Abstract:
The disclosed embodiments relate to reducing adjacent channel interference in an OFDM receiver. An error metric is monitored and the carrier frequency of the desired channel is slowly adjusted in response to the error metric. In this manner, the received OFDM signal, including the desired signal corrupted by the adjacent interfering channel, may be shifted until the zero crossings of the adjacent channel line up with the FFT bins. A multi-tap equalizer may then be used to remove the inter-bin interference that results from the frequency offset in the desired channel.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processing orthogonal frequency division multiplexed (OFDM) signals. 
     BACKGROUND OF THE INVENTION 
     This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     A wireless LAN (WLAN) is a flexible data communications system implemented as an alternative or extension to a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs. Some industries that have benefited from the productivity gains of using portable terminals (e.g., notebook computers) to transmit and receive real-time information are the digital home networking, health-care, retail, manufacturing, and warehousing industries. 
     Manufacturers of WLANs have a range of transmission technologies to choose from when designing a WLAN. Some exemplary technologies are multicarrier systems, spread spectrum systems, narrowband systems, and infrared systems. Although each system has its own benefits and detriments, one particular type of multicarrier transmission system, orthogonal frequency division multiplexing (OFDM), has proven to be exceptionally useful for WLAN communications. 
     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 data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplexing (FDM) which can waste 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 also provides several other advantages over more conventional transmission techniques. Some of these advantages are a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties. 
     In spite of these advantages, there are some problems with OFDM data transfer in systems that are adapted to process multiple signals from multiple users. One example of such a problem is adjacent channel interference (sometimes referred to as interchannel interference). One of the main reasons for adjacent channel interference is the windowing that occurs as an inherent part of the Fast Fourier Transform (FFT) processing of signals that is typical of all OFDM receivers. In processing signals using FFT algorithms, the tone of each OFDM sub-band may be spread across multiple FFT bins with periodically spaced zero crossings (e.g. a sinc function in the case of a rectangular window) in the frequency domain. Such a frequency domain structure allows adjacent channels to be placed at the multiples of the sampling rate with no adjacent channel interference because of the alignment of the zero crossing. Thus, even though the energy from an adjacent channel in the frequency range for the desired channel is non-zero, there is no adjacent channel interference because of the orthogonal nature of the two signals. 
     However, when a carrier frequency offset is present, the orthogonality between adjacent channel signals is not maintained and adjacent channel interference occurs. A method of reducing adjacent channel interference in OFDM receivers is desirable. 
     SUMMARY OF THE INVENTION 
     The disclosed embodiments relate to reducing adjacent channel interference in an OFDM receiver. An error metric is monitored and the carrier frequency of the desired channel is slowly adjusted in response to the error metric. In this manner, the received OFDM signal, including the desired signal corrupted by the adjacent interfering channel, may be shifted until the zero crossings of the adjacent channel line up with the FFT bins. A multi-tap equalizer may then be used to remove the intentionally introduced inter-bin interference that results from the frequency offset in the desired channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a block diagram of an exemplary OFDM receiver; 
         FIG. 2  is a diagram illustrating the placement of a training sequence, user data, and pilot signals within an OFDM symbol frame; 
         FIG. 3  is a block diagram of a circuit for reducing adjacent channel interference according to the present invention; 
         FIG. 4  is a block diagram of a multi-tap equalizer that may be used in conjunction with the present invention; and 
         FIG. 5  is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention. 
     
    
    
     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 the RF receiver  12  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 band pass filter  18 , an automatic gain control (AGC) circuit  20 , an RF mixer  22 , an RF carrier frequency local oscillator  24 , and an IF band pass filter  26 . 
     Through the antenna  14 , the 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 the RF local oscillator  24 , the 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 a mixer  28  and a 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 the 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 the IF local oscillator  32  by passing the in-phase IF signal through a 90° phase shifter  34  before providing it to the 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 . The 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 the ADCs  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  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 an FFT  46 . Note that in some conventional implementations of the 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. 
     The FFT  46  performs 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. The FFT  46  then delivers these sequences of sub-symbols to a decoder  48 . 
     The decoder  48  recovers the transmitted data bits from the sequences of frequency-domain sub-symbols that are delivered to it from the FFT  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. 
     Turning to  FIG. 2 , an exemplary OFDM symbol frame  50  of the present invention is shown. The symbol frame  50  includes a training sequence or symbol  52  containing known transmission values for each sub-carrier in the OFDM symbol, and a predetermined number of a cyclic prefix  54  and user data  56  pairs. For example, the proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless LAN standards, herein incorporated by reference, assign 64 known values or sub-symbols (i.e., 52 non-zero values and 12 zero values) to selected training symbols of a training sequence (e.g., “training symbol C” of the proposed ETSI standard and “long OFDM training symbol” of the proposed IEEE standard). The user data  56  has a predetermined number of pilots  58 , also containing known transmission values, embedded on predetermined sub-carriers. For example, the proposed ETSI and IEEE standards have four pilots located at bins or sub-carriers ±7 and ±21. Although the present invention is described as operating in a receiver that conforms to the proposed ETSI-BRAN HIPERLAN/2 (Europe) and IEEE 802.11a (USA) wireless LAN standards, it is considered within the skill of one skilled in the art to implement the teachings of the present invention in other OFDM systems. 
       FIG. 3  is a block diagram of a circuit for reducing adjacent channel interference according to the present invention. The adjacent channel interference reducing circuit is referred to generally by the reference numeral  60 . An input signal  62  is delivered to a carrier frequency adjustment module  64  before being processed by the FFT module  46 . As explained below, the carrier frequency adjustment module  64  generates a frequency adjustment based on feedback from an error metric computation module  72 . 
     The input signal  62  comprises the received OFDM signal which has been corrupted by tails of an adjacent channel interfering signal. In response to the output of the error metric computation module  72 , the carrier frequency adjustment module  64  changes the frequency offset. This change in frequency offset has the effect of introducing interbin interference into the input signal  62 . The rate of adjustment of the frequency offset must be sufficiently slower than the adaptation rate of an associated equalizer (described below with reference to  FIG. 4 ) to give the associated equalizer time to converge. The frequency adjustment may be done in either an open loop or dosed loop fashion. 
     The output of the carrier adjustment frequency module  64  is processed by the FFT module  46 . The output of the FFT module  46  is delivered to an equalizer module  68 . The output  70  of the equalizer module  68  is available for further processing. Also, the output  70  of the equalizer module  68  is delivered to the error metric computation module  72 , the output of which in turn is delivered back to the carrier adjustment frequency module  64 . 
     The error metric produced by the error metric computation module  72  is computed based on the equalized output of the FFT module  46 . The error metric is proportional to the adjacent channel interference in the input signal  62  so the value of the error metric is minimized when the adjacent channel interference is at its minimum. One example of an error metric that may be used is the standard deviation of the steady state decision-directed least mean squares (LMS) error. That LMS error is defined as the difference between the equalizer output sample and the corresponding hard decision (output of a symbol decision device). The trained LMS error can also be used for systems that have pilot sub-carriers or sub-bands inserted in the data stream. Other error metrics may be used if the value of the error metric is proportional to the degree of adjacent channel interference in the input signal  62 . 
     In systems where there are channels on either side of the desired channel, the removal of adjacent channel interference may be performed with respect to either channel. If there are two adjacent channels the adjacent channel interference may be minimized with respect to the stronger of the multiple adjacent channels. 
       FIG. 4  is a block diagram of a multi-tap equalizer that may be used in conjunction with the present invention. The equalizer module  68  removes the interbin interference that was introduced by the carrier frequency adjustment module  64 . The removal of the interbin interference results in a reduction in the adjacent channel interference associated with the signal. The output of the FFT module  46  is broken into three sub-bands: sub-band n, sub-band n−1 and sub-band n+1. The number of sub-bands may vary because of operational characteristics of a given application. The exact number of sub-bands into which the output of the FFT module  46  is broken is not a crucial aspect of the invention. 
     Each of the three sub-bands n, n−1 and n+1 are delivered to separate multiplier circuits where they are multiplied by corresponding equalizer coefficients. The coefficients for the equalizers may be chosen based on the known carrier frequency offset. The specific method of determining the coefficients is not a crucial aspect of the present invention. The sub-bands n−1, n and n+1 are delivered respectively to multipliers  76 ,  78  and  80 . One method of obtaining sub-band values may be to derive those values from the input signal  62 , as indicated by the dashed line  63 . 
     The output of the multipliers  76 ,  78  and  80  is delivered to a summing circuit  82 , which delivers an equalized output  84  for sub-band n. The multipliers  76 ,  78 ,  80  and the summing circuit  82  comprise the equalizer module  68 . 
     As set forth above, the multi-tap equalizer  68  cancels out interbin interference that is introduced by the carrier frequency offset from the carrier frequency adjustment module  64 . Although the intentional introduction of the frequency offset provided by the carrier frequency adjustment module may reduce the interference of the adjacent channel, the offset may also induce undesired interbin interference in the input signal because the sub-bands of the desired channel are no longer aligned with the FFT bins. The multi-tap equalizer  68  removes this interbin interference to achieve a reduction in adjacent channel interference. 
       FIG. 5  is a process flow diagram illustrating the operation of an exemplary embodiment of the present invention. The process is generally referred to by the reference numeral  86 . Those of ordinary skill in the field will appreciate that the functions and operations illustrated in  FIG. 5  may be accomplished using circuitry (hardware), software or combinations of hardware and software. 
     At  88 , the process begins. At  90 , a carrier frequency is performed based on an error metric. Computation of the error metric may be performed as described above with reference to  FIG. 3 . After adjusting the input signal by the frequency offset that has been changed based on the output of the error metric computation module, the FFT of the signal is taken at  92 . 
     The signal resulting from the FFT operation is equalized using a multi-tap equalizer as described with reference to  FIG. 4 . Thus, the input signal  62  has been processed to reduce the adjacent channel interference by the carrier frequency adjustment module  64 . The self-induced interbin interference has been removed by the equalizer module  68 . At  96 , the process ends. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.