WO2009107078A1

WO2009107078A1 - Methods, systems and arrangements for frequency shift compensation

Info

Publication number: WO2009107078A1
Application number: PCT/IB2009/050765
Authority: WO
Inventors: Junling Zhang
Original assignee: Nxp B.V.
Priority date: 2008-02-25
Filing date: 2009-02-25
Publication date: 2009-09-03

Abstract

Wireless communications devices, methods and systems are implemented in various fashions. According to one such method that is for use with an OFDM-based wireless communications system that is subject to time variation due to Doppler frequency shift, the following steps are implemented. A model of a channel of the system is generated from estimations of the number of multi-paths in the system (202) and their delays (204). Channel impulse responses are estimated for a first received symbol using first received pilot signals and the model and for a subsequently received symbol using subsequently received pilot signals and the model (212, 214). The time variation for the system is estimated as a linear function derived from the estimated channel impulse responses (216). Doppler frequency shift is compensated for using the estimated time variation (218).

Description

METHODS, SYSTEMS AND ARRANGEMENTS FOR FREQUENCY SHIFT COMPENSATION

The present invention relates generally to orthogonal-frequency-division multiplexing (OFDM), and more specifically, to mitigating frequency shift in OFDM channels.

Wireless communication systems are used in an ever growing number of applications. Many of such applications use high-bandwidth transmissions to convey large amounts of data. The increase in bandwidth is often at odds with a number of constraints, such as limited spectrum bandwidth, multi-path channels and radio- frequency interference. Orthogonal Frequency-Division Multiplexing (OFDM) is an example of one type of communicaiton system that addresses these and other issues. OFDM uses a large number of sub-carriers to carry data. While the sub- carriers can overlap in frequency, they are selected to be orthogonal to one another in order to avoid interference with each other. The orthogonality means that the spectrum of each carrier has a null at the centre frequency of each of the other carriers in the system due to each carrier having an integer number of cycles over a symbol period. As such, an ideal OFDM system sees no interference between the carriers, allowing them to be spaced as closely as theoretically possible. A modulation scheme, such as quadrature amplitude modulation (QAM) or quadrature phase-shift keying (QPSK), is implemented on each sub-carrier. Due to the high number of sub- carriers, high-throughput is possible with a relatively low symbol rate for each individual sub-carrier. In many instances data rates similar or greater than a single- carrier modulation scheme are possible even within the same bandwidth.

For these and other reasons OFDM systems are sometimes used in terrestrial broadcasting, such as digital television, where there are multi-path-channels causing the multiple versions of the signal to interfere with each other and to degrade the quality of the received signal. OFDM systems are often selected for such applications because of their high spectral efficiency, low multi-path distortion and resiliency to outside interference. Moreover, OFDM systems generally employ relatively simple channel-equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate allows for the use of a guard interval between symbols. The guard symbols make the systems less susceptible to time-spreading and inter-symbol interference (ISI).

OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access. While OFDM offers many advantages, there is room for improvement. For instance, mobile applications can be negatively affected by Doppler frequency shift. Such time-variation of multipath channels over an OFDM symbol leads to the loss of subchannel orthogonality which causes neighboring channels of the OFDM symbol to interfere with one another through what is sometimes called inter-carrier interference (ICI). Thus, the loss of orthogonality results in an increase in error floor due to the Doppler frequency. The ICI increases as the OFDM symbol duration increases and/or the vehicle speed increases. Various methods have been implemented attempting to compensate for ICI.

In one such method, the time variation of the channel impulse response (CIR) is linearized to reduce the complexity of channel estimation and the sequential ICI compensation. The time-variant multipath channel is first linearized and then estimated by the special pilot symbols comprised of a PN sequence. This method is not feasible, however, for some OFDM systems that do not contain special pilot symbols (e.g., Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB- T/H) or Satellite Terrestrial Interactive Multi-service Infrastructure (STiMi)). Another method involves the use of an iterative ICI cancellation method for DVB-T systems. This iterative method, however, uses a complex maximum-likelihood channel estimator.

Various aspects of the present invention are directed to a method for use in an OFDM-based wireless communications system subject to time variation due to Doppler frequency shift. A model is generated of a channel of the system. The model uses an estimation of the number of multi-paths in the system and of the delay of the multi-paths in the system. A channel impulse response for a first received symbol is estimated for using first received pilot signals and the model. A channel impulse response for a subsequently received symbol is estimated for using subsequently received pilot signals and the model. The time variation of a channel impulse response is estimated for as a linear function that is derived from the estimated channel impulse responses of the first and subsequent symbols. Doppler frequency shift in the first symbol and subsequently received symbol is compensated for using the estimated time variation.

Consistent with another embodiment of the present invention, a receiver is implemented for use in an OFDM-based wireless communications system subject to time variation due to Doppler frequency shift. The receiver includes a processor arrangement. The processor arrangement is configured to generate a model of a channel of the system by estimating the number of multi-paths in the system and estimating the delay of the multi-paths in the system. The processor arrangement estimates a channel impulse response for a first received symbol using first received pilot signals and the model and estimates a channel impulse response for a subsequently received symbol using subsequently received pilot signals and the model. The processor arrangement also estimates the time variation of a channel impulse response as a linear function derived from the estimated channel impulse responses of the first and subsequent symbols. Doppler frequency shift in the first symbol and subsequently received symbol is compensated for using the estimated time variation.

Consistent with another embodiment of the present invention, an OFDM- based wireless communications system is implemented that is subject to time variation due to Doppler frequency shift. A transmitter transmits an OFDM-based transmission. A receiver receives the OFDM-based transmission and includes a processor arrangement. The processor arrangement generates a model of a channel of the system by estimating the number of multi-paths in the system and by estimating the delay of the multi-paths in the system. The processor arrangement then estimates a channel impulse response for a first received symbol using first received pilot signals and the model and a channel impulse response for a subsequently received symbol using subsequently received pilot signals and the model. The time variation of a channel impulse response is estimated as a linear function derived from the estimated channel impulse responses of the first and subsequent symbols. Doppler frequency shift in the first symbol and subsequently received symbol are compensated for using the estimated time variation.

The above summary is not intended to describe each embodiment or every implementation of the present disclosure. The figures and detailed description that follow more particularly exemplify various embodiments. The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. IA shows a block diagram for an OFDM system, according to an example embodiment of the present invention;

FIG. IB shows a block diagram of a channel estimation system, according to an example embodiment of the present invention; and

FIG. 2 shows a block diagram of an OFDM-based system, according to an example embodiment of the present invention. While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention including aspects defined by the appended claims.

The present invention is believed to be applicable to a variety of different types of processes, devices and arrangements for use with Orthogonal Frequency- Division Multiplexing (OFDM), and in particular, to approaches to compensating for Doppler frequency shift. While the present invention is not necessarily so limited, various aspects of the invention may be appreciated through a discussion of examples using this context.

Embodiments of the present invention relate to a method for compensating for Doppler frequency shift across channels of an OFDM system that uses subcarriers on the channels. A model of the channels is generated from knowledge of channel parameters, such as the number of multi-paths and their delays. In a particular instance, the model is generated infrequently using pilot subcarriers to determine the parameters. The model is used to estimate the channel-impulse response (CIR) of the channels. The Doppler frequency shift is assumed to be linear over two consecutive symbols. Thus, the estimated frequency shift parameter is calculated by applying the received pilot subcarriers for the two consecutive symbols to the model. The estimated frequency shift parameter is then used to compensate for Doppler frequency shift. In a specific embodiment, an estimated frequency shift parameter is calculated for each subcarrier and multi-path thereof. FIG. IA shows a block diagram for an OFDM system, according to an example embodiment of the present invention. The OFDM system of FIG. IA includes OFDM transmitter/encoder 100, channel 110 and receiver/decoder 118. The transmitter 100 sends data across channel 110 using an OFDM signal. The OFDM signal is affected by various channel properties before being received by receiver/decoder 118. Receiver/decoder 118 interprets the received signal by decoding the OFDM signal and also by compensating for the channel properties.

Transmitter/encoder 100 formats the data according to the particular OFDM protocol. Example protocols include, but are not limited to, Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB-T/H) and Satellite Terrestrial Interactive Multi-service Infrastructure (STiMi). Serial-to-parallel converter 102 converts a serial data stream into multiple parallel data streams using, for example, data interleaving techniques. Modulator 104 modulates the parallel data streams according to the desired modulation scheme (e.g., quadrature amplitude modulation (QAM) or quadrature phase-shift keying (QPSK)). Inverse fast-Fourier transform (IFFT) block 106 transforms the modulated data streams onto the subcarriers according to the OFDM protocol. Parallel-to-serial converter 108 merges the subcarriers for transmission by an antenna.

Channel 110 represents modifications to the signal from transmitter 100 due to the medium over which the signal is transmitted and other factors including, but not limited to, noise from external sources and Doppler frequency shift. In particular, FIG. IA shows that the signal from transmitter 100 is affected by multi-path effects 114, Doppler-frequency shift 116 and Gaussian noise 112. These and other properties of the channel ultimately determine what signal is received at the receiver 118. Receiver 118 decodes the received signal by reversing the changes to the input data due to both the transmitter 100 and channel 110. More specifically, serial-to- parallel converter 120 separates the received OFDM signal into the subcarriers. Fast- Fourier-Transform (FFT) block 122 transforms the subcarriers into the time domain. Channel estimation block 124 attempts to estimate the effects of channel 110 on the transmitted signal and compensates for the estimated effects. Demodulation block 126 demodulates the signal to produce parallel data streams. Parallel-to-serial converter 128 reverses the interleaving of data performed by Serial-to-parallel converter 102 to produce a useful serial data stream. FIG. IA is merely an exemplary OFDM system, and for simplicity, some details of process are not shown. The skilled artisan would recognize that various other implementations of OFDM encoding and decoding schemes are also possible, as without consideration for the effects of channel 110 on the OFDM signal, OFDM processing is often relatively trivial. Channel estimation, however, can require large amounts of computational power. For example, the large number of subcarriers and/or multi-paths can significantly contribute to the complexity of any channel estimation.

FIG. IB shows a block diagram of a channel estimation system, according to an example embodiment of the present invention. Process steps 200 are such that they can be computed infrequently/offline relative to the received signals. This can be particularly useful for reducing the complexity of the real-time processing. Process steps 208 are performed frequently (e.g., for each received symbol).

Process steps 200 are implemented to generate a model to be used by the real- time process steps 208. An example of this model uses a matrix that is derived from channel properties that are relatively static. One such channel property is the number of multi-paths, which is estimated at block 202. Another relatively static channel property is the delay of each multipath, which is estimated at block 204. These properties can be estimated, for example, using an Inverse-Discrete Fourier Transform (IDFT) method. Another channel property (not shown) is that the number of pilot symbols is assumed to be greater than the number of multi-paths. These properties are used to generate a model as shown at block 206. A specific example of this model is discussed in more detail below with regards to the inverse matrix of (12).

Real-time process steps 208 begin as the OFDM signal is received (block 210). The pilot subcarriers in two (consecutive) symbols are used with model from block 206 to estimate the time-variation for each symbol (blocks 210 and 212, respectively). The estimated time variations are then used to estimate the time- variation (e.g., due to Doppler frequency shift) across both symbols (block 214). The estimation can be implemented as a linear function, where a separate estimation is determined for each multi-path in the system. A specific example of this linear determination is discussed in more detail below (see, e.g., h,^'(i) , ti,(i + Y) and a, in (15)). FIG. 2 shows a block diagram of an OFDM-based system, according to an example embodiment of the present invention. Transmitter 300 converts data into an OFDM signal for transmission over channel 320. In particular, a suitable encoder 302 converts the data into an electrical signal corresponding to the various OFDM sub- carrier frequencies. Antenna 304 converts the electrical signal from encoder 302 into an electromagnetic wave that is transmitted over channel 320. Channel 320 may distort the electromagnetic wave and the data carried therein (e.g., multi-path effects, Doppler-frequency shift or Gaussian noise).

Receiver 330 uses antenna 306 to convert the transmitted electromagnetic wave back into an electrical signal. Due to the distortion of the transmitted signal from channel 320, the electrical signal is not identical to the electrical signal that was sent to antenna 304. The receiver 330 first uses a front-end conditioning circuit/processor 308. Conditioning circuit 308 can vary depending upon the particular OFDM protocol implemented. In a specific example, conditioning circuit 308 includes a filter circuit, low-noise amplifier(s) (LNA), a serial-to-parallel converter and fast-Fourier transforms.

To compensate for the distortion of the transmitted signal from channel 320, processor arrangement 310 is arranged and configured to estimate the properties of channel 320 and to adjust the received signal accordingly. If so desired, processor arrangement can use known channel estimation and compensation techniques. Regardless, processor arrangement 310 uses various embodiments of the present invention to estimate and compensate for Doppler frequency shift due to movement of receiver 306 and/or transmitter 300. Processor arrangement generates a model to represent various properties of channel 320. The generated model can be stored in memory/database 314. Through use of embodiments of the present invention, the generation of the model can be accomplished infrequently. Specifically, the model is generated using properties of channel 320 that are relatively static. Processor arrangement 310 receives multiple symbols and estimates respective channel impulse responses by applying properties of the received symbols to the model. Processor arrangement 310 estimates a Doppler frequency shift for the channel, including estimations for multi-paths therein, by approximating the Doppler frequency shift as a linear function that corresponds to the estimated responses. Processor arrangement 310 uses estimated Doppler frequency shift response as a compensation for actual Doppler frequency shift of the channel. The resulting conditioned and compensated signal is then transformed into a serial data stream using back-end/decoding block 312. This decoding varies according to the specific type of encoding scheme/protocol implemented. Two example protocols are quadrature amplitude modulation (QAM) and quadrature phase-shift keying (QPSK). The resulting data stream can be processed, stored or used by the application.

In one instance, processing arrangement can be implemented using one or more processors programmed with software. In another instance, programmable logic can be used to implement one or more of the processing functions. Various hardware logic devices and circuits can also be used to implement various processing functions. In other instances, processors, software, programmable logic devices and hardware circuits can be used in various combinations to accomplish the functionality discussed herein.

The following discussion outlines a specific implementation of the invention. Variations on the specific algorithms and methods that follow are possible without departing from the spirit of the invention. The algorithm can be applied by recognizing that the linearization of the time variation of CIR can be used over 9.0e-4 seconds where the Doppler frequency is around 300 Hz or less. In a STiMi system, for example, the time duration of one OFDM symbol, which includes a guard interval and a cyclic prefix, is 4.632e-4 seconds. Accordingly, the linearization of the time variation of the CIR can be used for two consecutive OFDM symbols where the Doppler frequency is around 300 Hz or less. Using a similar comparison, this ratio can be applied for other Doppler frequencies and OFDM symbol lengths.

The time domain transmitted OFDM signal at the «-th time of the z^'-th OFDM symbol is

N-I 2π j m n x(n,ϊ) = x(n + i- N_s) = —_j=^'∑X(m,i) -e ^N

V N m=0

(1)

Where N is the size of FFT, X(m, ϊ) is the frequency domain transmitted signal at the m-th subcarrier of the z^'-th OFDM symbol. The time domain CIR at the rø-th time of the Mh OFDM symbol is h(n,i) = Y £-1h,(n + i- N_s) - δ(n -τ_ι)

1=0

(2) Where L is the number of multipath, and H₁ (n + i-N_s) is the time domain CIR of the /-th path at the «-th time of the /-OFDM symbol with N_s = N + N_GI denoting the total length of an OFDM symbol including the length of guard interval N_G/ . Assuming h_t(-) of (2) will change linearly in two consecutive OFDM symbols, i.e., h_ι(n + i-N_s) = h_r(l + a_rri),VnG[n_s,n_s+2-N_s),n_s≥0 (3)

Then, the received signal after channel is

L-I r(n,i) = ∑h,(n + i -N_s)-X(K-T₁)

1=0 (4)

Note the AWGN component is ignored in (4) to simplify the mathematics below.

Apply N-point FFT on the both sides of (4)

(5)

Where the first item on the right side is the useful signal R(m,i) \_useful , and the second item is the inter-carrier interference, N_{o ICI} (m, i) caused by time-variant multipath channel (2).

The useful signal of (5) can be rewritten as

= X(m,i)-H(m,i)

(6) Where

2π

= ^:∑∑h_ι(i- N_s)- (\ + a_rn) -e -^JΥ^{m τ}'

-* * 1=0 n=0

L-I 2π ϊ— ϊ /V — - / ™ r,

= ∑h_ι(i. N_s) -(\ + a_ι ^-) -e ^N

1=0

L-I 2π

= ∑>^'0> JV '

1=0

(7) Where h_l ^'(f) = h_l(i- N_s)- (l + a_l ^-)

(8)

The transmitted signal X(m,ϊ) is known to the receiver over the pilot subcarriers, thus the frequency domain CIR can be estimated by RO, 0

HO, o =

XO, O

= H(m,i) + N_o ^' _ICI(m,i)

(9) Ignore the ICI in (9) and substitute (7) into (9)

H(m,i) = ∑h_ι(i)-e ^» ι=o

(10)

The total number of multipath L and the delay of each multipath T₁ are estimated by, for example, IDFT method. Recognizing that these parameters are fixed for a relatively long time, L and T₁ are assumed to be known in (10). Another assumption is that the number pilot in one OFDM symbol is larger than the number of multi-path L. Namely, there are at least L independent equations of (10) with respect to the variable m to solve each H₁(P) , 0 ≤ / < L -1. (10), written in matrix format, is:

(H)

Thus Ii₁(V) can be solved as:

(12)

In (12), because Z and r, are already known and m_p,p = 0,l,---,L-l can be chosen over pilots offline, the matrix inverse of (12) can be pre-calculated as well. In the m^th OFDM symbol, i;(i + l) = A₍((i + l).f_s).(l + α₍— ), 0≤/<L-l (13) can be solved as well by using the same assumptions above. Furthermore, considering the assumption (3) in (8) and (13), the first item on the right side of (13) can be rewritten as

MQ + iy N_s) = H₁Q-N_Sy(Ua₁-N₃) (14)

Substitute (14) into (13): h](i + l) = h₁((i + l)-N_s)-(l + a₁^-)

= (l + a_rN_s)-h₁(i-N_s)-(l +

= (l + a₁-N_s)-h₁ ^'(i)

(15) Since both h_t (i) and h_t (i + 1) are known in (15), each a_t , 0 ≤ / < L -X can be solved, and then each H₁ (i ^■ N_s ) and H₁ ((i + Y) ^■ N_s ) , 0 ≤ / < L - 1 can be solved in (8) and (13), respectively. Accordingly, all parameters for the time-variant multipath channel (2) have been solved.

A significant computation load for above algorithm involves solving H₁(V)

(and h,(i + Y) ) through (12). This computational load can be offset by calculating the matrix inverse in (12) offline or infrequently (e.g., during initialization, periodically or over couples of OFDM symbols). While the present invention has been described above and in the claims that follow, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention.

Claims

What is Claimed is:

1. A method for use in an OFDM-based wireless communications system subject to time variation due to Doppler frequency shift, the method comprising: generating a model of a channel of the system (206) by estimating the number of multi-paths in the system (202) and estimating the delay of the multi -paths in the system (204); estimating a channel impulse response for a first received symbol using first received pilot signals and the model (212); estimating a channel impulse response for a subsequently received symbol using subsequently received pilot signals and the model (214); estimating a time variation for the system as a linear function derived from the estimated channel impulse responses of the first symbol and subsequently received symbol, respectively (216), and compensating for Doppler frequency shift in the first symbol and subsequently received symbol using the estimated time variation (218).

2. The method of claim 1, wherein estimating a channel impulse response for a first received symbol includes using the model to estimate the first received pilot signals and comparing the estimation of the received pilot symbol to the first received pilot signal.

3. The method of claim 3, wherein the step of generating the model occurs relatively infrequently with respect to the frequency of estimating the time variation.

4. The method of claim 1, wherein the OFDM-based wireless communications system does not use pseudorandom number (PN) sequences.

5. The method of claim 1, wherein the Doppler frequency shift is less than about

300 Hz and a length of symbols used in the OFDM-based wireless communications system are less than about 5e^~4 seconds.

6. The method of claim 1, wherein the first received symbol and the subsequently received symbol are consecutively received symbols.

7. The method of claim 6, wherein the estimated time variation is applied across the consecutively received symbols.

8. For use in an OFDM-based wireless communications system subject to time variation due to Doppler frequency shift, a receiver comprising: a processor arrangement (310) for generating a model (31) of a channel of the system by estimating the number of multi-paths in the system (202) and estimating the delay of the multi-paths in the system (204), and for estimating a channel impulse response for a first received symbol using first received pilot signals and the model (212); estimating a channel impulse response for a subsequently received symbol using subsequently received pilot signals and the model (214); estimating a time variation for the system as a linear function derived from the estimated channel impulse responses of the first symbol and subsequently received symbol, respectively (216), and compensating for Doppler frequency shift in the first symbol and subsequently received symbol using the estimated time variation (218).

9. The receiver of claim 8, wherein estimating a channel impulse response for a first received symbol includes using the model to estimate the first received pilot signals and comparing the estimation of the received pilot symbol to the first received pilot signal.

10. The receiver of claim 8, wherein the step of generating the model occurs relatively infrequently with respect to the frequency of estimating the time variation.

11. The receiver of claim 8, wherein the OFDM-based wireless communications system does not use pseudorandom number (PN) sequences.

12. The receiver of claim 8, wherein the Doppler frequency shift is less than about

13. The receiver of claim 8, wherein the first received symbol and the subsequently received symbol are consecutively received symbols.

14. The receiver of claim 13, wherein the estimated time variation is applied across the consecutively received symbols.

15. An OFDM-based wireless communications system subject to time variation due to Doppler frequency shift, the system comprising: a transmitter for transmitting an OFDM-based transmission (300); a receiver for receiving the OFDM-based transmission (330) and including a processor arrangement (310) for generating a model of a channel of the system by estimating the number of multi -paths in the system (202) and estimating the delay of the multi-paths in the system (204), and for estimating a channel impulse response for a first received symbol using first received pilot signals and the model (212); estimating a channel impulse response for a subsequently received symbol using subsequently received pilot signals and the model (214); estimating a time variation for the system as a linear function derived from the estimated channel impulse responses of first symbol and subsequently received symbol, respectively (216), and compensating for Doppler frequency shift in the first symbol and subsequently received symbol using the estimated time variation (218).

16. The system of claim 15, wherein estimating a channel impulse response for a first received symbol includes using the model to estimate the first received pilot signals and comparing the estimation of the received pilot symbol to the first received pilot signal.

17. The system of claim 15, wherein the step of generating the model occurs relatively infrequently with respect to the frequency of estimating the time variation.

18. The system of claim 15, wherein the OFDM-based wireless communications system does not use pseudorandom number (PN) sequences.

19. The system of claim 15, wherein the Doppler frequency shift is less than about 300 Hz and a length of symbols used in the OFDM-based wireless communications system are less than about 5e^~4 seconds.

20. The system of claim 15, wherein the first received symbol and the subsequently received symbol are consecutively received symbols.

21. The system of claim 20, wherein the estimated time variation is applied across the consecutively received symbols.