Abstract:
The present invention provides an improved channel quality indicator indicia for OFDM communication environments. In addition to taking into consideration carrier-to-interference ratios, the present invention also takes into consideration the degree to which the channel response varies among the sub-carriers throughout the OFDM frequency band. The carrier-to-interference ratio and the degree of channel response variation are directly or indirectly used by a base station to select coding and modulation schemes for transmissions from the base station to the mobile terminal reporting these factors. Further, scheduling of data sent to the mobile terminal and other mobile terminals competing for the same channel resources may also be based in part on the carrier-to-interference ratio and the degree to which the channel response varies.

Description:
This application claims the benefit of U.S. provisional patent application Ser. No. 60/495,944, filed Aug. 18, 2003, the disclosure of which is hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to communications, and in particular to providing an improved channel quality indicator for an orthogonal frequency division multiplexing communication environment. 
   BACKGROUND OF THE INVENTION 
   Today&#39;s advanced wireless packet data CDMA systems, such as High Speed Data Packet Access (HSDPA) systems, measure the carrier-to-interference ratio (CIR) at a mobile terminal, and based on this measurement, send channel quality indicator (CQI) information to the base station. The CQI information, as defined using the CIR measurement for CDMA systems, is a reliable indication of the error rate expected at the mobile terminal. CQI information is continuously reported back to the base station because the transmission channel conditions change as the mobile terminal and objects around the mobile terminal move. Therefore, the CQI information reporting occurs on a regular basis, continuously updating the base station of the changing channel conditions experienced by the mobile terminal. The base station receives CQI information for each active mobile terminal that is attempting to share a common transmission channel. 
   The base station uses the CQI information for two primary purposes. The first purpose is to select from various levels of modulation and coding to use for transmissions to the mobile terminal in an effort to maximize the throughput to that particular mobile terminal or for the system in general. The second purpose is to assist in scheduling when and how much information should be sent to the mobile terminal and other mobile terminals competing for channel resources. Depending on the scheduling criteria, the base station may choose to send more data to those mobile terminals experiencing good channel conditions to maximize the system&#39;s overall throughput. For example, the scheduling criteria may dictate sending information only to those mobile terminals having channel conditions sufficient to support the highest data rate or rates. 
   In orthogonal frequency division multiplexing (OFDM) systems, the CIR may also be used as a CQI in the same fashion as it is used for CDMA. When one plots the average error rate at the mobile terminal as a function of the CIR, the result is a monotonically decreasing function. However, OFDM and CDMA communications differ in some fundamental ways. One particular difference is the mechanism by which frequency diversity is achieved in each system. Due to this difference, CIR alone is not the most reliable indication of the error rate to be expected at the mobile terminal. As illustrated in  FIG. 1 , for a given CIR, there is a fairly large variance in the error rate. A large variance in the error rate for a given CIR implies that there are other factors that influence the error rate, not just the CIR. In contrast, CDMA systems have a much smaller variance in the error rate at a particular CIR at the output of a receiver, and as such, the CIR alone has proven to be a good CQI. Since CIR alone is not sufficient for efficient OFDM systems, there is a need for an improved CQI for OFDM systems that takes into account the frequency response of the channel. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved channel quality indicator indicia for OFDM communication environments. In addition to taking into consideration carrier-to-interference ratios, the present invention also takes into consideration the degree to which the channel response varies among the sub-carriers throughout the OFDM frequency band. The carrier-to-interference ratio and the degree of channel response variation are directly or indirectly used by a base station to select coding and modulation schemes for transmissions from the base station to the mobile terminal reporting these factors. Further, scheduling of data sent to the mobile terminal and other mobile terminals competing for the same channel resources may also be based in part on the carrier-to-interference ratio and the degree to which the channel response varies. 
   The variation of the channel gain throughout the OFDM frequency band may be calculated by determining the standard deviation of the channel gain throughout the sub-carriers of the OFDM frequency band. Preferably, standard deviation is calculated using the logarithm of the various channel gains. The sub-carriers for which the variation is considered are preferably those sub-carriers used for data transmission, wherein those sub-carriers on the outside boundaries of the OFDM frequency band used for interference buffering are not considered. 
   The carrier-to-interference ratio and the degree to which the sub-carriers vary are generally measured at the mobile terminal and may be sent to the base station in any number of formats. Measures of each of these indicia may be sent to the base station, which will take the two factors and determine a channel quality indicator. Alternatively, the mobile terminal can arrive at a channel quality indicator value, which is sent to the base station and is then used to select coding and modulation, as well as to assist in scheduling data for transmission. Those skilled in the art will recognize various ways for providing such channel quality indicator indicia from the mobile terminal to the base station. 
   Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. 
       FIG. 1  illustrates the variation in bit error rates for a given carrier-to-interference ratio. 
       FIG. 2  is a block representation of a cellular communication system. 
       FIG. 3  is a block representation of a base station according to one embodiment of the present invention. 
       FIG. 4  is a block representation of a mobile terminal according to one embodiment of the present invention. 
       FIG. 5  is a logical breakdown of an OFDM transmitter architecture according to one embodiment of the present invention. 
       FIG. 6  is a logical breakdown of an OFDM receiver architecture according to one embodiment of the present invention. 
       FIG. 7  illustrates a pattern of sub-carriers for carrying pilot symbols in an OFDM environment. 
       FIG. 8  illustrates an example channel frequency response and a standard deviation associated therewith. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
   The present invention adaptively controls coding and modulation techniques for transmission as well as multi-user scheduling based on an improved channel quality indicator (CQI) in an orthogonal frequency division multiplexing (OFDM) environment. The channel quality indicator is determined based on a carrier to interference ratio (CIR) and a measure of the degree to which the channel response varies across the band of sub-carriers for the OFDM frequency band. The latter measure is referred to as a variation measure for simplicity and ease of understanding. In one embodiment, the channel gains for each of the sub-carriers used for transmission are used to determine the variation measure. For example, the variation measure may be the standard deviation of the logarithm of the channel gains. The measurements required to determine the CIR and variation measure are measured at a mobile terminal; however, the CQI may be determined at the mobile terminal or at a serving base station depending on the desired implementation of the invention. The following description initiates with an overview of a wireless communication environment and the architecture of a base station, or like access point, and a mobile terminal. 
   With reference to  FIG. 2 , a base station controller (BSC)  10  controls wireless communications within multiple cells  12 , which are served by corresponding base stations (BS)  14 . In general, each base station  14  facilitates communications using OFDM with mobile terminals  16 , which are within the cell  12  associated with the corresponding base station  14 . The movement of the mobile terminals  16  in relation to the base stations  14  results in significant fluctuation in channel conditions. As illustrated, the base stations  14  and mobile terminals  16  may include multiple antennas to provide spatial diversity for communications. 
   A high level overview of the mobile terminals  16  and base stations  14  of the present invention is provided prior to delving into the structural and functional details of the preferred embodiments. With reference to  FIG. 3 , a base station  14  configured according to one embodiment of the present invention is illustrated. The base station  14  generally includes a control system  20 , a baseband processor  22 , transmit circuitry  24 , receive circuitry  26 , multiple antennas  28 , and a network interface  30 . The receive circuitry  26  receives radio frequency signals bearing information from one or more remote transmitters provided by mobile terminals  16  (illustrated in  FIG. 4 ). Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
   The baseband processor  22  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  22  is generally implemented in one or more digital signal processors (DSPs) or application-specific integrated circuits (ASICs). The received information is then sent across a wireless network via the network interface  30  or transmitted to another mobile terminal  16  serviced by the base station  14 . 
   On the transmit side, the baseband processor  22  receives digitized data, which may represent voice, data, or control information, from the network interface  30  under the control of control system  20 , and encodes the data for transmission. The encoded data is output to the transmit circuitry  24 , where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  28  through a matching network (not shown). Modulation and processing details are described in greater detail below. 
   With reference to  FIG. 4 , a mobile terminal  16  configured according to one embodiment of the present invention is illustrated. Similarly to the base station  14 , the mobile terminal  16  will include a control system  32 , a baseband processor  34 , transmit circuitry  36 , receive circuitry  38 , multiple antennas  40 , and user interface circuitry  42 . The receive circuitry  38  receives radio frequency signals bearing information from one or more base stations  14 . Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove broadband interference from the signal for processing. Downconversion and digitization circuitry (not shown) will then downconvert the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
   The baseband processor  34  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation decoding and error correction operations, as will be discussed on greater detail below. The baseband processor  34  is generally implemented in one or more digital signal processors (DSPs) and application specific integrated circuits (ASICs). 
   For transmission, the baseband processor  34  receives digitized data, which may represent voice, data, or control information, from the control system  32 , which it encodes for transmission. The encoded data is output to the transmit circuitry  36 , where it is used by a modulator to modulate a carrier signal that is at a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas  40  through a matching network (not shown). Various modulation and processing techniques available to those skilled in the art are applicable to the present invention. 
   In OFDM modulation, the transmission band is divided into multiple, orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used. 
   OFDM modulation requires the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. For demodulation, the performance of a Fast Fourier Transform (FFT) on the received signal is required to recover the transmitted information. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform (DFT), respectively. Accordingly, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands within a transmission channel. The modulated signals are digital signals having a relatively low transmission rate and capable of staying within their respective bands. The individual carrier waves are not modulated directly by the digital signals. Instead, all carrier waves are modulated at once by IFFT processing. 
   In the preferred embodiment, OFDM is used for at least the downlink transmission from the base stations  14  to the mobile terminals  16 . Each base station  14  is equipped with n transmit antennas  28 , and each mobile terminal  16  is equipped with m receive antennas  40 . Notably, the respective antennas can be used for reception and transmission using appropriate duplexers or switches and are so labeled only for clarity. 
   With reference to  FIG. 5 , a logical OFDM transmission architecture is provided according to one embodiment. Initially, the base station controller  10  will send data to be transmitted to various mobile terminals  16  to the base station  14 . The base station  14  may use the CQIs associated with the mobile terminals to schedule the data for transmission as well as select appropriate coding and modulation for transmitting the scheduled data. The CQIs may be directly from the mobile terminals  16  or determined at the base station  14  based on information provided by the mobile terminals  16 . In either case, the CQI for each mobile terminal  16  is a function of the degree to which the channel amplitude (or response) varies across the OFDM frequency band. 
   The scheduled data  44 , which is a stream of bits, is scrambled in a manner reducing the peak-to-average power ratio associated with the data using data scrambling logic  46 . A cyclic redundancy check (CRC) for the scrambled data is determined and appended to the scrambled data using CRC adding logic  48 . Next, channel coding is performed using channel encoder logic  50  to effectively add redundancy to the data to facilitate recovery and error correction at the mobile terminal  16 . Again, the channel coding for a particular mobile terminal  16  is based on the CQI. The channel encoder logic  50  uses known Turbo encoding techniques in one embodiment. The encoded data is then processed by rate matching logic  52  to compensate for the data expansion associated with encoding. 
   Bit interleaver logic  54  systematically reorders the bits in the encoded data to minimize the loss of consecutive data bits. The resultant data bits are systematically mapped into corresponding symbols depending on the chosen baseband modulation by mapping logic  56 . Preferably, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Key (QPSK) modulation is used. The degree of modulation is preferably chosen based on the CQI for the particular mobile terminal The symbols may be systematically reordered to further bolster the immunity of the transmitted signal to periodic data loss caused by frequency selective fading using symbol interleaver logic  58 . 
   At this point, groups of bits have been mapped into symbols representing locations in an amplitude and phase constellation. When spatial diversity is desired, blocks of symbols are then processed by space-time block code (STC) encoder logic  60 , which modifies the symbols in a fashion making the transmitted signals more resistant to interference and more readily decoded at a mobile terminal  16 . The STC encoder logic  60  will process the incoming symbols and provide n outputs corresponding to the number of transmit antennas  28  for the base station  14 . The control system  20  and/or baseband processor  22  will provide a mapping control signal to control STC encoding. At this point, assume the symbols for the n outputs are representative of the data to be transmitted and capable of being recovered by the mobile terminal  16 . See A. F. Naguib, N. Seshadri, and A. R. Calderbank, “Applications of space-time codes and interference suppression for high capacity and high data rate wireless systems,” Thirty-Second Asilomar Conference on Signals, Systems &amp; Computers, Volume 2, pp. 1803-1810, 1998, which is incorporated herein by reference in its entirety. 
   For the present example, assume the base station  14  has two antennas  28  (n=2) and the STC encoder logic  60  provides two output streams of symbols. Accordingly, each of the symbol streams output by the STC encoder logic  60  is sent to a corresponding IFFT processor  62 , illustrated separately for ease of understanding. Those skilled in the art will recognize that one or more processors may be used to provide such digital signal processing, alone or in combination with other processing described herein. The IFFT processors  62  will preferably operate on the respective symbols to provide an inverse Fourier Transform. The output of the IFFT processors  62  provides symbols in the time domain. The time domain symbols are grouped into frames, which are associated with a prefix by like insertion logic  64 . Each of the resultant signals is up-converted in the digital domain to an intermediate frequency and converted to an analog signal via the corresponding digital up-conversion (DUC) and digital-to-analog (D/A) conversion circuitry  66 . The resultant (analog) signals are then simultaneously modulated at the desired RF frequency, amplified, and transmitted via the RF circuitry  68  and antennas  28 . Notably, pilot signals known by the intended mobile terminal  16  are scattered among the sub-carriers. The mobile terminal  16 , which is discussed in detail below, will use the pilot signals for channel estimation. 
   Reference is now made to  FIG. 6  to illustrate reception of the transmitted signals by a mobile terminal  16 . Upon arrival of the transmitted signals at each of the antennas  40  of the mobile terminal  16 , the respective signals are demodulated and amplified by corresponding RF circuitry  70 . For the sake of conciseness and clarity, only one of the two receive paths is described and illustrated in detail. Analog-to-digital (A/D) converter and down-conversion circuitry  72  digitizes and downconverts the analog signal for digital processing. The resultant digitized signal may be used by automatic gain control circuitry (AGC)  74  to control the gain of the amplifiers in the RF circuitry  70  based on the received signal level. 
   Initially, the digitized signal is provided to synchronization logic  76 , which includes coarse synchronization logic  78 , which buffers several OFDM symbols and calculates an auto-correlation between the two successive OFDM symbols. A resultant time index corresponding to the maximum of the correlation result determines a fine synchronization search window, which is used by fine synchronization logic  80  to determine a precise framing starting position based on the headers. The output of the fine synchronization logic  80  facilitates frame acquisition by frame alignment logic  84 . Proper framing alignment is important so that subsequent FFT processing provides an accurate conversion from the time to the frequency domain. The fine synchronization algorithm is based on the correlation between the received pilot signals carried by the headers and a local copy of the known pilot data. Once frame alignment acquisition occurs, the prefix of the OFDM symbol is removed with prefix removal logic  86  and resultant samples are sent to frequency offset correction logic  88 , which compensates for the system frequency offset caused by the unmatched local oscillators in the transmitter and the receiver. Preferably, the synchronization logic  76  includes frequency offset and clock estimation logic  82 , which is based on the headers to help estimate such effects on the transmitted signal and provide those estimations to the correction logic  88  to properly process OFDM symbols. 
   At this point, the OFDM symbols in the time domain are ready for conversion to the frequency domain using FFT processing logic  90 . The results are frequency domain symbols, which are sent to processing logic  92 . The processing logic  92  extracts the scattered pilot signal using scattered pilot extraction logic  94 , determines a channel estimate based on the extracted pilot signal using channel estimation logic  96 , and provides channel responses for all sub-carriers using channel reconstruction logic  98 . In order to determine a channel response for each of the sub-carriers, the pilot signal is essentially multiple pilot symbols that are scattered among the data symbols throughout the OFDM sub-carriers in a known pattern in both time and frequency.  FIG. 7  illustrates an exemplary scattering of pilot symbols among available sub-carriers over a given time and frequency plot in an OFDM environment. Continuing with  FIG. 6 , the processing logic compares the received pilot symbols with the pilot symbols that are expected in certain sub-carriers at certain times to determine a channel response for the sub-carriers in which pilot symbols were transmitted. The results are interpolated to estimate a channel response for most, if not all, of the remaining sub-carriers for which pilot symbols were not provided. The actual and interpolated channel responses are used to estimate an overall channel response, which includes the channel responses for most, if not all, of the sub-carriers in the OFDM channel. 
   The frequency domain symbols and channel reconstruction information, which are derived from the channel responses for each receive path are provided to an STC decoder  100 , which provides STC decoding on both received paths to recover the transmitted symbols. The channel reconstruction information provides equalization information to the STC decoder  100  sufficient to remove the effects of the transmission channel when processing the respective frequency domain symbols 
   The recovered symbols are placed back in order using symbol de-interleaver logic  102 , which corresponds to the symbol interleaver logic  58  of the transmitter. The de-interleaved symbols are then demodulated or de-mapped to a corresponding bitstream using de-mapping logic  104 . The bits are then de-interleaved using bit de-interleaver logic  106 , which corresponds to the bit interleaver logic  54  of the transmitter architecture. The de-interleaved bits are then processed by rate de-matching logic  108  and presented to channel decoder logic  110  to recover the initially scrambled data and the CRC checksum. Accordingly, CRC logic  112  removes the CRC checksum, checks the scrambled data in traditional fashion, and provides it to the de-scrambling logic  114  for de-scrambling using the known base station de-scrambling code to recover the originally transmitted data  116 . 
   In parallel to recovering the data  116 , a CQI, or at least information sufficient to create a CQI at the base station  14 , is determined and transmitted to the base station  14 . As noted above, the CQI in a preferred embodiment is a function of the carrier-to-interference ratio (CIR), as well as the degree to which the channel response varies across the various sub-carriers in the OFDM frequency band. For this embodiment, the channel gain for each sub-carrier in the OFDM frequency band being used to transmit information are compared relative to one another to determine the degree to which the channel gain varies across the OFDM frequency band. Although numerous techniques are available to measure the degree of variation, one technique is to calculate the standard deviation of the channel gain for each sub-carrier throughout the OFDM frequency band being used to transmit data. 
   Continuing with  FIG. 6 , a relative variation measure may be determined by providing the channel response information from the channel estimation function  96  to a channel variation analysis function  118 , which will determine the variation and channel response for each of the sub-carriers in the OFDM frequency band, and if standard deviation is used, calculate the standard deviation associated with the frequency response. As noted, channel gain is a preferred measure of the channel response for calculating a CQI. The channel gain may be quantified based on a relative amplitude of the channel frequency response in decibels (dB), and as such, the amplitude of the channel frequency response may be represented by H dB (k), which is a function of a sub-carrier index k, where k=1 . . . k MIN , . . . k MAX , . . . k FFT . Notably, k FFT  is the number of sub-carriers in the entire OFDM frequency band, and the sub-carriers k MIN  through k MAX  represent the sub-carriers within the OFDM frequency band that are actually used to transmit data. Typically, a range of sub-carriers at either end of the range of sub-carriers are not used, in order to minimize interference with other transmissions. As such the degree of variation of the amplitude of the channel response may be determined only for the range of sub-carriers being used to transmit data (k MIN  through k MAX ). The standard deviation of the channel response across the usable range of sub-carriers is calculated as follows: 
                   std   =         1       N   u     -   1       ⁢       ∑     k   MIN       K   MAX       ⁢           ⁢       (         H     d   ⁢           ⁢   B       ⁡     (   k   )       -       H   _       d   ⁢           ⁢   B         )     2             ,           Eq   .           ⁢   1               
where N u  is the number of usable sub-carriers, H dB (k) is the log amplitude of the channel frequency response, and  H   db  is the mean of the log amplitude of the channel response across the usable range of sub-carriers or a subset thereof.
 
   In a multiple-input multiple-output (MIMO) system where there are multiple transmit and multiple receive antennas  28 ,  40  each link corresponding to transmit/receive antenna pairs will have a unique CQI. An aggregate CQI, or set of aggregate CQIs, may be required for the overall MIMO set of links. To determine the aggregate CQIs, the channel frequency response and CIR for each transmit and receive antenna pair is determined. 
   For multiple receive antennas  40 , the multiple channel frequency responses are combined, to provide for the diversity achieved from the multiple receive antennas  40 . This combining is an averaging of the power of the respective channel frequency responses across the OFDM frequency band. The channel variation measure is then determined across the combined channel frequency response. The CIR values for the respective multiple receive antennas  40  are combined by summing. 
   For multiple transmit antennas  28 , the modification to the CQI will depend on the particular space time coding technique employed to reflect the method by which the transmit diversity is being achieved by the code and used by the system. Some schemes, such as transmit diversity, will require that the respective channel frequency responses from the multiple transmit antennas  28  be combined as described for the multiple receive antennas  40  by averaging the power of the channel frequency responses across the OFDM frequency band. The channel variation measure is made across the combined frequency response. Further, the CIR values for the multiple transmit antennas  28  are also combined. For other schemes, a separate CQI may be determined for each transmit antenna  28  and relayed back to the base station  14 . The base station  14  may use the CQI per transmit antenna  28  to separately adapt the modulation and coding on the data transmitted on the respective transmit antennas  28 . 
   With reference to  FIG. 8 , an example channel frequency response is illustrated, wherein channel gain is represented by the log amplitude of the channel gain across the k sub-carriers for the entire OFDM frequency range. In this case, the entire OFDM frequency range has 1,024 sub-carriers, and the standard deviation for the given example is 4.7 dB. Again, although standard deviation is illustrated as a technique for determining the degree of variation throughout the range of usable sub-carriers, those skilled in the art will recognize numerous techniques for determining the relative variation thereof. 
   Continuing with  FIG. 6 , once the channel variation analysis is provided, a variation measure is provided to a CQI function  120  or to the baseband processor  34  for transmission back to the base station  14  via the transmit circuitry  36 , depending on the configuration of the embodiment. If the CQI is determined at the base station  14 , then the mobile terminal  16  will provide information indicative of the CIR as well as the variation analysis to the base station  14 , which will calculate a CQI and control scheduling as well as coding and modulation for subsequent transmissions to the mobile terminal  16 . If the CQI is generated at the mobile terminal  16  and transmitted to the base station  14 , the CQI function  120  will receive a CIR from a CIR function  122  and will use the CIR and the variation measurement to either calculate or look up through a look-up table an appropriate CQI, which is then transmitted to the base station  14  via the transmit circuitry  36 . 
   The CIR function  122  will preferably receive channel response information from the channel estimation function  96  and determine the CIR based on the relative strengths of the desired carrier in light of other interferers in traditional fashion When the pilot symbols are passed through the channel estimation function  96 , the pilot symbols are filtered in a manner exploiting the known pilot symbols to remove noise and interference. The output of the channel estimation function  96  is intended to be a noiseless replica of the pilot symbol. With this replica, the carrier power may be determined, as well as subtracted from the received pilot symbol to yield a noise plus interference signal. This resulting signal is computed to provide an interference power, which is compared to the carrier power to determine the CIR. One example of determining a CIR is provided in co-assigned U.S. patent application Ser. No. 10/038,916 filed Jan. 8, 2002. Those skilled in the art will recognize numerous techniques for determining the CIR. Importantly, the CQI, whether calculated at the mobile terminal  16  or at the base station  14 , is based on the variation measure indicia, preferably in light of a CIR. Since the CIR for an OFDM system fails to account for the respective responses for each of the sub-carriers used for transmission, providing a CQI based on the CIR and the variation measure indicia significantly improves the performance of the OFDM system by allowing the base station  14  to better predict an appropriate coding and modulation technique, as well as to provide scheduling among the multiple users. 
   Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.