RECEIVER AND METHOD FOR DETECTING FREQUENCY AND TIMING OFFSETS IN MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) SYSTEM

Abstract

Frequency and timing offsets compensation in a Multiple Input Multiple Output (MIMO) system are provided. A receiver includes one or more antennas for receiving one or more signals using the same radio resource; a channel estimator for estimating a channel using the received signals; a transmit signal candidate pre-compensator for compensating a frequency offset and a timing offset of transmit signal candidates by estimating a frequency offset and a timing offset of one or more transmit antennas; and a demodulator for demodulating the received signals using the estimated channel information and the transmit signal candidates of the compensated frequency and timing offsets. The service coverage of the cellular system can be expanded and the transmit power of the terminal can be saved by enhancing the demodulation performance by compensating for the offset influence of each transmitter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a simplified diagram of a typical CSM MIMO system;

FIGS. 2A and 2B are diagrams of a typical PUSC tile structure;

FIG. 3 is a block diagram of a conventional receiver in an OFDM system;

FIG. 4 is a block diagram of a receiver in a MIMO system according to the present invention;

FIG. 5 is a detailed block diagram of a transmit signal candidate pre-compensator according to the present invention; and

FIG. 6 is a flowchart of a frequency and timing offset compensating procedure in the MIMO system according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

The present invention provides a technique for compensating for frequency offset and timing offset in a receiver of a Multiple Input Multiple Output (MIMO) system; that is, a technique for compensating for frequency and timing offsets with respect to a plurality of received signals by compensating for a phase of a transmit candidate group in the receiver of the MIMO system. Since the influence of the frequency and timing offsets can be modeled as a phase rotation which linearly increases in a time-axis index and a frequency-axis index in the MIMO system, the phase of the transmit candidate group can be compensated.

The following explanation describes the receiver, which receives a plurality of signals using the same radio resource, compensates for the frequency and timing offsets of the received signals in the MIMO system. The frequency offset impedes the orthogonality by causing Inter Carrier Interference (ICI) by the frequency offset with the linear phase rotation in the same time axis. The timing offset causes the phase rotation between the subcarriers and Inter Symbol Interference (ISI) in the direction of the timing offset, and thus spoils the orthogonality. Hence, the present invention provides the technique for compensating for the frequency and timing offsets of the received signals in the receiver of the MIMO system.

Now, a radio communication system using a MIMO-Orthogonal Frequency Division Multiplexing (OFDM) scheme is explained by way of example.

Particularly, Partially Used Sub-Carrier (PUSC) Collaborative Spatial Multiplexing (CSM) scheme is described in the MIMO-OFDM system. Accordingly, it is assumed that the MIMO-OFDM system includes two transmitters and a receiver having a plurality of receive antennas as shown in FIG. 1. Alternatively, the MIMO-OFDM system can include a transmitter having a plurality of transmit antennas and a receiver having a plurality of receive antennas.

The transmitters send different signals to the receiver using the same radio resource. The receiver receives the different signals from the transmitters through the same resource on the plurality of the receive antennas.

Hereafter, it is assumed that the receiver compensates for frequency and timing offsets in the received signals.

The receiver of FIG. 4 includes Cyclic Prefix (CP) eliminators 401, 411 and 421, Fast Fourier Transform (FFT) processors 403, 413 and 423, channel estimators 405, 415 and 425, a transmit signal candidate pre-compensator 430, and a Maximum Likelihood (ML) demodulator 440.

CP eliminators 401, 411 and 421 remove a guard interval CP from signals received on antennas 400, 410 and 420. FFT processors 403, 413 and 423 and convert a time-domain signal fed from CP eliminators 401, 411 and 421 to a frequency-domain signal.

Channel estimators 405, 415 and 425 estimate a channel using pilots in the signal provided from FFT processors 403, 413 and 423. For instance, in a PUSC subchannel using the CSM, transmitters of the MIMO-OFDM system put the pilot tones to alternate with each other in the time axis and the frequency axis as shown in FIG. 2B.

Channel estimators 405, 415 and 425 estimate a channel for one tile using a mean value with respect to the pilots of the transmitters based on Equation (2).

$\begin{array}{cc}{H}_{1,R,T}=\frac{1}{2}\left(Y_{R,T,1}+Y_{R,T,3}\right)H_{2,R,T}=\frac{1}{2}\left(Y_{R,T,2}+Y_{R,T,4}\right)& \left(2\right)\end{array}$

In Equation (2), H_k,R,T indicates an estimated channel value of T-th tile with respect to k-th transmitter and R-th antenna, and Y_R,T,i indicates a received signal of i-th pilot tone contained in T-th tile of R-th antenna of the transmitter. For example, when the tile is constituted as shown in FIG. 2B, transmitter 1 uses pilot tones #1 and #3 and transmitter 2 uses pilot tones #2 and #4.

The channel estimated based on Equation (2) can be expressed as a matrix represented by Equation (3).

$\begin{array}{cc}H=\left[\begin{array}{cc}{H}_{1,1,T}& H_{2,1,T}\\ H_{1,2,T}& H_{2,2,T}\\ ⋮& ⋮\\ H_{1,N_R,T}& H_{2,N_R,T}\end{array}\right]& \left(3\right)\end{array}$

In Equation (3), H_k,R,T indicates an estimated channel value of T-th tile with respect to k-th transmitter and R-th antenna. For example, when two transmitters include a single antenna communicating with a receiver including N_R-ary receive antennas as shown in FIG. 1, H has a matrix N_R×2.

Transmit signal candidate pre-compensator 430 compensates frequency and timing offsets of the transmit signal candidate group by estimating frequency and timing offsets with respect to the transmitters. For instance, transmit signal candidate pre-compensator 430 estimates the frequency and timing offsets of each transmitter using the pilot in a ranging signal channel or a Channel Quality Information (CQI) channel. Since the influence of the frequency and timing offsets in the transmitters can be modeled as the phase rotation which linearly increases in the time-axis index and the frequency-axis index, transmit signal candidate pre-compensator 430 compensates the frequency and timing offsets by phase-rotating the transmit signal candidate group.

Transmit signal candidate pre-compensator 430 can be constructed as shown in FIG. 5.

ML demodulator 440 demodulates the received signals using the channel estimate values fed from channel estimators 405, 415 and 425 and the transmit signal candidate with the compensated frequency and timing offsets fed from transmit signal candidate pre-compensator 430.

Transmit signal candidate pre-compensator 430 of FIG. 5 includes an offset estimator 501, a phase compensation value calculator 503, a transmit signal candidate storage 505, and a phase rotator 507.

Offset estimator 501 estimates frequency and timing offsets Δf_k and Δt_k of each transmitter using the pilot in the ranging signal channel or the CQI channel.

Phase compensation value calculator 503 calculates a phase compensation value for compensating the frequency and timing offsets of the transmit signal candidate group using the estimated values of the frequency and timing offsets of the transmitters which are provided from offset estimator 501. For instance, phase compensation value calculator 503 calculates the phase compensation value for every frequency index and every time index in the tile using the estimate values of the frequency and timing offsets of the transmitters. That is, phase compensation value calculator 503 calculates the phase compensation value for compensating the frequency and timing offsets of each tone in the tile based on Equation (4).

e ^{jθ k,t,s} =e ^{jΔf k {circle around (x)}t{circle around (x)}s} ·e ^{jΔf k {circle around (x)}t{circle around (×)}s}   (4)

In Equation (4), k indicates the transmitter index, t indicates the frequency-axis index of each tone in the tile, and s indicates the time-axis index of each tone in the tile. Hence, e_jθ^k,t,s indicates the phase compensation value of the tone having the frequency-axis index t and the time-axis index s k-th transmitter. Δf_k indicates a frequency offset of the k-th transmitter and Δt_k indicates a timing offset of the k-th transmitter.

Transmit signal candidate storage 505 stores a table of the transmit signal candidates according to a modulation scheme of the transmitters. The transmit signal candidate storage 505 selects the transmit signal candidate group

$\left[\begin{array}{c}{x}_i\\ x_j\end{array}\right]$

in the table and sends it to phase rotator 507.

Phase rotator 507 phase-rotates the transmit signal candidate group fed from transmit signal candidate storage 505 using the phase compensation value provided from phase compensation value calculator 503 based on Equation (5).

$\begin{array}{cc}{X}_{i,j}\left[\begin{array}{cc}{}^{{\mathrm{j\theta }}_1}& 0\\ 0& {}^{{\mathrm{j\theta }}_2}\end{array}\right]=\left[\begin{array}{c}{x}_i\\ x_j\end{array}\right]\left[\begin{array}{cc}{}^{{\mathrm{j\theta }}_1}& 0\\ 0& {}^{{\mathrm{j\theta }}_2}\end{array}\right]=\left[\begin{array}{c}{x}_i{}^{{\mathrm{j\theta }}_1}\\ x_j{}^{{\mathrm{j\theta }}_2}\end{array}\right]& \left(5\right)\end{array}$

In Equation (5), X_i,j indicates a transmit signal candidate vector indicative of a transmittable candidate i-th value of the first transmitter and a transmittable candidate j-th value of the second transmitter, and e^jθk indicates a phase compensation value of k-th transmitter.

Phase rotator 507 compensates the frequency and timing offsets of the transmitters by phase-rotating the transmit signal candidate groups of the transmitters using the phase compensation value produced at phase compensation value calculator 503 based on Equation (5).

As such, the receiver compensates for the frequency offset and the timing offset of the transmit signal candidate groups of the transmitters using transmit signal candidate pre-compensator 430.

ML demodulator 440 can demodulate the received signals using the ML scheme based on Equation (6).

$\begin{array}{cc}X+\mathrm{argmin}\left\{{Y-{\mathrm{HX}}_{i,j}\left[\begin{array}{cc}{}^{{\mathrm{j\theta }}_1}& 0\\ 0& {}^{{\mathrm{j\theta }}_2}\end{array}\right]}^2\right\}& \left(6\right)\end{array}$

In Equation (6), X indicates a final demodulated signal which is the value having the smallest error among all possible transmit signal candidate groups X_i,j, Y indicates a receive signal vector, and H indicates a radio channel response matrix. X_i,j indicates a transmit signal candidate vector indicative of a transmittable candidate i-th value of the first transmitter and a transmittable candidate j-th value of the second transmitter, and e^jθk indicates a phase compensation value of the k-th transmitter.

In FIG. 6, the receiver estimates the channel using the pilots of the signals received on the antennas in step 601. For instance, the receiver estimates the channel using the mean value of the pilots for each transmitter based on Equation (2).

After estimating the channel, the receiver calculates the phase compensation value for compensating the frequency and timing offsets of the frequency-axis index T and the time-axis index S in the tile in step 603. In more detail, the receiver calculates the phase compensation value for compensating the frequency and timing offsets by applying the frequency and timing offsets of each transmitter, which is estimated using the pilots in the ranging signal channel or the CQI channel, to Equation (4). Herein, T and S have ‘1’ as their initial value.

In step 605, the receiver compensates for the frequency offset and the timing offset by rotating the phase of the transmit signal candidate group of each transmitter using the calculated phase compensation value. For instance, the receiver compensates for the frequency and timing offsets by rotating the phase of the transmit signal candidate group using the phase compensation value based on Equation (5).

After compensating the frequency and timing offsets of the transmit signal candidate group, the receiver performs the ML demodulation with respect to the received signals using the channel estimate value and the transmit signal candidate group of the compensated frequency and timing offsets in step 607.

In step 609, the receiver compares T with the frequency axis magnitude N_T to check whether every tone of the frequency axis in the tile is ML-demodulated in step 609.

When T is smaller than N_T (T<N_T); that is, when the ML demodulation is not performed for every tone of the frequency axis, the receiver increases T by one stage (T=T+1) in step 611.

Next, the receiver calculates the phase compensation value of the tone having the frequency axis index T in step 603.

By contrast, when T is greater than or equal to N_T (T≧N_T ); that is, when the ML demodulation is performed on every tone of the frequency axis, the receiver compares S with a time axis magnitude N_S of the tile to check whether every tone of the time axis in the tile is ML-demodulated in step 613.

When S is smaller than N_S (S<N_S); that is, when the tones of the time axis are not ML-demodulated, the receiver increases S by one stage (S=S+1) in step 615.

Next, the receiver calculates the phase compensation value of the tone having the time axis index S in step 603.

By contrast, when S is greater than or equal to N_S (S≧N_S); that is, when the tones of the time axis are ML-demodulated, the receiver ends the process.

As set forth above, the frequency and timing offsets are compensated by rotating the phase of the transmit signal candidate group with the phase compensation value for the frequency and timing offset estimate values of each transmitter or each transmit antenna in the MIMO system. Therefore, by enhancing the demodulation performance, the service coverage of the cellular system can be expanded and the transmit power of the terminal can be saved. Additionally, the capacity can be increased by lowering the operating point of the high order modulation scheme.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as further defined by the appended claims.

Claims

1. A receiver in a communication system, comprising: one or more antennas for receiving one or more signals using the same radio resource;a channel estimator for estimating a channel using the received signals;a transmit signal candidate pre-compensator for compensating a frequency offset and a timing offset of transmit signal candidates by estimating a frequency offset and a timing offset of one or more transmit antennas; anda demodulator for demodulating the received signals using the estimated channel information and the transmit signal candidates of the compensated frequency and timing offsets.

2. The receiver of claim 1, wherein the channel estimator estimates the channel using pilots contained in the received signals.

3. The receiver of claim 1, wherein the transmit signal candidate pre-compensator comprises: an offset estimator for estimating the frequency offset and the timing offset of the transmit antennas;a phase compensation value calculator for calculating a phase compensation value of each tone in a tile using the estimated frequency and timing offsets;a storage for storing the transmit signal candidates according to a modulation scheme of the transmit antennas; andan offset compensator for compensating the frequency and timing offsets by rotating a phase of the transmit signal candidate with the calculated phase compensation value.

4. The receiver of claim 3, wherein the offset estimator estimates the frequency and timing offsets of the transmit antennas using a channel with respect to a sync acquisition signal.

5. The receiver of claim 4, wherein the sync acquisition signal is a ranging signal or Channel Quality Information (CQI).

6. The receiver of claim 3, wherein the offset compensator compensates the frequency and timing offsets of each transmit antenna by rotating a phase of a transmit signal candidate vector using a phase compensation vector of each tone which is calculated at the phase compensation value calculator.

7. The receiver of claim 3, wherein the offset compensator compensates the frequency and timing offsets by phase-rotating the transmit signal candidates based on the following equation:

8. The receiver of claim 1, wherein the demodulator is a Maximum Likelihood (ML) demodulator.

9. The receiver of claim 8, wherein the ML demodulator demodulates the received signals based on the following equation:

10. A method for compensating a frequency offset and a timing offset in a Multiple Input Multiple Output (MIMO) system, the method comprising: estimating a channel using signals received on one or more receive antennas using the same radio resource;compensating for a frequency offset and a timing offset of transmit signal candidates by estimating a frequency offset and a timing offset of one or more transmit antennas; anddemodulating the received signals using the estimated channel information and the transmit signal candidates of the compensated frequency and timing offsets.

11. The method of claim 10, wherein the channel estimating step comprises: estimating the channel using pilots contained in the received signals.

12. The method of claim 10, wherein the frequency and timing offset compensating step comprises: estimating the frequency offset and the timing offset of the transmit antennas;calculating a phase compensation value of each tone in a tile using the estimated frequency and timing offsets; andcompensating the frequency and timing offsets by rotating a phase of the transmit signal candidates with the calculated phase compensation value.

13. The method of claim 12, wherein the frequency and timing offsets are estimated using a channel relating to a sync acquisition signal.

14. The method of claim 13, wherein the sync acquisition signal is a ranging signal or Channel Quality Information (CQI).

15. The method of claim 12, wherein the frequency and timing offset compensating step comprises: compensating the frequency and timing offsets of each transmit antenna by rotating a phase of a transmit signal candidate vector using the calculated phase compensation vector of each tone.

16. The method of claim 12, wherein the frequency and timing offsets are compensated by phase-rotating the transmit signal candidates based on the following equation:

17. The method of claim 10, wherein the demodulation uses a Maximum Likelihood (ML) scheme.

18. The method of claim 17, wherein the ML demodulation demodulates the signal based on the following equation:

19. An apparatus for compensating a frequency offset and a timing offset in a communication system, the apparatus comprising: means for estimating a channel using signals received on one or more receive antennas using the same radio resource;means for compensating for a frequency offset and a timing offset of transmit signal candidates by estimating a frequency offset and a timing offset of one or more transmit antennas; andmeans for demodulating the received signals using the estimated channel information and the transmit signal candidates of the compensated frequency and timing offsets.