Abstract:
A method and apparatus for detecting transmission of a packet in a wireless communication system are disclosed. A packet includes a preamble which comprises multiple repetition of a training sequence. A signal is detected and the detected signal is correlated with a delayed version of the detected signal which is delayed by a predetermined delay time to generate a sequence of correlations. The correlations are normalized. A difference between a first normalized correlation and a second normalized correlation that are separated by the predetermined delay time is calculated. The difference is compared to a threshold. A transmission of the packet is detected based on the comparison. The first and second normalized correlations may be averaged over first and second averaging periods, respectively. A point generating a maximum difference may be identified, and packet synchronization may be performed based on the identified point.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/751,331 filed Dec. 16, 2005, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is related to wireless communication systems. More particularly, the present invention is related to a method and apparatus for detecting transmission of a packet in a wireless communication system.  
       BACKGROUND  
       [0003]     Synchronization is an essential task for any digital communication system. Without accurate synchronization, it is not possible to reliably receive a transmitted packet. For packet switched networks, the synchronization has to be achieved during a very short time after the start of an incoming packet. To facilitate fast synchronization, under the current wireless local area network (WLAN) standards, such as IEEE 802.11a/g/n, a preamble is included in the beginning of each packet. The length and contents of the preamble are carefully designed to provide enough information for signal detection and synchronization without any unnecessary overhead.  
         [0004]     An IEEE 802.11a/g/n WLAN system is essentially a random access system, in which a receiver does not know exactly when a packet starts. The first task of the receiver is to detect a signal and the start of an incoming packet. Generally, the signal detection can be performed as a binary hypothesis test where a decision variable is compared to a threshold. If the decision variable is smaller than the threshold, it is determined that a packet is not present in the signal. If the decision variable is greater than or equal to the threshold, it is determined that a packet is present in the signal.  
         [0005]      FIG. 1  is a functional block diagram of a conventional detector  100 . The conventional detector  100  is a delay and correlator type detector. The detector  100  includes a delay unit  102 , a complex conjugate unit  104 , a multiplier  106 , an integration unit  108 , a magnitude square unit  110 , a divider  116 , an auto-correlation unit  112  and a square unit  114 . The conventional detector  100  takes advantage of the periodicity of short training sequences at the start of an IEEE 802.11a/g/n preamble  202 .  FIG. 2  shows an exemplary packet  200  including the preamble  202 . The preamble  202  includes a short training sequence (t 1 -t 10 ) and a long training sequence (T 1 , T 2 ). The short training sequence is repeated ten (10) times and the long training sequence is repeated two (2) times.  
         [0006]     Referring to  FIG. 1 , a received signal  101  is delayed by the delay unit  102  by a predetermined period of time, (e.g., 0.8 μs), and a complex conjugate of the delayed signal  103  is generated by the complex conjugate unit  104 . The received signal  101  and the complex conjugate  105  of the delayed received signal  103  are multiplied by the multiplier  106 . The multiplied result is integrated over a first sliding window by the integration unit  108  to generate a cross-correlation  109 . A magnitude square  111  of the cross-correlation  109  is calculated by the magnitude square unit  110 .  
         [0007]     The delayed received signal  103  is also processed by the auto-correlation unit  112  to compute a signal energy value  113  over a second sliding window. The signal energy value  113  is squared by the square unit  114 . A normalized correlation is then computed using the divider  116  which divides the cross-correlation magnitude square  111  with the squared signal energy value  115 . The normalized correlation  117  is compared to a threshold to determine a presence of a transmitted packet.  
         [0008]     As mentioned above, there are two sliding windows used in the conventional detector  100  of  FIG. 1 . The first sliding window is for calculating the cross-correlation between the received signal  101  and the delayed received signal  103 . The second sliding window is for calculating the received signal energy. The calculated received signal energy is used to normalize the cross-correlation, so that the cross-correlation is not dependent on an absolute received power level.  
         [0009]     When signal strength is low, the noise variance contributes to the signal energy calculation significantly differently to the cross-correlation. Therefore, the signal detection decision variable is dependent to a signal-to-noise ratio (SNR) and the threshold should be set differently based on the SNR.  
       SUMMARY  
       [0010]     The present invention is related to a method and apparatus for detecting transmission of a packet in a wireless communication system. A packet includes multiple repetition of a training sequence. A signal is detected and the detected signal is correlated with a delayed version of the detected signal delayed by a predetermined delay time to generate a sequence of correlations. The correlations are normalized. A difference between a first normalized correlation and a second normalized correlation that are separated by the predetermined delay time is calculated. The difference is compared to a threshold. A transmission of the packet is detected based on the comparison. The first and second normalized correlations may be averaged over first and second averaging periods, respectively. A point generating a maximum difference may be identified, and packet synchronization may be performed based on the identified point. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a functional block diagram of a conventional detector.  
         [0012]      FIG. 2  shows an exemplary frame including a preamble processed by the convention detector of  FIG. 1 .  
         [0013]      FIG. 3  is a block diagram of an apparatus for detecting transmission of a packet in accordance with the present invention.  
         [0014]      FIG. 4  shows typical correlation values calculated from the detected signals.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Hereafter, the terminology “wireless transmit/receive unit” (WTRU) includes but is not limited to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “access point” (AP) includes but is not limited to a Node-B, a base station, a site controller, or any other type of interfacing device capable of operating in a wireless environment.  
         [0016]     The present invention may be implemented in a WTRU, a base station or a WLAN system at the physical layer in the radio and digital baseband. The implementation may be in the form of an application specific integrated circuit (ASIC), digital signal processor (DSP), middleware or hardware. The present invention is applicable to IEEE 802.11/16/20. The present invention may be implemented in a smart antenna, an enhanced uplink or orthogonal frequency division multiplexing (OFDM)/multiple-input multiple-output (MIMO) capable system, as well as a non-cellular system.  
         [0017]     The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
         [0018]      FIG. 3  is a functional block diagram of an apparatus  300  for detecting transmission of a packet in accordance with the present invention. The apparatus  300  is an improvement over the conventional detector  100  of  FIG. 1 . The apparatus  300  includes a correlation unit  310 , a signal energy calculator  320 , a divider  330 , a difference calculator  340 , a comparator  350  and a maximum identifier  360  (optional).  
         [0019]     A variable, y(n), represents a signal  302  detected from each receive antenna (not shown). The detected signal  302  is fed to the correlation unit  310  and the signal energy calculator  320 . The correlation unit  310  calculates correlations of detected signal and a delayed version of the detected signal. The correlation unit  310  may include a delay unit  312 , a complex conjugate unit  314 , a multiplier  316 , an integrator  318  and a magnitude calculation unit  319 . The delay unit  312  delays the detected signals  302  by a predetermined delay time, preferably of a duration of one training sequence (T slot ). For example, in an IEEE 802.11a system, a 0.8 μs short training sequence, (i.e., 16 symbols), is repeated ten (10) times. Therefore, the delay time may be set to 0.8 μs, (i.e., 16 symbols). The complex conjugate unit  314  generates a complex conjugate of the delayed detected signal  313 . The detected signal  302  and the complex conjugate  315  of the delayed detected signal  313  are then multiplied by the multiplier  316 . The output  317  of the multiplier  316  is integrated by the integrator  318  over an integration interval to generate correlations, C(n). The integration interval may also be set to 0.8 μs. The magnitude of the correlations is computed by the magnitude calculation unit  319 .  
         [0020]     The signal energy calculator  320  calculates signal energy of the detected signal  302  for the correlation calculation interval, (e.g., 0.8 μs). The signal energy calculator  320  may include a complex conjugate unit  322 , a multiplier  324  and an integrator  326 . A complex conjugate  323  of the detected signals  302  is generated by the complex conjugate unit  322 . The detected signals  302  and the complex conjugate  323  of the detected signals are multiplied by the multiplier  324 . The output  325  of the multiplier  324  is then integrated by the integrator  326  over an integration interval to calculate signal energy, C 1 (n), over the correlation calculation interval.  
         [0021]     It should be noted that the configuration of the correlation unit  310  and the signal energy calculation unit  320  shown in  FIG. 3  is provided as an example and any other configuration may be implemented.  
         [0022]     The divider  330  divides the magnitude of the correlations, |C(n)|, with the signal energy C 1 (n), to generate normalized correlations, P(n). The correlations C(n), the signal energy C 1 (n) and the normalized correlations P(n) may be written as follows:  
                 C   ⁡     (   n   )       =       ∫   n     n   +   T       ⁢       y   ⁡     (   t   )       ⁢   y   *     (     t   -   D     )     ⁢     ⅆ   t           ;           Equation   ⁢           ⁢     (   1   )                         C   1     ⁡     (   n   )       =       ∫   n     n   +   T       ⁢       y   ⁡     (   t   )       ⁢   y   *     (   t   )     ⁢     ⅆ   t           ;     ⁢           ⁢     
     ⁢   and           Equation   ⁢           ⁢     (   2   )                   P   ⁡     (   n   )       =         C   ⁡     (   n   )           C   1     ⁡     (   n   )         .             Equation   ⁢           ⁢     (   3   )               
 
         [0023]      FIG. 4  shows normalized correlation values, P(n), calculated from the simulation. In the simulation, the integration interval is set to 0.8 μs, an SNR is set to 20 dB, and received signals are 5× over-sampled. In a conventional method, the normalized correlation, P(n), is searched over the time index, n. If P(n) is greater than or equal to a threshold, a signal detection is declared and the corresponding n is the starting point of the packet. The conventional method has a disadvantage that the threshold should be set differently depending on an SNR. The present invention alleviates this problem.  
         [0024]     The present invention utilizes a differential detection method. At the starting point of the packet, n 1 , the difference between P(n 1 ) and P(n 1 +T slot ) is maximum. As shown in  FIG. 4 , the normalized correlation rises from the minimum at a time index  500  to the maximum at a time index  580 , which are separated by 80 samples, (i.e., 16 delay×5). Before the starting point of the packet, the normalized correlation values includes only noise and after the starting point of the packet the normalized correlation values increase to around one (1). Therefore, the difference of the two normalized correlation values P(n 1 ) and P(n 1 +T slot ) that are separated by the T slot  is maximum at the starting point of the packet.  
         [0025]     Referring back to  FIG. 3 , the difference calculator  340  includes a subtractor  346  which calculates a difference between P(n) and P(n+T slot ) that are separated by a delay time, preferably T slot , (e.g., 16 samples), over n. Optionally, the difference calculator  340  may include two averaging units  342 ,  344  such that, before calculating the difference, the P(n 1 ) and P(n 1 +T slot ) are averaged over averaging periods L and L′, respectively, as follows:  
               S   ⁡     (   n   )       =         1   L     ⁢       ∑     l   =   0       L   -   1       ⁢     P   ⁡     (     n   +   D   +   l     )           -       1     L   ′       ⁢       ∑       l   ′     =   0         L   ′     -   1       ⁢       P   ⁡     (     n   -     l   ′       )       .                   Equation   ⁢           ⁢     (   4   )               
 
 L and L′ may be same. The purpose of calculating an average is to reduce the effect of noise. 
 
         [0026]     The comparator  350  compares the difference with a threshold and outputs a signal  352 . If the difference is greater than or equal to the threshold, the signal  352  indicates a detection of the packet. If the difference is smaller than the threshold, the signal  352  indicates no detection of the packet.  
         [0027]     The maximum identifier  360  identifies a local maximum for the difference, (or optionally S(n)), that are greater than or equal to the threshold. The maximum identifier  360  outputs a signal  362  indicating the point with the maximum value of the difference, (or optionally S(n)), as the starting point of the packet.  
         [0028]     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The methods provided in the present invention may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs).  
         [0029]     Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any integrated circuit, and/or a state machine.  
         [0030]     A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, user equipment, terminal, base station, radio network controller, or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a videocamera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a handsfree headset, a keyboard, a Bluetooth module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.