the block of OFDM symbols. The term {tilde over (H)}·{tilde over (S)} denotes the element-wise multiplication of {tilde over (H)} and {tilde over (S)}. Because the known pilots are equally placed in frequency (and time) in {tilde over (S)}, and because the number of pilots is greater than the length of the channel impulse response (IR) and number of transmit antennas so that the Nyquist criteria in frequency and time are satisfied, the columns and rows of *Discrete*-*Time Signal Processing*, p. 558, Prentice Hall, second edition, 1999.

This can be demonstrated in the following example. Assume **1** and A**2** are used.

Then

Assume that the channel matrix

The first and the second columns of

At the first transmission time instant t_{1}, OFDM symbol s_{1 }is transmitted from the transmit antenna A**1**, and OFDM symbol s_{2 }is transmitted from transmit antenna A**2**. At the next transmission time instant t_{2}, OFDM symbol s_{2 }is transmitted from transmission antenna A**1** and OFDM symbol s_{1 }is transmitted from antenna A**2**. At the receiver, the received signal corresponding to the first and second OFDM symbols can be written as:

are the channel matrices corresponding to channel impulse responses h_{1 }and h_{2}, respectively, (again the values for H_{1 }and H_{2 }are selected randomly for this example). Replacing the channel matrices and the transmit OFDM symbols by their respective values, the received signals at the receiver are given

The 2-D FFT of _{1 }y_{2}] produces

Working from equation (1) above, and given that the 2-D FFT of

the product

which is identical to {tilde over (

The above-described channel estimation technique has many advantages. Conventional MIMO OFDM technology requires transmission of significant “overhead” information which reduces throughput. In the approach described above, there is no need to associate a unique pilot to each transmit antenna. Nor does the receiver need to know the number of transmit antennas. So neither piece of overhead information needs to be sent over the wireless channel. Another advantage is that the channel is accurately estimated at the borders of the time-frequency grid. Moreover, and in contrast to channel estimation techniques like linear MMSE, the present channel estimation approach needs only relatively small amounts of memory and is substantially less computationally complex.

Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential such that it must be included in the claims scope. The scope of patented subject matter is defined only by the claims. The extent of legal protection is defined by the words recited in the allowed claims and their equivalents. No claim is intended to invoke paragraph 6 of 35 USC §112 unless the words “means for” are used.

1. A method for use in a transmitter having multiple antennas in a OFDM-based radio communications system, comprising:

determining one or more known pilot symbols;

determining a periodic pilot transmission pattern;

forming a block of OFDM symbols that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern;

using a space-time coder to perform space and time coding by assigning certain ones of the OFDM symbols to be transmitted during a particular time slot from a selected one of the multiple antennas; and

transmitting OFDM symbols from the antennas,

wherein the space-time coder provides a first block of multiple OFDM symbols in a first sequential order to a first antenna for transmission during a number of time slots and a second block of the same multiple OFDM symbols in a second sequential order different from the first sequential order to a second antenna for transmission during the number of time slots.

determining one or more known pilot symbols;

determining a periodic pilot transmission pattern;

forming a block of OFDM symbols that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern;

using a space-time coder to perform space and time coding by assigning certain ones of the OFDM symbols to be transmitted during a particular time slot from a selected one of the multiple antennas; and

transmitting OFDM symbols from the antennas,

wherein the space-time coder provides a first block of multiple OFDM symbols in a first sequential order to a first antenna for transmission during a number of time slots and a second block of the same multiple OFDM symbols in a second sequential order different from the first sequential order to a second antenna for transmission during the number of time slots.

2. The method in claim 1, further comprising:

prior to transmitting, adding a cyclic prefix to each block of OFDM symbols.

prior to transmitting, adding a cyclic prefix to each block of OFDM symbols.

3. The method in claim 1, wherein the predetermined pattern is a symmetric pattern.

4. The method in claim 1, wherein the predetermined pattern is not symmetric, the method further comprises:

transforming the predetermined pattern into a symmetric pattern where the pilot symbols are interspersed symmetrically with the data symbols in the OFDM symbol block.

transforming the predetermined pattern into a symmetric pattern where the pilot symbols are interspersed symmetrically with the data symbols in the OFDM symbol block.

5. The method in claim 1, wherein the predetermined pattern is such that the pilot symbols are transmitted at periodic times on periodic sub-carriers.

6. The method in claim 1, wherein the second sequential order is a cyclically shifted version of the first sequential order.

7. The method in claim 1, wherein during a first time slot, providing a first symbol from the first block in the first sequential order and a first symbol from the second block in the second sequential order to the first antenna and the second antenna for transmission, respectively, and during a second subsequent time slot, providing a second symbol from the first block in the first sequential order and a second symbol from the second block in the second sequential order to the first antenna and the second antenna for transmission, respectively.

8. Apparatus for use in a transmitter in a OFDM-based radio communications system, comprising:

multiple antennas;

electronic circuitry configured to:

determine one or more known pilot symbols;

determine a periodic pilot transmission pattern;

form a block of OFDM symbols that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern;

perform space-time coding by assigning certain ones of the OFDM symbols to be transmitted during a particular time slot from a selected one of the multiple antennas; and

a transmitter, coupled to the electronic circuitry, for transmitting OFDM symbols from the antennas,

wherein the electronic circuitry is configured to perform the space-time coding to provide a first block of multiple OFDM symbols in a first sequential order to a first antenna for transmission during a number of time slots and a second block of the same multiple OFDM symbols in a second sequential order different from the first sequential order to a second antenna for transmission during the number of time slots.

multiple antennas;

electronic circuitry configured to:

determine one or more known pilot symbols;

determine a periodic pilot transmission pattern;

form a block of OFDM symbols that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern;

perform space-time coding by assigning certain ones of the OFDM symbols to be transmitted during a particular time slot from a selected one of the multiple antennas; and

a transmitter, coupled to the electronic circuitry, for transmitting OFDM symbols from the antennas,

wherein the electronic circuitry is configured to perform the space-time coding to provide a first block of multiple OFDM symbols in a first sequential order to a first antenna for transmission during a number of time slots and a second block of the same multiple OFDM symbols in a second sequential order different from the first sequential order to a second antenna for transmission during the number of time slots.

determine one or more known pilot symbols;

determine a periodic pilot transmission pattern;

form a block of OFDM symbols that includes the one or more pilot symbols and data symbols in accordance with a predetermined pattern;

perform space-time coding by assigning certain ones of the OFDM symbols to be transmitted during a particular time slot from a selected one of the multiple antennas; and

9. The apparatus in claim 8, wherein the electronic circuitry is configured to add a cyclic prefix to each block of OFDM symbols prior to transmission.

10. The apparatus in claim 8, wherein the predetermined pattern is a symmetric pattern.

11. The apparatus in claim 8, wherein when the predetermined pattern is not symmetric, the electronic circuitry is configured to transform the predetermined pattern into a symmetric pattern where the pilot symbols are interspersed symmetrically with the data symbols in the OFDM symbol block.

12. The apparatus in claim 8, wherein the predetermined pattern is such that the pilot symbols are transmitted at periodic times on periodic sub-carriers.

13. The apparatus in claim 8, wherein the second sequential order is a cyclically shifted version of the first sequential order.

14. The apparatus in claim 8, wherein during a first time slot, the electronic circuitry is configured to provide a first symbol from the first block in the first sequential order and a first symbol from the second block in the second sequential order to the first antenna and the second antenna for transmission, respectively, and during a second subsequent time slot, the electronic circuitry is configured to provide a second symbol from the first block in the first sequential order and a second symbol from the second block in the second sequential order to the first antenna and the second antenna for transmission, respectively.