MXPA01001707A - Frequency offset correction in a multicarrier receiver - Google Patents

Abstract

An Orthogonal Frequency Division Multiplexing (OFDM) receiver (10) that detects and corrects a carrier frequency offset of a received signal is provided. The OFDM receiver (10) samples an incoming signal in the time domain and correlates (68) the samples with a stored version of a training or reference symbol to generate a correlation sequence. A correlation peak is detected (70) in the correlation sequence and the index of the correlation peak is set as a reference point (72, 74). The OFDM receiver acquires a sample of the incoming signal that is a predetermined distance from the reference point (74). Next, the phase difference between the acquired sample and the local oscillator is computed (74). Afterwards, the frequency of the local oscillator is adjusted to reduce the computed phase difference (76, 80). 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

LOCAL OSCILLATOR FREQUENCY CORRECTION IN A MULTIPLEXON SYSTEM OF ORTHOGONAL FREQUENCY DIVISION DESCRIPTION OF THE INVENTION The present invention relates to the processing of orthogonal frequency division multiplexed signals (OFDM, according to its acronym in English). Orthogonal frequency division multiplexing (OFDM) is a robust technique to efficiently transmit data over a channel. The technique uses a plurality of sub-carrier frequencies (sub-bearers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimum bandwidth efficiency compared to more conventional transmission aspects, such as frequency division multiplexing (FDM), which spends large portions of the bandwidth of channel in order to separate and isolate sub-carrier frequency spectra and thus avoid inter-carrier interference (ICI). In contrast, although the frequency spectra of the OFDM subcarriers overlap significantly within the bandwidth of the OFDM channel, the OFDM however allows the resolution and retrieval of the information that has been modulated on each subcarrier.

The transmission of data through a channel through OFDM signals provides several advantages over more conventional transmission techniques. An advantage is a tolerance to the spread of multiple path delay. 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 intersymbol interference (ISI). Another advantage is a tolerance to selective frequency fading. Including redundancy in the OFDM signal, the data encoded on fading sub-bearers can be reconstructed from the data retrieved from the other sub-bearers. Another advantage is the efficient use of the spectrum. Since the OFDM sub-bearers are placed very close to each other without the need to leave a frequency space unused between them, the OFDM can efficiently fill a channel. One more advantage is the simplified sub-channel equalization. OFDM shifts the equalization of the time domain channel (as in single carrier transmission systems) to the frequency domain, where a bank of single equalizers of a bypass can individually adjust the phase and amplitude distortion of each sub-channel . Another advantage is the good interference properties. It is possible to modify the OFDM spectrum to represent the energy distribution of an interference signal. Also, it is possible to reduce out-of-band interference by avoiding the use of OFDM sub-bearers near the edges of the channel bandwidth. Although the OFDM exhibits these advantages, the prior art implementations of OFDM also exhibit several difficulties and practical limitations. One difficulty is the emission of determining and correcting the frequency deviation of the carrier, a main aspect of OFDM synchronization. Ideally, the frequency of the reception bearer, fcr, must exactly match the frequency of the transmission bearer fct. If this condition is not satisfied, however, the mismatch contributes to a bearer frequency deviation that is not zero, delta fc, in the received OFDM signal. The OFDM signals are very susceptible to said bearer frequency deviation, which causes a loss of orthogonality between the OFDM subcarriers and results in inter-carrier interference (ICI) and a severe increase in the bit error rate ( BER) of the data recovered in the receiver. The present invention is directed to the correction of this problem. An OFDM receiver corrects a carrier frequency deviation by calculating 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 calculated phase difference.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a block diagram of a conventional OFDM receiver; Figure 2 illustrates a typical arrangement of OFDM symbols and their corresponding security intervals within a data frame; Figure 3 is a block diagram of a local oscillator frequency correction system illustrative of the present invention; Figure 4 is a block diagram illustrating the present invention integrated with the conventional OFDM receiver of the Figure 1; Figure 5 is a diagram of an illustrative training sequence in the frequency domain; and Figure 6 is a time domain representation of the training sequence of Figure 5. The features and advantages of the present invention will be more apparent from the following description, given by way of example. Referring to Figure 1, the first element of a typical OFDM receiver 10 is an RF receiver 12. Many variations of the RF receiver exist and are well known in the art, but typically, the RF receiver 12 includes an antenna 14. a low noise amplifier (LNA) 16, an RF bandpass filter 18, an automatic gain control circuit (AGC) 20, an RF mixer 22, a RF carrier frequency local oscillator 24, and a bandpass IF 26. Through antenna 14, RF receiver 12 couples the OFDM modulated carrier, RF, after it passes through the channel. Then, by mixing it with a frequency receiver carrier fcr generated by the local oscillator RF 24, the RF receiver 12 converts the OFDM modulated carrier, RF, to obtain a received OFDM IF signal. The frequency difference between the receiving carrier and the transmitting carrier contributes to the bearer frequency deviation, delta fc. This received OFDM IF signal is coupled to the mixer 28, and the mixer 30 will be mixed with an IF signal in phase and an IF signal phase shifted to 90 ° (quadrature), respectively, to produce OFDM signals in phase and quadrature, respectively. The IF signal in phase that is fed to the mixer 28 is produced by a local oscillator IF 32. The signal IF of phase shifted to 90 ° which is fed to the mixer 30 is derived from the IF signal in phase of the local oscillator IF 32 passing the signal IF in phase through a 90 ° phase shifter 34 before being supplied to the mixer 30. The phase and quadrature OFDM signals then pass to analog to digital converters (ADCs) 36 and 38, respectively, where they are digitized at a sampling rate fck_r as determined by a clock circuit 40. Analog to digital converters 36 and 38 produce digital samples that form a phase OFDM signal and discrete quadrature time signal, respectively. The difference between the sampling rates of the receiver and those of the transmitter is the sampling velocity deviation, delta fck = fck_r-fck_t. OFDM signals not filtered in phase and discrete-time quadrature of the analog-to-digital converters 36 and 38 then pass through digital low-pass filters 42 and 44, respectively. The output of the low pass digital filters 42 and 44 is filtered in phase and quadrature samples, respectively, of the received OFDM signal. In this way, the received OFDM signal is converted to phase (qi) and quadrature (pi) samples that represent the real and imaginary value components, respectively, of the complex value OFDM signal, ri = qi + jpi . These phase and quadrature samples (of real value and imaginary value) of the OFDM signal received afterwards are sent by the DSP 46. Note that in some conventional implementations of the receiver 10, the analog to digital conversion is done before the process of mixed IF. In such implementation, the mixing process involves the use of digital mixers and a digital frequency synthesizer. Also note that in many conventional implementations of the receiver 10, digital to analog conversion is done after filtering. DSP 46 performs a variety of operations on the phase and quadrature samples of the received OFDM signal. These operations may include: a) synchronizing the receiver 10 for time control of the symbols and data frames within the received OFDM signal, b) removing the cyclic prefixes from the received OFDM signal, c) calculating the discrete transformation Fourier (DFT) or preferably Fast Fourier Transform (FFT) of the received OFDM signal in order to recover the frequency domain subsymbol sequences that were used to modulate the subcarriers during each interval of the OFDM symbol; ) performing any channel equalization required on the subcarriers, e) calculating a sequence of subsymbols of frequency domain, and k, of each symbol of the OFDM signal by demodulating the subcarriers of the OFDM signal through the calculation of FFT. DSP 46 then sends these subsymbol sequences to a decoder 48. The decoder 48 retrieves the data bits transmitted from the subsymbol sequences of sequence domains that are delivered thereto from the DSP 46. This retrieval is performed by decoding the domain subsymbols. of frequency to obtain a stream of data bits, which ideally must match the data bit stream that was fed to! OFDM transmitter. This decoding process may include soft decoding of Viterbi and / or Reed-Solomon decoding, for example, to retrieve convolutionally encoded block data and / or subsymbols.

In a typical OFDM data transmission system, such as one for implementing digital television or a wireless local area network (WLAN), the data is transmitted in the OFDM signal in groups of symbols known as data frames. This concept is shown in Figure 2, wherein a data frame 50 includes consecutive symbols 52a, 52b,. . ., 52M, each of which includes a security interval, Tg, as well as the OFDM symbol interval, Ts. Therefore, each symbol has a total duration of Tg + Ts seconds. Depending on the application, the data frames can be transmitted continuously, such as in digital television broadcasting, or the data frames can be transmitted in random time in bursts, such as in the implementation of a WLAN. Referring now to Figure 3, an illustrative embodiment of the present invention is illustrated. Although the present invention is illustrated as being distinct from the elements of the OFDM receiver of Figure 1, one skilled in the art will readily advise that the present invention can be integrated with the elements of the OFDM receiver, as shown in Figure 4 and it is discussed later. However, the present invention is illustrated as a different local oscillator frequency correction loop for clarity, ease of reference, and to facilitate an understanding of the present invention. The present invention operates on a receiver conforming to the ETSI-BRAN wireless LAN standards HIPERLAN / 2 (Europe) and IEEE 802.11a (E.U.A.), incorporated herein by reference. However, it is considered within the skill in the art to implement the teachings of the present invention in other OFDM systems. The previously identified wireless LAN standards propose the use of a training sequence for the detection of OFDM transmissions. In summary, the training frequency (eg, 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 tanks ( for example, 12 pilot sub-carriers). All other sub-carriers (for example, 52 sub-carriers) remain at zero during the transmission of the training sequence. Although the use of the training sequence of the above-identified LAN standards is discussed below, the use of alternative training sequences and symbols is considered within the scope of the invention as defined by the appended claims. The frequency domain and time domain representations of the illustrative training sequence are shown in Figures 5 and 6. Referring now to Figure 3, a network or frequency correction system of oscillator 60 is shown. Note that the system 60 may be modalized in software, hardware, or any combination thereof. System 60 includes a complex de-rotator or multiplier 66 that receives a sample OFDM signal sampled through a sample selection loop 62 and a phase closure loop 64. As discussed above, the sampled OFDM signal contains in-phase (qi) and quadrature (pi) samples representing the real-value and imaginary-value components, respectively, of the value OFDM signal complex, ri = qi + jpi. Ideally, the des-rotator 66 multiplies the OFDM signal sampled or digitized with a local signal (i.e., carrier signal) generated by a numerically controlled local oscillator 80 to carry the digitized OFDM signal below the baseband . However, the output of the des-rotator may not be exactly in the baseband. One reason for this discrepancy is that the frequency of the local oscillator 80 may not coincide with the frequency of the transmitter oscillator. In this way, there may be a local oscillator frequency deviation (ie, bearer frequency deviation) with respect to the transmitter oscillator frequency. The present invention is directed to compensate for the local oscillator frequency deviation through the operation of the sample selection loop 62 and the phase closure loop 64. The sample selection loop 62 includes a correlator module 68, a module peak detector 70 and a sample selector module 72. More specifically, the correlator module 68 is coupled to a source of a sampled OFDM signal and an input of the peak detector module 70. An output of the peak detector module 70 is coupled to an input of the sample selector module 72 which, in turn, is coupled to the source of a sampled OFDM signal and inputs the des-rotator 66 and phase closure loop 64. The phase closure loop 64 includes a module phase detector 74, a loop filter 76 and a numerically controlled oscillator 80. More specifically, the phase detector module 74 is coupled to an output of the sample selector module 72 and an output of a numerically controlled oscillator 80, as well as a module input. loop filter 76. The loop filter module 76 is coupled to an input of the numerically controlled oscillator 80, which, in turn, is coupled to an input of the rotator 66 and is fed to an input of the detector. phase 74. During operation, the 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 closure loop 64 to analyze the phase of a shows that it is located at a predetermined site within the training symbol. More specifically, the correlator module 68 correlates the received digitized OFDM signal with time domain samples of a known training sequence (e.g., the training sequence B of the aforementioned wireless LAN standards) stored in a local memory. A maximum correlation will occur when the stored training sequence matches a training sequence contained in the digitized signal. In this way, a peak in the energy of the correlation output can be used to determine when the received signal matches the stored training sequence. The peak detector module 70 searches for the correlation sequence received from the correlator module 68 for a peak in the energy of the correlation sequence. The output of the correlator module 68 is a complex signal, since the inputs (i.e., the stored training sequence and the digitized signal) are complex. The peak detector module 70 can calculate the energy or magnitude of each sample of the correlated signal in one of two ways according to the design of a particular OFDM receiver. First, the peak detector module 70 can calculate the square magnitude (i.e., energy) of each complex sample of the correlated signal to generate a real number indicating the energy of the correlated signal. Secondly, the peak detection module 70 can obtain the magnitude (opposite to the square magnitude) of each complex sample of the correlated signal. Next, the peak detector module 70 searches for the correlation energy sequence to identify the sample that has the largest energy or magnitude value. Once the largest value has been identified, the peak detector module 70 outputs the peak location index to the sample selector module 72. The index is used by the system 60 as a reference point. Within the training sequence, it is known that certain samples have the same phase as the local oscillator 80 without any local oscillator frequency deviation being present. However, if a frequency deviation is present, the samples will have a phase deviation with respect to the phase of the signal generated by the local oscillator 80. The phase deviation can be used by the phase closure loop 64 of the system 60 to generate a frequency error signal to adjust the frequency of the local oscillator 80, so that the local oscillator frequency deviation converges towards zero. The sample selection module 72 receives the index of the peak location from the peak detector module 70 and uses the index to delay the received digitized OFDM signal so that the predetermined samples, within the training sequence carried by the signal digitized, can be analyzed by the phase detector module 74 of the phase closure loop 64, as described in detail below. The predetermined samples are known to be located at a fixed distance or time period from the correlation peak and, absent a local oscillator frequency deviation, they have the same phase as the local oscillator 74. The predetermined OFDM samples and the phase of the Local oscillator are selected according to the design of a particular OFDM receiver. The sample selection module 72 may include a derivation delay line and a FIFO buffer arrangement or any selective delay arrangement as is known to one of ordinary skill in the art.

The phase detector module 74 follows the passage of the OFDM signal output digitized by the sample selector module 72 and analyzes predetermined samples after the passage of a number of samples. For example, the phase detector module 74 may include a counter that counts the number of samples exiting the sample selector module 72 and activates the phase detector module 74 to capture a sample after reaching a predetermined count. The period of time between activations is known and used by the sample selector 72 to delay the digitized OFDM signal, so that the phase detector module 74 acquires the predetermined samples of the training sequence. Once the sample is selected, the phase detector module 74 calculates the phase of the sample and the phase of the signal generated by the numerically controlled oscillator 80. Then, the phase detector module 74 generates a phase shift error by calculating the difference in phase between the selected sample and the signal generated by the local oscillator 74. The phase shift error is provided to a filter 76 that generates a local oscillator frequency error. The local oscillator frequency error, in turn, is provided to the local oscillator 80 to adjust the frequency of the local oscillator 80, so that the frequency deviation of the local oscillator (i.e., the carrier frequency deviation) converges towards zero and the un-rotated signal output of des-rotator 66 approaches the baseband. The base deviation error is preferably kept constant by the phase closure loop 64 after the counter, within the phase detector module 74, is reset and is counting towards the predetermined activation value. It should be noted that the de-rotator 66 can also adjust (via internal or similar filters) the received phase error deviation to more accurately de-rotate the digitized signal from the passband to the baseband. Referring now to Figure 4, the present invention is integrated with the conventional OFDM receiver of Figure 3 as shown. More specifically, the system 60 is coupled to the LPFs 42 and 44 outputs and the DSP inputs 46. With this arrangement, the system 60 receives OFDM samples of LPFs 42 and 44, corrects any detected carrier frequency deviation, and gives output to the corrected OFDM samples to DSP 46 for further processing. Although the present invention has been described with reference to the preferred embodiments, it is clear that various changes in the embodiments can be made without departing from the spirit and scope of the invention, as defined by the appended claims.

Claims (20)

1. - A method for correcting a carrier frequency deviation in an orthogonal frequency division multiplexing (OFDM) receiver in the time domain, the method comprising the steps of receiving an OFDM signal having a reference symbol; correlating the OFDM signal with a stored copy of the reference symbol; output an index of a correlation peak; sampling the OFDM signal at a predetermined distance from the index of the correlation peak to produce a reference sample; calculating the phase difference between the reference sample and a locally generated carrier frequency; and generating a bearer frequency deviation error in response to the calculated phase difference.

2. The method according to claim 1, further comprising the steps of: adjusting the locally generated carrier frequency to correct the bearer frequency deviation error; and de-rotating the OFDM signal received from the passband to the baseband using the locally generated carrier frequency.

3. The method according to claim 1, wherein the step of correlating includes the steps of: outputting a sequence of correlation samples representing the correlation of the stored reference symbol with the OFDM signal; determine the energy of each correlation sample in the sequence; and determining the index of the correlation peak by locating a correlation sample in the sequence having a maximum energy value.

4. The method according to claim 3, wherein the step of determining the energy of each correlation sample includes the step of calculating a square quantity of each correlation sample.

5. The method according to claim 3, wherein the step of determining the energy of each correlation sample includes the step of obtaining a magnitude of each correlation sample.

6. The method according to claim 1, wherein the step of generating a bearer frequency deviation error includes the step of passing the calculated phase difference through a loop filter.

7. An orthogonal frequency division multiplexing receiver (OFDM) for receiving an OFDM signal having a training symbol, the OFDM receiver characterized by: an analog-to-digital converter (ADC) that converts a received OFDM signal to a plurality of digital samples, the plurality of digital samples including a plurality of training symbol samples; an oscillator that generates a digital signal; a des-rotator coupled to the oscillator, the des-rotator mixing the digital OFDM samples with the digital signal, so that the digital OFDM samples are converted from the band to the baseband; and an error module coupled to the analog-to-digital converter, the oscillator, and the des-rotator, the error module passing the plurality of digital samples from the ADC to the des-rotator, the error module deriving a phase difference between a sample of predetermined training symbol and a digital signal generated by the oscillator, the error module adjusting an oscillator frequency to reduce the derived phase difference.

8. - The OFDM receiver according to claim 7, wherein the error module comprises: a sample selection unit coupled to the ADC and the unloader, the sample selection unit selectively delaying the digital samples leaving the ADC to the de-rotator for a predetermined time; and a phase detection unit coupled to the sample selection unit and the oscillator, the phase detection unit acquiring the predetermined training symbol sample of the delayed digital samples, calculating the phase difference between the symbol sample of default training and the digital signal of the oscillator, and generating a control signal to adjust the frequency of the oscillator to reduce the calculated phase difference.

9. The OFDM receiver according to claim 8, wherein the predetermined time is selectively set by the sample selection unit, such that the phase selection unit acquires the predetermined training symbol sample.

10. The OFDM receiver according to claim 8, wherein the sample selection unit comprises: a correlator coupled to the ADC, the correlator correlating the digital sample output of the ADC with a stored copy of the training symbol to generate a plurality of correlation samples; a correlation peak detector coupled to the correlator, the correlation peak detector outputs an index of a correlation peak in response to the detection of a correlation peak in the plurality of correlation samples; and a sample selector coupled to the ADC, the correlation peak detector, and the phase detection unit, the sample selector selectively delaying the digital samples leaving the ADC for the predetermined time in response to receiving the peak index of correlation of the peak correlation detector.

11. The OFDM receiver according to claim 10, wherein the correlation peak detector calculates a correlation energy for each correlation sample and detects the correlation peak by detecting a maximum energy value in the plurality of samples of correlation.

12. The OFDM receiver according to claim 11, wherein the correlation energy is a square quantity of each correlation.

13. The OFDM receiver according to claim 11, wherein the correlation energy is a magnitude of each correlation.

14. The OFDM receiver according to claim 7, wherein the predetermined training symbol sample is in phase with the digital signal generated by the oscillator when the oscillator is synchronized with a carrier frequency of the OFDM signal.

15. An apparatus for synchronizing a local oscillator frequency of an orthogonal frequency division multiplexing receiver (OFDM) with a carrier frequency generated by an OFDM transmitter, the apparatus characterized by having: means for receiving an OFDM signal transmitted at a carrier frequency; means for extracting a reference point from the OFDM signal; means for sampling the OFDM signal at a predetermined distance from the reference point; means for calculating a phase difference between the sample and the local oscillator frequency; and means for synchronizing the local oscillator frequency with the carrier frequency of the OFDM signal by adjusting the local oscillator frequency so that the phase difference is reduced.

16. - The apparatus according to claim 15, wherein the apparatus is incorporated into a receiver operating in a wireless LAN.

17. The apparatus according to claim 15, wherein the means for extracting comprises: means for correlating the OFDM signal with a stored copy of a reference symbol to generate a plurality of correlation samples; and means for detecting a site of a correlation peak in the plurality of correlation samples; and means to fix the site of the correlation peak as the reference point.

18. The apparatus according to claim 17, wherein the means for detecting comprises: means for determining the energy of each correlation sample in the plurality of correlation samples; and means for determining the correlation peak site by locating a correlation sample in the sequence that has a maximum energy value.

19. The apparatus according to claim 18, wherein the means for determining the energy of each correlation sample include at least one of the means for calculating a square quantity for each correlation sample and a means for obtaining a quantity of each correlation sample.

20. The apparatus according to claim 15, wherein the predetermined distance is set so that at least one reference symbol within the OFDM signal is sampled, the reference symbol sample being in phase with the frequency of oscillator when the oscillator is synchronized with the carrier frequency.