Patent Publication Number: US-7724849-B2

Title: Methods and apparatus for noise estimation in a communication system

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
   The present Application for Patent claims priority to Provisional Application No. 60/756,071 entitled “Method and Apparatus of Noise and Interference Estimation in an OFDM Communication System” filed Jan. 3, 2006, and assigned to the assignee hereof and hereby expressly incorporated by reference herein. 

   BACKGROUND 
   1. Field 
   The present application relates generally to the operation of communication systems, and more particularly, to methods and apparatus for noise estimation in a communication system. 
   2. Background 
   Data networks, such as wireless communication networks, have to trade off between services customized for a single terminal and services provided to a large number of terminals. For example, the distribution of multimedia content to a large number of resource limited portable devices (subscribers) is a complicated problem. Therefore, it is very important for network administrators, content retailers, and service providers to have a way to distribute content and/or other network services in a fast and efficient manner and in such a way as to increase bandwidth utilization and power efficiency. 
   In current content delivery/media distribution systems, real time and non real time services are packed into a transmission frame and delivered to devices on a network. For example, a communication network may utilize Orthogonal Frequency Division Multiplexing (OFDM) to provide communications between a network server and one or more mobile devices. This technology provides a plurality of subcarriers that are modulated with data representing services to be delivered over a distribution network as a transmit waveform. Thus, the ability to receive and accurately process the transmit waveform determines how well the system will perform. 
   The signal to interference/noise ratio (SINR) is one of the most important parameters that characterize the signal quality and the receiver performance in a communication system. The SINR is defined as the quotient of the signal power divided by the power of noise and interference. 
   The estimation of the signal power is well known and relatively simple to determine. Conventionally, a received signal strength indicator (RSSI) is measured and used as an estimate of the signal power. Such an estimate can be quite accurate especially when the interference/noise power is relatively low, i.e., in a communication environment with high SINR. However, it is more difficult to accurately estimate the interference/noise power in such an environment because the weak noise and interference energy is “buried” in the strong signal. 
   One approach to estimate the interference/noise power in such an environment is to stop the transmission of the signal so the received signal strength measured in such a condition will represent the power of the noise and interference. However, such an approach may not be appropriate for some applications. First of all, during the disruption of transmission, no useful information is transmitted by the transmitter and this reduces the channel&#39;s utilization. Moreover, in some communication systems, e.g., an OFDM multimedia broadcasting system, stopping or cycling transmissions from high power transmitters may damage equipment and is not recommended. 
   Therefore, it would be desirable to have a system that operates to accurately determine an interference and noise estimate at a receiving device in a communication network without disrupting normal network operations, thereby allowing a SINR to be determined so that the performance of the network can be determined and/or optimized. 
   SUMMARY 
   In one or more aspects, a noise estimation system is provided that operates to provide noise estimates in a communication system. For example, the system is operable to provide a signal to interference and noise ratio in an OFDM communication system. In an aspect, the system operates to measure noise and interference power at unmodulated subcarriers. The measured values are then used to determine a SINR of the communication system as experienced by a receiving device. As a result, the performance of the communication system can be determined and/or optimized. 
   In an aspect, a method for noise and interference estimation is provided. The method comprises identifying one or more unmodulated subcarriers in a received waveform, processing the one or more unmodulated subcarriers to produce a demodulated output, and determining a noise variance based on the demodulated output. 
   In another aspect, an apparatus for providing a noise and interference estimation is provided. The apparatus comprises selection logic configured to identify one or more unmodulated subcarriers in a received waveform, a processor configured to demodulate the one or more unmodulated subcarriers to produce a demodulated output, and variance determination logic configured to determine a noise variance based on the demodulated output. 
   In another aspect, an apparatus for providing a noise and interference estimation is provided. The apparatus comprises means for identifying one or more unmodulated subcarriers in a received waveform, means for processing the one or more unmodulated subcarriers to produce a demodulated output, and means for determining a noise variance based on the demodulated output. 
   In another aspect, a computer-readable medium is provided that has a computer program comprising instructions, which when executed, operate to provide a noise and interference estimation. The computer program comprises instructions for identifying one or more unmodulated subcarriers in a received waveform, instructions for processing the one or more unmodulated subcarriers to produce a demodulated output, and instructions for determining a noise variance based on the demodulated output. 
   In another aspect, at least one processor is provided that is configured to perform a method for providing a noise and interference estimation. The method comprises identifying one or more unmodulated subcarriers in a received waveform, processing the one or more unmodulated subcarriers to produce a demodulated output, and determining a noise variance based on the demodulated output. 
   In still another aspect, a method for providing a noise and interference estimation is provided. The method comprises generating an OFDM transmission frame that comprises one or more unmodulated subcarriers, encoding location information associated with the one or more unmodulated subcarriers into the transmission frame, and transmitting the transmission frame. 
   Other aspects will become apparent after review of the hereinafter set forth Brief Description of the Drawings, Description, and the Claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects described herein will become more readily apparent by reference to the following description when taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  shows a network that comprises an aspect of a noise estimation system; 
       FIG. 2  shows an aspect of a transmission frame for use in a noise estimation system; 
       FIG. 3  shows an aspect of a TDM symbol in the frequency domain comprising unmodulated subcarriers for use in a noise estimation system; 
       FIG. 4  shows a receiver that comprises an aspect of a noise estimation system; 
       FIG. 5  shows an aspect of noise estimation logic for use in a noise estimation system; 
       FIG. 6  shows an aspect of a method for providing a noise estimation system; and 
       FIG. 7  shows an aspect of a noise estimation system. 
   

   DETAILED DESCRIPTION 
   In one or more aspects, a noise estimation system is provided that operates to determine noise estimates in a communication network. For example, in an aspect, the power levels at known unmodulated subcarriers in an OFDM communication system are measured to determine noise variance at a receiving device. The determined noise variance is used to determine a SINR that indicates the performance of the communication system. Furthermore, the ability to estimate the noise variance also has advantages with respect to system performance optimization as well. In particular, various algorithms that perform functions such as channel estimation or log likelihood ratio (LLR) computations can benefit from accurate noise variance estimation, which results in better performance. 
   Aspects of the noise estimation system are described herein with reference to a communication system that utilizes OFDM to provide communications between a network server and one or more mobile devices. In an OFDM communication system, an OFDM symbol is defined that comprises multiple subcarriers. The subcarriers that are modulated with data or other non-zero energy are referred to as modulated subcarriers. The subcarriers that are not modulated with data or other non-zero energy are referred to as unmodulated subcarriers (or “null” subcarriers). For example, unmodulated subcarriers have no energy in the frequency “bins” corresponding to these subcarriers. Thus, aspects of the noise estimation system operate to determine noise and/or interference levels by measuring the power levels at known unmodulated subcarriers. The power levels at the unmodulated subcarriers are then used to determine a noise variance and a SINR at a receiving device. 
   For the purpose of this description, a specific OFDM network implementation is presented in order to simplify and clarify the aspects. However, it should be noted that aspects of the noise estimation system are suitable for use in other network implementations. In a particular implementation of an OFDM system, a transmission frame is defined that comprises time division multiplex (TDM) pilot signals, frequency division multiplex (FDM) pilot signals, wide area identifiers (WIC), local area identifiers (LIC), positioning signals (PPC), overhead information symbols (OIS), and data symbols. The data symbols are used to transport content and/or services from a server to receiving devices. 
   Within the transmission frame there are a number of OFDM symbols having known unmodulated subcarriers. For example, the TDM pilot signals, the WIC/LIC symbols, the PPC symbols, and other symbols within the transmission frame comprise some number of known unmodulated subcarriers. Aspects of the noise estimation system utilize these known unmodulated subcarriers to measure noise and interference power at a receiving device. 
     FIG. 1  shows a network  100  that comprises an aspect of a noise estimation system. The network  100  comprises mobile devices  102 ,  104 ,  106 , a server  108 , and a communication network  110 . For the purpose of this description, it will be assumed that the network  110  operates to provide communications between the server  108  and one or more of the mobile devices using OFDM technology; however, aspects of the noise estimation system are suitable for use with other transmission technologies as well. 
   In one aspect, the server  108  operates to provide services that may be subscribed to by devices in communication with the network  110 . The server  108  is coupled to the network  110  through the communication link  112 , which comprises any suitable communication link. The network  110  comprises any combination of wired and/or wireless networks that allows services to be delivered from the server  108  to devices in communication with the network  110 , such as the device  102 . 
   The devices  102 ,  104 , and  106  in this aspect comprise mobile telephones that communicate with the network  110  through the wireless links  114 . The wireless links  114  comprises forward communication links based on OFDM technology and reverse communication links based on any suitable transmission technology. For the remainder of this description, aspects of the noise estimation system are described with reference to the device  102 ; however, the aspects are equally applicable to the devices  104  and  106 . 
   It should be noted that the network  110  may communicate with any number and/or types of devices. For example, other devices suitable for use in aspects of the noise estimation system include, but are not limited to, a personal digital assistant (PDA), email device, pager, a notebook computer, mp3 player, video player, or a desktop computer. 
   The server  108  comprises content that includes real time and non real time services. For example, the services comprise multimedia content that includes news, sports, weather, financial information, movies, and/or applications, programs, scripts, or any other type of suitable content or service. Thus, the services may comprise video, audio or other information formatted in any suitable format. 
   The content is input to transmission logic  116 , which processes the content to produce a transmission frame as described above. The transmission logic  116  operates to transmit the transmission frame over the network  110  as a transmit waveform using OFDM technology, as shown by path  118 . In addition to modulated subcarriers comprising data and other information, the transmit waveform comprises unmodulated subcarriers at known locations. For example, the unmodulated subcarriers may be contained in one or more special symbols, such as TDM pilot symbols, WIC/LIC symbols or PPC symbols. 
   In an aspect, the locations of unmodulated subcarriers in the transmission frame are pre-determined and known to devices in communication with the network  100 . For example, the locations of the unmodulated subcarriers in the WIC/LIC symbols are known to the devices in the network  100 . In another aspect, the locations of unmodulated subcarriers are dynamically communicated to devices in the network  100 . For example, in an aspect, the transmission logic  116  comprises subcarrier location encoder  120  that operates to encode subcarrier location information in the transmission frame. The location information operates to identify the location of unmodulated subcarriers in the transmission frame. Thus, it is possible to dynamically communicate the location of unmodulated subcarriers in the transmission frame to devices in the network  100 . It should also be noted that the location of the unmodulated subcarriers may also be communicated using any other suitable technique, such as by transmitting the location information in an out-of-band transmission. 
   The device  102  receives the transmit waveform at estimation logic  122 . The estimation logic  122  operates to determine a noise variance and provide an estimate of the SINR of the communication system experienced at the device  102  by measuring the power at the unmodulated subcarriers. For example, the system utilizes the fact that not all of the subcarriers in the transmit waveform are modulated. As described above, the transmit waveform comprises a few special OFDM symbols, including the TDM, WIC/LIC, and PPC symbols, which are used for synchronization and other purposes. Unlike the normal data OFDM symbols, in which all of the subcarriers are modulated by data, these special symbols comprise a number of known unmodulated subcarriers (i.e., they do not carry any signal power). 
   When these unmodulated subcarriers are processed (or “demodulated”), the demodulated output should be zero if there is no additive noise and/or interference. Any non-zero values at these unmodulated subcarriers are due to noise and/or interference experienced at the receiving device. The variance of the received noise and interference can be estimated by computing the variance of the demodulated output at these null subcarriers. By dividing the received signal power by the variance of the received noise, the SINR of the communication system as experienced by the device  102  can be determined. 
   In an aspect, the location of the unmodulated subcarriers is known in advance. For example, the estimation logic  122  at the device  102  knows that the unmodulated subcarriers are located in the special OFDM symbols. However, in another aspect, the location of the unmodulated subcarriers is provided to a receiving device dynamically. For example, the subcarrier location encoder  120  operates to encode the locations of unmodulated subcarriers in the transmit waveform. In another aspect, the subcarrier location encoder  120  communicates the locations of the unmodulated subcarriers in an out-of-band channel, or provides this information to receiving devices in any other fashion. 
   In an aspect, the estimation logic  122  optionally comprises subcarrier selection logic  124 . The selection logic  124  operates to detect the location of unmodulated subcarriers. For example, the selection logic  124  operates to decode the received transmit waveform to determine the location of the unmodulated subcarriers. In another aspect, the selection logic  124  operates to receive the locations of the unmodulated subcarriers in an out-of-band transmission. Thus, the selection logic  124  operates to determine the location of the unmodulated subcarriers when their locations are dynamically changing. 
   Once the locations of the unmodulated subcarriers are determined, the estimation logic  122  operates to demodulate these subcarriers to measure noise/interference power. A received signal power is also determined from modulated subcarriers, which may be data symbols. The SINR value can be computed by dividing the received signal power by the measured interference/noise power. Similarly, the SINR distribution vs. frequency can be computed the by dividing the signal power received at different frequencies by the measured interference/noise power at corresponding frequencies. Preferably, the unmodulated subcarriers are distributed over the entire OFDM signal bandwidth so that it will be possible to compute the distribution of the noise/interference power vs. frequency. The computed SINR values can be used as an indicator of the signal quality and/or for scaling the LLR values as a decoding metric. In a system with feedback, e.g. OFDMA systems, computed SINR values can be used for rate adaptation and bin energy/bit loading to improve system capacity. 
   Therefore, aspects of a noise estimation system operate to determine a SINR by performing one or more of the following functions at a transmitting device.
     a. Encode content/services into an OFDM transmission frame that comprises known unmodulated subcarriers.   b. Optionally encode the location of unmodulated subcarriers into the transmission frame.   c. Transmit the transmission frame over a network to receiving devices.   

   Therefore, aspects of a noise estimation system operate to determine a SINR by performing one or more of the following functions at a receiving device.
     a. Receive a transmit waveform that comprises unmodulated subcarriers.   b. Determine the locations of unmodulated subcarriers.   c. Measure noise/interference power at the unmodulated subcarriers.   d. Measure received signal power.   e. Determine an SINR from the noise/interference power and the signal power.   

   As described above, the variance of the demodulated output at the unmodulated subcarriers provides a good estimate of the variance of the noise/interference received by the receiver. In an aspect, the unmodulated subcarriers are evenly distributed over the entire band, which allows a determination of the noise/interference distribution. This information can be useful to further improve the receiver performance. The noise estimation system operates by taking advantage of the properties of the special OFDM symbols that are constructed to comprise unmodulated subcarriers. As a result, there is no disruption of the normal transmitter/receiver operation. Furthermore, the system is resource efficient since computing the noise/interference variance is straight forward and simple to implement in a receiver 
   Therefore, aspects of a noise estimation system operate to efficiently determine noise variance and corresponding SINR levels at a receiving device. It should be noted that the noise estimation system is not limited to the implementations described with reference to  FIG. 1 , and that other implementations are possible. 
     FIG. 2  shows an aspect of a transmission frame  200  for use in a noise estimation system. For example, the transmission frame  200  may be used in a forward link only (FLO) communication system. The frame  200  comprises time division multiplex (TDM) pilot signals  202 , WIC symbols  204 , LIC symbols  206 , frequency division multiplex (FDM) pilot signals  208 , overhead information symbols (OIS)  210 , data symbols  212 , and PPC symbols  214 . The data symbols  212  are used to transport services from a server to receiving devices. 
   In an aspect, the TDM  202 , WIC  204 , LIC  206  and PPC  214  symbols represent special symbols in which not all of their subcarriers are modulated. Thus, these symbols are already designed to include known unmodulated subcarriers and are therefore suitable for use in aspects of a noise estimation system. It is also possible that other symbols in the transmission frame  200  contain unmodulated subcarriers which could be utilized. Furthermore, in a dynamic implementation, the noise estimation system operates to dynamically determine any subcarriers in the frame  200  that are unmodulated and encode location information into the frame  200  that identifies these unmodulated subcarriers. 
     FIG. 3  shows an aspect of a TDM symbol  300  comprising unmodulated subcarriers for use in a noise estimation system. For example, the TDM symbol  300  may be one of the TDM symbols  202  shown in  FIG. 2 . 
   The TDM symbol  300  comprises modulated subcarriers  302  spaced over selected frequency intervals. Between the modulated subcarriers are unmodulated subcarriers  304 . The unmodulated subcarriers  304  contain no signal power so that any power received at these subcarrier locations is due to noise and interference. 
     FIG. 4  shows an aspect of a receiver  400  for use in a noise estimation system. For example, the receiver  400  is suitable for use at the devices  102 ,  104 , and  106  shown in  FIG. 1 . 
   The receiver  400  comprises radio frequency (RF) processing logic  402  that operates to receive and process a transmit waveform. For example, the RF processing logic  402  is suitable to receive and process the transmit waveform shown at path  118  in  FIG. 1 . The RF processing logic  402  operates to produce a time domain waveform  404  comprising a transmission frame that is input to fast Fourier transform (FFT) logic  406 . 
   The FFT logic  406  operates to transform the time domain waveform  404  to produce a frequency domain waveform  408  that comprises symbols having a plurality of subcarriers. The frequency domain waveform  408  is input to channel estimation logic  410  that operates to provide channel estimates. The frequency domain waveform  408  is also input to data demodulator  412  that operates to demodulate modulated subcarriers in data symbols to produce data that is passed to user applications. 
   The frequency domain waveform  408  is also input to estimation logic  414 . For example, the estimation logic  414  is suitable for use as the estimation logic  122  shown in  FIG. 1 . The estimation logic  414  comprises subcarrier selection logic  416 . The subcarrier selection logic  414  operates to identify unmodulated subcarriers in the frequency domain waveform  408 . For example, the unmodulated subcarriers may be contained in special symbols as described above, or may be identified in the transmission frame. The estimation logic  414  also comprises estimator  418 . In an aspect, the estimator  418  operates to process the unmodulated subcarriers to estimate the noise variance as experienced at the receiver  400 . The noise variance is then used to determines a SINR estimate. 
     FIG. 5  shows an aspect of estimation logic  500  for use in a noise estimation system. For example, the estimation logic  500  is suitable for use as the estimation logic  122  shown in  FIG. 1  or the estimation logic  414  shown in  FIG. 4 . The estimation logic  500  comprises subcarrier selection logic  502 , signal power determination logic  504 , demodulator  506 , variance determination logic  508 , and signal to noise determination logic  510 . 
   The signal power determination logic  504  comprises any suitable hardware and/or software that operates to receive a frequency domain waveform  512  comprising a plurality of subcarriers and determine signal power. For example, the frequency domain waveform  512  may be the waveform  408  shown in  FIG. 4 . The signal power determination logic  504  outputs a signal power indicator  514  to the signal to noise determination logic  510 . 
   The subcarrier selection logic  502  comprises a CPU, processor, gate array, hardware logic, virtual machine, software, and/or any combination of hardware and software. The selection logic  502  operates to identify the location of unmodulated subcarriers in the received waveform  512 . In an aspect, the subcarrier selection logic  502  identifies known unmodulated subcarriers in special symbols included in the received waveform  512 . For example, the known unmodulated subcarriers may be part of TDM, WIC, LIC, PPC or any other symbol in a received waveform  512 . For example, the known unmodulated symbols may be part of TDM symbols as illustrated in  FIG. 3 . The subcarrier selection logic  502  provides a selection signal  516  to the demodulator that indicates the location of the unmodulated subcarriers. 
   In another aspect, the subcarrier selection logic  502  operates to determine the location of unmodulated subcarriers by decoding the received waveform  512 . For example, in an aspect, the location of the unmodulated subcarriers is encoded into the received waveform  512  by the transmission logic  116 . In another aspect, the selection logic  502  receives the location of the unmodulated subcarriers in an out-to-band communication  518 . 
   The demodulator  506  comprises a CPU, processor, gate array, hardware logic, virtual machine, software, and/or any combination of hardware and software. The demodulator  506  operates to receive the selection signal  516  and the received waveform  512  to select and process (i.e., demodulate) unmodulated subcarriers. For example, the selection signal  516  identifies subcarriers in special symbols in the received waveform  512 . The demodulator  506  then demodulates these subcarriers to produce a demodulated output  520  that is input to the variance determination logic  508 . 
   The variance determination logic  508  comprises a CPU, processor, gate array, hardware logic, virtual machine, software, and/or any combination of hardware and software. The variance determination logic  508  operates to receive the demodulated output  520  and determine a variance  522  that is input to the signal to noise determination logic  510 . In an aspect, the variance determination logic  508  operates to perform an algorithm to determine the variance  522 . A more detailed description of the algorithm is provided in another section of this document. 
   The signal to noise determination logic  510  comprises a CPU, processor, gate array, hardware logic, virtual machine, software, and/or any combination of hardware and software. The signal to noise determination logic  510  operates to receive the variance  522  and the signal power indicator  514  and computers a SINR  524 . For example, in an aspect, the signal to noise determination logic  510  operates to divide the signal power  514  by the variance  522  to determine the SINR  524 . 
   In an aspect, the noise estimation system comprises a computer program having one or more program instructions (“instructions”) stored on a computer-readable medium, which when executed by at least one processor, provides the functions of the noise estimation system described herein. For example, instructions may be loaded into the estimation logic  500  from a computer-readable media, such as a floppy disk, CDROM, memory card, FLASH memory device, RAM, ROM, or any other type of memory device. In another aspect, the instructions may be downloaded into the estimation logic  500  from an external device or network resource. The instructions, when executed by the estimation logic  500  operate to provide aspects of a noise estimation system as described herein. 
   Thus, the estimation logic  500  operates to determine a noise variance and SINR of a communication network as experienced at a receiving device. It should be noted that the estimation logic  500  is just one implementation and that other implementations are possible within the scope of the aspects. For example, the functions of estimation logic  500  may be embodied in a computer program that is executed by one or more processors. 
   Noise Variance Algorithm 
   In one or more aspects, the variance determination logic  508  operates to perform an algorithm to compute a noise variance based on received unmodulated subcarriers. The following is a description of one aspect of the algorithm. 
   It will be assumed that y 0, y 1,  . . . y N  correspond to the time domain samples of a special symbol that is identified to contain unmodulated carriers. The k th  frequency domain subcarrier can be obtained via a discrete Fourier transform as follows. 
   
     
       
         
           
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   If the subcarrier selection logic  502  has identified the subcarriers k 1 , k 2 , . . . k P  to correspond to unmodulated subcarriers in the frequency domain, then the noise and interference variance estimate can be obtained from the following expression. 
   
     
       
         
           
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   It should be noted that other possible implementations of this algorithm as well as other noise and interference variance estimations are also possible. In the case of WIC/LIC symbols, only selected subcarriers may be unmodulated. In an aspect, an interlace is defined that identifies selected subcarriers in the symbol. For example, interlace 0 defines a collection of subcarriers with indices of the form 8 m (where m=0,1,2, . . . 511). This collection of subcarriers is modulated with non-zero energy, while all the remaining subcarriers in the symbol are unmodulated subcarriers. For reduced implementation complexity as well as providing sufficiently reliable noise and interference estimation, the estimation logic  500  operates to compute the noise and interference variance based only on interlace (s) where interlace s corresponds to subcarriers with indices of the form 8m+s (where s=1,2 . . . 7, and m=0,1,2 . . . 511). Similarly, interlaces or part of interlaces can be chosen for noise and variance estimation in the case of TDM and PPC symbols as well. 
     FIG. 6  shows an aspect of a method  600  for providing a noise estimation system. For example, the estimation logic  500  is configured to perform the method  600  as describe below. 
   At block  602 , a transmit waveform is received that comprises a transmission frame having special symbols that contain one or more unmodulated sub-carriers. In an aspect, the transmit waveform is received at a device from an OFDM network. For example, the received transmit waveform may be the waveform  512  shown in  FIG. 5 . 
   At block  604 , the location of unmodulated subcarriers is determined. In an aspect, the selection logic  502  identifies the location of the unmodulated subcarriers in the transmit waveform  512 . For example, the unmodulated subcarriers are located at fixed locations in special symbols of the waveform  512  that are known to the selection logic  502 . In another aspect, the locations of the unmodulated subcarriers are encoded into the waveform  512  and decoded by the selection logic  502 . In another aspect, the locations of the unmodulated subcarriers are provided to the selection logic  502  in an out-of-band transmission  518 . The selection logic  502  operates to determine the locations of the unmodulated subcarriers in any of the above implementations and provides the selection signal  516  that identifies the locations of the unmodulated subcarriers. 
   At block  606 , the unmodulated subcarriers are processed (i.e., demodulated). In an aspect, the demodulator  506  operates to demodulate the unmodulated subcarriers to produce the demodulated output  520 . For example, the demodulator  506  receives the selection signal  516 , which identifies the locations of unmodulated subcarriers to be processed. 
   At block  608 , a variance of the demodulated output is determined. For example, the variance determination logic  508  operates to determine the variance of the demodulated output  520  to produce the variance output  522 . In an aspect, the variance determination logic  508  operates to perform the noise variance algorithm described above to determine the variance output  522 . 
   At block  610 , a signal power associated with the received waveform is determined. In an aspect, the signal power determination logic  504  operates to determine the signal power  514  of the received waveform  512  using any suitable technique. 
   At block  612 , a SINR is determined. In an aspect, the signal to noise determination logic  510  operates to determine the SINR  524  from the variance output  522  and the signal power  514 . For example, the signal power  514  is divided by the variance output  522  to determine the SINR  524 . 
   Thus, the method  600  operates to determine a noise variance and SINR of a communication system as experienced by a receiving device. It should be noted that the method  600  represents just one implementation and the changes, additions, deletions, combinations or other modifications of the method  600  are possible within the scope of the aspects. 
     FIG. 7  shows an aspect of a noise estimation system  700 . The noise estimation system  700  comprises means ( 702 ) for identifying unmodulated subcarriers, means ( 704 ) for processing the unmodulated subcarriers, means ( 706 ) for determining a noise variance, and means ( 708 ) for determining a SINR. 
   In an aspect, the means  702  comprises the subcarrier selection logic  502 , the means  704  comprises the demodulator  506 , the means  706  comprises the variance determination logic  508 , and the means  708  comprises the signal to noise determination logic  510 . In another aspect, the means  702 - 708  are implemented by at least one processor configured to execute program instructions to provide aspects of a noise estimation system as described herein. 
   Therefore various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
   The steps of a method or algorithm described in connection with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor, such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
   The description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects, e.g., in an instant messaging service or any general wireless data communication applications, without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. The word “exemplary” is used exclusively herein to mean “serving as an example, instance, or illustration.” Any aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects. 
   Accordingly, while aspects of a noise estimation system have been illustrated and described herein, it will be appreciated that various changes can be made to the aspects without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.