Abstract:
An Orthogonal Frequency Division Multiplexing (OFDM) receiver that detects and corrects a carrier frequency offset of a received signal is provided. The OFDM receiver samples an incoming signal in the time domain and correlates the samples with a stored version of a training or reference symbol to generate a correlation sequence. A correlation peak is detected in the correlation sequence and the index of the correlation peak is set as a reference point. The OFDM receiver acquires a sample of the incoming signal that is a predetermined distance from the reference point. Next, the phase difference between the acquired sample and the local oscillator is computed. Afterwards, the frequency of the local oscillator is adjusted to reduce the computed phase difference. The acquired sample has a known phase that is equal to the phase of the local oscillator in the absence of a carrier frequency offset. Thus, reducing the phase difference between the predetermined sample and the local oscillator causes the carrier frequency offset to converge towards zero.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processing of 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 sub-carriers 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 determining and correcting for carrier frequency offset, a major aspect of OFDM synchronization. Ideally, the receive carrier frequency, f cr , should exactly match the transmit carrier frequency, f ct . If this condition is not met, however, the mis-match contributes to a non-zero carrier frequency offset, delta f c , in the received OFDM signal. OFDM signals are very susceptible to such carrier frequency offset which causes a loss of orthogonality between the OFDM sub-carriers and results in inter-carrier interference (ICI) and a severe increase in the bit error rate (BER) of the recovered data at the receiver. The present invention is directed to the correction of this problem. 
     SUMMARY OF THE INVENTION 
     An OFDM receiver corrects a carrier frequency offset by computing a phase difference between a predetermined sample of a training sequence or reference symbol and a local oscillator, and adjusting the frequency of the local oscillator to reduce the computed phase difference. 
    
    
     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 local oscillator frequency correction system of the present invention; 
     FIG. 4 is a block diagram illustrating the present invention as integrated with the conventional OFDM receiver of FIG. 1; 
     FIG. 5 is a diagram of an exemplary training sequence in the frequency domain; and 
     FIG. 6 is a time domain representation of the training sequence of FIG.  5 . 
    
    
     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 receiver carrier and the transmitter carrier contributes to the carrier frequency offset, delta f c . 
     This received IF OFDM signal is coupled to 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 providing 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. Although the present invention is illustrated as being distinct from the elements of OFDM receiver of FIG. 1, one skilled in the art will readily devise that the present invention may be integrated with the elements of the OFDM receiver, as shown in FIG.  4  and discussed below. However, the present invention is illustrated as a distinct local oscillator frequency 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 802.11 a  (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. Briefly, 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 predetermined 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 below, 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 are shown in FIGS. 5 and 6. 
     Referring now to FIG. 3, an oscillator frequency correction network or system  60  is shown. It should be noted that system  60  may be embodied in software, hardware, or some combination thereof. System  60  includes a derotator or complex multiplier  66  that receives a sampled OFDM signal via a sample selection loop  62  and a phase lock loop  64 . As discussed above, the sampled OFDM signal contains 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 . Ideally, derotator  66  multiplies the sampled or digitized OFDM signal with a local signal (i.e., carrier signal) generated by a numerically controlled local oscillator  80  to bring the digitized OFDM signal down to baseband. However, the derotator output may not be exactly at baseband. One reason for this discrepancy is that the frequency of local oscillator  80  may not match the transmitter oscillator frequency. Thus, there may be a local oscillator frequency offset (i.e., carrier frequency offset) with respect to the transmitter oscillator frequency. The present invention is directed to compensating for the local oscillator frequency offset through the operation of sample selection loop  62  and phase lock loop  64 . 
     Sample selection loop  62  includes a correlator module  68 , a peak detector module  70 , and a sample selector module  72 . More specifically, correlator module  68  is coupled to a source of a sampled OFDM signal and an input of peak detector module  70 . An output of peak detector module  70  is coupled to an input of sample selector module  72  which, in turn, is coupled to the source of a sampled OFDM signal and to inputs of derotator  66  and phase locked loop  64 . 
     Phase locked loop  64  includes a phase detector module  74 , a loop filter  76 , and a numerically controlled oscillator  80 . More specifically, phase detector module  74  is coupled to an output of sample selector module  72  and an output of a numerically controlled oscillator  80  as well as an input of a loop filter module  76 . Loop filter module  76  is coupled to an input of numerically controlled oscillator  80  which, in turn, is coupled to an input of derotator  66  and fed back to an input of phase detector  74 . 
     In operation, sample selection loop  62  extracts the location of a training symbol in the received OFDM signal and delays the OFDM signal in order for the phase locked loop  64  to analyze the phase of a sample that is located at a predetermined location within the training symbol. More specifically, correlator module  68  correlates the received digitized OFDM signal with time-domain samples of a known training sequence (e.g., training sequence B of the above-mentioned wireless LAN standards) stored in a local memory. A maximum correlation will occur when the stored training sequence matches up with a training sequence contained in the digitized 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. 
     Peak detector module  70  searches the correlation sequence received from correlator module  68  for a peak in the power of the correlation sequence. The output of correlator module  68  is a complex signal since the inputs (i.e., the stored training sequence and the digitized signal) are complex. Peak detector module  70  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, peak detector module  70  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, peak detection module  70  may obtain the magnitude (as opposed to the squared magnitude) of each complex sample of the correlated signal. Afterwards, peak detector module  70  searches the correlation power sequence to identify the sample having the largest power or magnitude value. Once the largest value has been identified, peak detector module  70  outputs the index of the peak location to sample selector module  72 . The index is used by system  60  as a reference point. Within the training sequence certain samples are known to have the same phase as local oscillator  80  if there is not a local oscillator frequency offset present. However, if a frequency offset is present the samples will have a phase offset with respect to the phase of the signal generated by local oscillator  80 . The phase offset can be used by the phase locked loop  64  of system  60  to generate a frequency error signal to adjust the frequency of local oscillator  80  such that the local oscillator frequency offset converges towards zero. 
     Sample selection module  72  receives the index of the peak location from peak detector module  70  and uses the index to delay the received digitized OFDM signal such that predetermined samples, within the training sequence carried by the digitized signal, can be analyzed by the phase detector module  74  of phase locked loop  64 , as described in further detail below. The predetermined samples are known to be located a fixed distance or time period from the correlation peak and, absent a local oscillator frequency offset, have the same phase as local oscillator  74 . The predetermined OFDM samples and the phase of the local oscillator are selected in accordance with the design of a particular OFDM receiver. Sample selection module  72  may include a tap delay line and FIFO buffer arrangement or any similar selective delay arrangement as known by those skilled in the art. 
     Phase detector module  74  tracks the passage of the digitized OFDM signal output by sample selector module  72  and analyzes predetermined samples after the passage of a number of samples. For example, phase detector module  74  may include a counter that counts the number of samples output from sample selector module  72  and triggers the phase detector module  74  to capture a sample after reaching a predetermined count. The time period between triggers is known and utilized by sample selector  72  to delay the digitized OFDM signal such that phase detector module  74  acquires the predetermined samples of the training sequence. Once a sample is selected, phase detector module  74  computes the phase of the sample and the phase the signal generated by local numerically controlled oscillator  80 . Afterwards, phase detector module  74  generates a phase offset error by calculating the difference in phase between the selected sample and the signal generated by the local oscillator  74 . The phase offset error is provided to a filter  76  that generates a local oscillator frequency error. The local oscillator frequency error, in turn, is provided to local oscillator  80  to adjust the frequency of local oscillator  80  such that the local oscillator frequency offset (i.e., carrier frequency offset) converges towards zero and the derotated signal output from derotator  66  approaches baseband. The phase offset error is preferably held constant by phase lock loop  64  after the counter within phase detector module  74  resets and is counting towards the predetermined trigger value. 
     It should be noted that derotator  66  may further adjust (via internal filters or the like) the received phase error offset to more precisely derotate the digitized signal from passband to baseband. 
     Referring now to FIG. 4, the present invention is integrated with the conventional OFDM receiver of FIG. 3 as shown. More specifically, system  60  is coupled to the outputs of LPFs  42  and  44  and to the inputs of DSP  46 . With this arrangement, system  60  receives OFDM samples from LPFs  42  and  44 , corrects any detected carrier frequency offset, and outputs the corrected OFDM samples to DSP  46  for further processing. 
     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.