Abstract:
An Orthogonal Frequency Division Multiplexing (OFDM) receiver detects and corrects sampling offsets in the time domain. The OFDM receiver oversamples a training sequence or symbol in a received OFDM signal, correlates the oversampled training sequence with a stored copy of a truncated version of the training sequence, locates a correlation peak, and derives a sampling offset by calculating a difference in magnitude of correlation samples in the vicinity of the correlation peak.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals. 
     BACKGROUND OF THE INVENTION 
     Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. 
     The transmission of data through a channel via OFDM signals provides several advantages over more conventional transmission techniques. One advantage is a tolerance to multipath delay spread. This tolerance is due to the relatively long symbol interval Ts compared to the typical time duration of the channel impulse response. These long symbol intervals prevent inter-symbol interference (ISI). Another advantage is a tolerance to frequency selective fading. By including redundancy in the OFDM signal, data encoded onto fading sub-carriers can be reconstructed from the data recovered from the other sub-carriers. Yet another advantage is efficient spectrum usage. Since OFDM subcarriers are placed in very close proximity to one another without the need to leave unused frequency space between them, OFDM can efficiently fill a channel. A further advantage is simplified sub-channel equalization. OFDM shifts channel equalization from the time domain (as in single carrier transmission systems) to the frequency domain where a bank of simple one-tap equalizers can individually adjust for the phase and amplitude distortion of each sub-channel. Yet another advantage is good interference properties. It is possible to modify the OFDM spectrum to account for the distribution of power of an interfering signal. Also, it is possible to reduce out-of-band interference by avoiding the use of OFDM sub-carriers near the channel bandwidth edges. 
     Although OFDM exhibits these advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. One difficulty is the issue of synchronizing the transmitter&#39;s sample rate to the receiver&#39;s sample rate to eliminate sampling rate offset. Any mis-match between these two sampling rates results in a rotation of the 2 m -ary sub-symbol constellation from symbol to symbol in a frame for smaller frequency offsets. However, for larger frequency offsets, the result is a contraction or expansion of the frequency spectrum of the received signal. Both of these can contribute to increased BER. One cause of sampling rate offset is the presence of a sampling frequency offset. A sampling frequency offset occurs when the receiver samples the received signal at a frequency that is either higher or lower than the sample rate used at the transmitter. Another cause of sampling rate offset is the presence of a sampling phase offset. A sampling phase offset occurs when the receiver samples the received signal at a phase offset from sample rate of the transmitter. Both the sampling frequency and sampling phase offsets can be detrimental to the performance of the receiver, and must be corrected for in order for the receiver to be properly synchronized. The present invention is directed to the correction of this problem. 
     SUMMARY OF THE INVENTION 
     An Orthogonal Frequency Division Multiplexing (OFDM) receiver detects and corrects sampling offsets in the time domain. The OFDM receiver oversamples a training sequence or symbol in a received OFDM signal, correlates the oversampled training sequence with a stored copy of a truncated version of the training sequence, locates a correlation peak, and derives a sampling offset by calculating a difference in magnitude of correlation samples in the vicinity of the correlation peak. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram of a conventional OFDM receiver; 
     FIG. 2 illustrates a typical arrangement of OFDM symbols and their corresponding guard intervals within a data frame; 
     FIG. 3 is a block diagram of an exemplary sampling offset correction system of the present invention; 
     FIG. 4 is an illustration of a correlation power peak when there is a sampling offset (phase and/or frequency); 
     FIG. 5 is a block diagram illustrating the present invention as integrated with the conventional OFDM receiver of FIG. 1; 
     FIG. 6 is a diagram of an exemplary training sequence in the frequency domain; and 
     FIG. 7 is a time domain representation of the training sequence of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The characteristics and advantages of the present invention will become more apparent from the following description, given by way of example. 
     Referring to FIG. 1, the first element of a typical OFDM receiver  10  is an RF receiver  12 . Many variations of RF receiver  12  exist and are well known in the art, but typically, RF receiver  12  includes an antenna  14 , a low noise amplifier (LNA)  16 , an RF bandpass filter  18 , an automatic gain control (AGC) circuit  20 , an RF mixer  22 , an RF carrier frequency local oscillator  24 , and an IF bandpass filter  26 . 
     Through antenna  14 , RF receiver  12  couples in the RF OFDM-modulated carrier after it passes through the channel. Then, by mixing it with a receiver carrier of frequency f cr  generated by RF local oscillator  24 , RF receiver  12  downconverts the RF OFDM-modulated carrier to obtain a received IF OFDM signal. The frequency difference between the receive carrier and the transmit carrier contributes to the carrier frequency offset, delta f c . 
     This received IF OFDM signal then feeds into both mixer  28  and mixer  30  to be mixed with an in-phase IF signal and a 90° phase-shifted (quadrature) IF signal, respectively, to produce in-phase and quadrature OFDM signals, respectively. The in-phase IF signal that feeds into mixer  28  is produced by an IF local oscillator  32 . The 90° phase-shifted IF signal that feeds into mixer  30  is derived from the in-phase IF signal of IF local oscillator  32  by passing the in-phase IF signal through a 90° phase shifter  34  before feeding it to mixer  30 . 
     The in-phase and quadrature OFDM signals then pass into analog-to-digital converters (ADCs)  36  and  38 , respectively, where they are digitized at a sampling rate f ck     —     r  as determined by a clock circuit  40 . ADCs  36  and  38  produce digital samples that form an in-phase and a quadrature discrete-time OFDM signal, respectively. The difference between the sampling rates of the receiver and that of the transmitter is the sampling rate offset, delta f ck =f ck     —     r −f ck     —     t . 
     The unfiltered in-phase and quadrature discrete-time OFDM signals from ADCs  36  and  38  then pass through digital low-pass filters  42  and  44 , respectively. The output of lowpass digital filters  42  and  44  are filtered in-phase and quadrature samples, respectively, of the received OFDM signal. In this way, the received OFDM signal is converted into in-phase (q i ) and quadrature (p i ) samples that represent the real and imaginary-valued components, respectively, of the complex-valued OFDM signal, r i =q i +jp i . These in-phase and quadrature (real-valued and imaginary-valued) samples of the received OFDM signal are then delivered to DSP  46 . Note that in some conventional implementations of receiver  10 , the analog-to-digital conversion is done before the IF mixing process. In such an implementation, the mixing process involves the use of digital mixers and a digital frequency synthesizer. Also note that in many conventional implementations of receiver  10 , the digital-to-analog conversion is performed after the filtering. 
     DSP  46  performs a variety of operations on the in-phase and quadrature samples of the received OFDM signal. These operations may include: a) synchronizing receiver  10  to the timing of the symbols and data frames within the received OFDM signal, b) removing the cyclic prefixes from the received OFDM signal, c) computing the discrete Fourier transform (DFT) or preferably the fast Fourier transform (FFT) of the received OFDM signal in order to recover the sequences of frequency-domain sub-symbols that were used to modulate the sub-carriers during each OFDM symbol interval, d),performing any required channel equalization on the sub-carriers, and e) computing a sequence of frequency-domain sub-symbols, y k , from each symbol of the OFDM signal by demodulating the sub-carriers of the OFDM signal by means of the FFT calculation. DSP  46  then delivers these sequences of sub-symbols to a decoder  48 . 
     Decoder  48  recovers the transmitted data bits from the sequences of frequency-domain sub-symbols that are delivered to it from DSP  46 . This recovery is performed by decoding the frequency-domain sub-symbols to obtain a stream of data bits which should ideally match the stream of data bits that were fed into the OFDM transmitter. This decoding process can include soft Viterbi decoding and/or Reed-Solomon decoding, for example, to recover the data from the block and/or convolutionally encoded sub-symbols. 
     In a typical OFDM data transmission system such as one for implementing digital television or a wireless local area network (WLAN), data is transmitted in the OFDM signal in groups of symbols known as data frames. This concept is shown in FIG. 2 where a data frame  50  includes M consecutive symbols  52   a ,  52   b , . . . ,  52 M, each of which includes a guard interval, T g , as well as the OFDM symbol interval, Ts. Therefore, each symbol has a total duration of T g +T s  seconds. Depending on the application, data frames can be transmitted continuously, such as in the broadcast of digital TV, or data frames can be transmitted at random times in bursts, such as in the implementation of a WLAN. 
     Referring now to FIG. 3, an exemplary embodiment of the present invention is shown. The FIG. 3 arrangement may be employed in the receiver of FIG. 1, as illustrated in FIG.  5 . However, the present invention is illustrated as a distinct sampling offset correction loop for clarity, ease of reference, and to facilitate an understanding of the present invention. 
     The present invention operates in a receiver that conforms to the proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless LAN standards, herein incorporated by reference. However, it is considered within the skill of one skilled in the art to implement the teachings of the present invention in other OFDM systems. 
     The above-identified wireless LAN standards propose the use of a training sequence for detection of OFDM transmissions. The training sequence (e.g., training sequence A or B) includes a series of short OFDM training symbols (having known amplitudes and phases) that are transmitted over a pre-determined number of pilot sub-carriers or bins (e.g., 12 pilot sub-carriers). All the other sub-carriers (e.g., 52 sub-carriers) remain at zero during the transmission of the training sequence. Although use of the training sequence of the above-identified LAN standards is discussed, use of alternative training sequences and symbols is considered within the scope of the invention as defined by the appended claims. Frequency domain and time domain representations of an exemplary training sequence B of HIPERLAN/2 are shown in FIGS. 5 and 6. As shown in FIG. 6, the training sequence has a block of 16 samples that is repeated 4 times per training symbol. This repetitive block or time period is utilized by the present invention, as discussed in further detail below. 
     Returning now to FIG. 3, a sampling offset correction system  60  is shown. It should be noted that system  60  may be embodied in software, hardware, or some combination thereof. A pair of samplers (e.g., ADCs)  62  and  78  sample a received OFDM signal. As discussed above, the received OFDM signal contains in-phase (q i ) and quadrature (p i ) portions that represent the real and imaginary-valued components, respectively, of the complex-valued OFDM signal, r i =q i +jp i . Sampler  78  samples the OFDM signal at a given sample rate (selected to be near the sampling rate of the transmitter) and passes the sampled OFDM signal through a sampling rate converter  76  for downstream processing (e.g., FFT and the like), as discussed in further detail below. Sampler  62  upsamples or oversamples the received OFDM signal by a predetermined factor (e.g., a factor of 2) and passes the upsampled signal to a correlator module  64 . Oversampling the received OFDM signal provides a resolution of the OFDM signal that is necessary to derive a meaningful error, as discussed in further detail below. It should be noted that sampler  78  and sampler  62  may be interconnected in a number of different ways, as known by one skilled in the art. For example, sampler  78  and sampler  62  may be driven by a clock circuit (not shown) that drives both samplers  78  and  62  to oversample the OFDM signal by a factor of 2. In this case, sampler  62  would pass every sample to a correlator module  64  and sampler  78  would pass every other sample to sampling rate converter  76 . 
     Correlator module  64  correlates the upsampled signal received from sampler  62  with time-domain samples of the training sequence (e.g., training sequence B of the above-mentioned wireless standards) stored in a local memory  66 . Each sample in the exemplary training sequence has a value of sqrt(13/6)*[(1+j) or (−1−j)]. The memory allocated for storing each sample value will depend on the design of a particular OFDM receiver. The stored version of the training sequence is, preferably, a truncated version of the training sequence corresponding to one of the repetitive blocks of samples (e.g., 16 samples) of training sequence B. More specifically, the stored version of the truncated training sequence, preferably, corresponds to an oversampled version (e.g., 32 samples) of the repetitive block that is oversampled by the same predetermined factor (e.g., a factor of 2) as used in sampler  62 . By only storing a truncated, albeit oversampled, version of the training sequence, memory space is efficiently utilized in local memory  66  since the entire training sequence (i.e., 64 samples if the training sequence is not oversampled) is not stored in local memory  66 . 
     A maximum correlation will occur between the oversampled OFDM signal and the truncated version of the training sequence when the stored training sequence coincides with a training sequence contained in the OFDM signal. Thus, a peak in the power of the correlation output may be utilized to determine when the received signal coincides with the stored training sequence. 
     The output of correlator module  64  is a complex signal since the inputs (i.e., the stored training sequence and the OFDM signal) are complex. Power module  68  may compute the power or magnitude of each sample of the correlated signal in one of two ways in accordance with the design of a particular OFDM receiver. First, power module  68  may compute the squared magnitude (i.e., the power) of each complex sample of the correlated signal to generate a real number indicating the power of the correlated signal. Second, power module  68  may obtain the magnitude (as opposed to the squared magnitude) of each complex sample of the correlated signal. 
     A peak locator module  70  searches the correlation power sequence output from power module  68  in order to locate the sample in the correlation power sequence having the largest power or magnitude value. Once the largest value is identified, peak locator module  70  outputs the index of the peak location to an error computation module  72 . The index is used by error computation module  72  as a reference point. 
     As discussed above, oversampling the OFDM signal increases the number of correlation samples such that error computation module  72  can derive a meaningful sampling error. For example, FIG. 4 shows a main correlation peak  80  and a pair of smaller correlation peaks  82  and  84  on either side of main correlation peak  80 . If the OFDM signal was not oversampled by sampler  62 , it is likely that only main correlation peak  80  would be present and error computation module  72  would not be able to determine a sampling error  86  derived from the magnitude of correlation peaks in the vicinity of main peak  80 , as discussed in further detail below. 
     When the main peak of the correlation samples is detected, error computation module  72  analyzes correlation samples  82  and  84  on either side of main peak  80 . When there is no sampling offset the frequency correlation samples  82  and  84  will have the same magnitude (not shown). However, if there is a sampling offset the correlation samples  82  and  84  will have different magnitudes, as shown in FIG.  4 . 
     Computation module  72  computes an error value by calculating the difference in magnitude between the correlation samples  82  and  84  on either side of correlation peak  80 . The difference in magnitude may be positive or negative. The magnitude of the difference indicates the degree that the stored training sequence and the received training sequence are out of synch. The sign of the difference indicates whether to increase or decrease the sampling frequency. For a given sampling offset, the magnitude of the sample to the left of a main correlation peak (e.g., main peak index−1) minus the value of the sample to the right of the main correlation peak (e.g., main peak index+1) will produce the error value. Alternatively, the error value may be computed as the difference between the right sample and the left sample depending on the requirements of a particular system. 
     Returning to FIG. 3, error computation module  72  outputs the computed error value to a second order loop filter  74  that adjusts the sampling rate such that the sampling error is driven towards zero and the sampling rate of the receiver synchronizes with the sampling rate of the transmitter. More specifically, second order loop filter  74  adjusts the sampling rate of a sampler  78  via a conventional sampling rate converter  76  or, in the alternative, may adjust the sampling rate of sampler  78  and associated upsampler  62 . 
     Referring now to FIG. 5, an integration of the present invention and conventional OFDM receiver  10  of FIG. 1, is shown. More specifically, sampling offset correction system  60  may be coupled to the outputs of mixers  28  and  30  and to the inputs of DSP  46 . With this arrangement, sampling offset correction system  60  receives the in-phase and quadrature OFDM signals from mixers  28  and  30 , digitizes the received signals at a corrected sampling rate that matches the sampling rate of the transmitter, and outputs the digitized signals to DSP  46  for further processing. It should be noted that LPF  42  and LPF  44  of FIG. 1 may be coupled to the outputs of sampling offset correction system  60  and to the inputs of DSP  46  for filtering the digitized OFDM signals although such an arrangement is not shown in FIG.  5 . 
     Thus according to the principle of the present invention, there is provideda method of correcting a sampling offset in an OFDM receiver. The method includes sampling a received OFDM signal, the OFDM signal containing a reference symbol, correlating the sampled OFDM signal with a stored symbol, locating a correlation peak, calculating a difference in magnitude of correlation samples on either side of the correlation peak, and deriving a sampling offset error from the calculated difference. 
     While the present invention has been described with reference to the preferred embodiments, it is apparent that that various changes may be made in the embodiments without departing from the spirit and the scope of the invention, as defined by the appended claims.