Abstract:
Briefly, a method and apparatus to detect and acquire a frame within a transmission are provided. The apparatus may be coupled to at least two antennas and may include a frame acquisition module to detect and acquire the frame received within a transmission by one of the antennas.

Description:
BACKGROUND OF THE INVENTION 
   A receiver and, more particularly, a receiver that may be used with a wireless local area network (WLAN) may utilize a predetermined time to detect a transmission. A transmission may include a preamble signal of 16 microsecond (uSec). The first half of the preamble signal may include 10 repetitions of a short training sequence of 0.8 uSec. The second half of the preamble signal may be used for acquiring a frequency of the transmission and may be used to estimate channel parameters. For example, the predetermined time may be the time of the preamble signal, which is the time required by some WLAN standards to acquire the frequency of the transmission. 
   Disadvantageously, the detection of the transmission and the acquisition of the frequency in a such short time interval may cause miss detection and/or false alarm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanied drawings in which: 
       FIG. 1  is a schematic illustration of a wireless communication system that may include a receiver according to an exemplary embodiment of the present invention; 
       FIG. 2  is a schematic illustration of a frame that may be helpful in understanding some embodiments of the present invention; and 
       FIG. 3  is a block diagram of a receiver according to an exemplary embodiment of the present invention. 
   

   It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However it will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
   Some portions of the detailed description, which follow, are presented in terms of algorithms and symbolic representations of operations on data bits or binary digital signals within a computer memory. These algorithmic descriptions and representations may be the techniques used by those skilled in the data processing arts to convey the substance of their work to others skilled in the art. 
   Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” or the like, refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulates and/or transforms data represented as physical, such as electronic, quantities within the computing system&#39;s registers and/or memories into other data similarly represented as physical quantities within the computing system&#39;s memories, registers or other such information storage, transmission or display devices. 
   It should be understood that the present invention may be used in a variety of applications. Although the present invention is not limited in this respect, the circuits and techniques disclosed herein may be used in many apparatuses such as receivers of a radio system. Receivers intended to be included within the scope of the present invention include, by way of example only, wireless local area network (WLAN) receivers, two-way radio receivers, digital system receivers, analog system receivers, cellular radiotelephone receivers and the like. 
   Types of WLAN receivers intended to be within the scope of the present invention include, although are not limited to, receivers for receiving spread spectrum signals such as, for example, Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Orthogonal frequency-division multiplexing (OFDM) and the like. 
   Turning to  FIG. 1 , a wireless communication system  100 , for example, a WLAN communication system, is shown. Although the scope of the present invention is not limited in this respect, the exemplary WLAN communication system  100  may include at least one access point (AP)  110  and at least one mobile unit (MU)  130 . Mobile unit  130  may include a receiver  140 , a transmitter  150 , and at least two antennas, antennas  160  and  170 . In some embodiments of the present invention, MU  130  may include an antenna array and antennas  160  and  170  may comprise antenna arrays. 
   In operation, according to an embodiment of the invention, a wireless communication link  180  may be used to transport transmissions between AP  110  and MU  130 . MU  130  may establish connection to the Internet and/or to a local area network (LAN)  190  via AP  110 . 
   In embodiments of the present invention, the communications may be modulated and transmitted with an OFDM signal. The OFDM signal may be split into several channels at different frequencies. For example, in some WLAN communication systems, wireless communication link  180  may include 52 sub-channels at different frequencies, if desired. 
   Turning to  FIG. 2 , a portion of an exemplary transmission  200  is shown. Transmission  200  may include a portion of a frame  210  that may be detected and/or acquired by receiver  140 . Although the scope of the present invention is not limited in this respect, the portion of frame  210  may include a short preamble  220 , a long preamble  230  and a first physical layer convergence procedure (PLCP) time interval  240 . Furthermore, a predetermined silence period  250  may be positioned at the beginning of frame  200 . For example, in some embodiments of the present invention the silence period  250  may be 16 microsecond. The short preamble  220  may include 10 repetitions of training sequence  211 . For example, training sequence  211  may be 0.8 microsecond long, although the scope of the present invention is not limited in this respect. 
   Turning to  FIG. 3 , a block diagram of a receiver  300  according to an exemplary embodiment of the present invention is shown. Although the scope of the present invention is not limited in this respect, receiver  300  may include an antenna  310 , an antenna  315 , an antenna selector  320 , an automatic gain controller (AGC)  330 , an analog to digital converter (ADC)  340 , a controller  350 , an energy measurement unit  360 , a correlation module  370 , a periodicity module  380  and a frame acquisition module  390 . In some embodiments of the present invention, energy measurement unit  360 , correlation module  370 , periodicity module  380  and frame acquisition module  390  may be implemented in software or in hardware or as a combination of software and hardware, if desired. 
   In operation, although the scope of the present invention is not limited in this respect, controller  350  may command antenna selector  320  to switch periodically between antennas  310  and  315  within a predetermined interval of 0.8 microseconds, if desired. Accordingly, during a first interval antenna  310  may receive at least one transmission and during a second interval antenna  315  may receive at least one other transmission. The transmission, for example transmission  200  may be received by antenna  310  and/or antenna  315  and may include, for example, frame  210 . Although the scope of the present invention is not limited in this respect, antenna  310  and/or antenna  315  may include antenna arrays, if desired. Furthermore, in some embodiments of the present invention three or more antennas may be used. 
   Although the scope of the present invention is not limited in this respect, AGC  330  may adjust the amplitude level of the received transmission. In some embodiments of the present invention, transmission  200  may be an analog signal and ADC  340  may convert the analog signal into a digital signal. In an embodiment of the invention, energy measurement unit  360  may measure and/or estimate an energy of transmission  200  to provide an energy value to frame acquisition module  390 . 
   Although the scope of the present invention is not limited in this respect, energy measurement unit  360  may measure and/or estimate an energy value of the transmission received by antenna  310  and may measure and/or estimate an energy value of the transmission received by antenna  315 . Furthermore, energy measurement unit  360  may measure and/or estimate a predetermined silence period, e.g. silence period  250 , on one of antennas  310 ,  315  to enable the detection of transmission  200  and the acquisition of frame  210  by frame acquisition module  390 , if desired. 
   Although the scope of the present invention is not limited in this respect, correlation module  370  may provide a score to frame acquisition module  390  based on a correlation between a received sequence of frame  210  with a training sequence (not shown). For example, the score may be the peak value of the correlation and/or sum of at least some of the samples in the proximity of the peak level of the correlation. Additionally or alternatively, correlation module  370  may estimate and/or measure the correlation of transmission  200  and may provide a correlation value  375  to frame acquisition module  390 . 
   Although the scope of the present invention is not limited in this respect, periodicity module  380  may provide a periodicity score  385  to frame acquisition module  390  based on the periodicity of preamble sequence  211  of frame  210 . For example, the calculation of periodicity score  385  may be based on the periodicity of preamble sequence  211 , for example a 0.8 microsecond period. More specifically, the periodicity score may be calculated using the following function |∫input_signal(t)·input_signal*(t−0.8 μS)| wherein, input_signal may be a signal that carries the transmission received by at least one of antennas  310  and  315 , although the scope of the present invention is not limited in this respect. Periodicity module  380  may provide periodicity score  385  to frame acquisition module  390 . 
   Although the scope of the present invention is not limited in this respect, energy value  365 , correlation value  375  and/or the periodicity score  385  that may be generated based on the transmission that received by antennas  310 ,  315  may be used to generate a receive signal indicator  395 . For example, energy value  365 , correlation value  375  and/or the periodicity score  385  that may be generated based on the transmission received by antennas  310  may be used to generate a first value of receive signal indicator  395 . In addition, energy value  365 , correlation value  375  and/or the periodicity score  385  that may be generated based on the transmission received by antennas  315  may be used to generate a second value of receive signal indicator  395 . 
   In some alternative embodiments of the present invention, frame acquisition module  390  may generate first and second values of receive signal indicator  395  for antennas  310  and  315  respectively and provide the first and second values of received signal indicator  395  to controller  350 . Controller  350  may be, for example, a processor capable of performing control functions and may control antenna selector  320  to select an antenna based on the values of received signal indicator value  395 . For example, if the value of received signal indicator  395  generated for antenna  310  is higher than the value of received signal indicator  395  generated for antenna  315 , then antenna selector  320  may select antenna  310 , and vice versa. Furthermore, controller  350  may instruct AGC  330  to set its gain based on the selected value of received signal indicator  395 , if desired. 
   Although the scope of the present invention is not limited in this respect, in some embodiments of the present invention the antenna selection may be done based on a rule and/or criterion. For example, the rule and/or criterion may be ‘start of transmission detected if correlation_measure&gt;b+energy*a or if energy&gt;c’. In addition, the rule and/or criterion may include a predetermined threshold. The predetermined threshold may include a first threshold level to indicate a high energy of the transmission and a second threshold to indicate a low energy of the transmission, although the scope of the present invention is not limited in this respect. 
   Additionally or alternatively, frame acquisition module  390  may instruct controller  350  to adjust AGC  330  based on the energy value and the operation mode of receiver  300 . For example, a first mode may be selection between antennas  310  and  315  and a second mode may be receiving transmissions with a single antenna. Thus, in the first mode frame acquisition module  390  may instruct controller  350  to adjust AGC  330  with a two-stage adjustment algorithm, and in the second mode, frame acquisition module  390  may instruct controller  350  to adjust AGC  330  with a three-stage adjustment algorithm, although the scope of the present invention is not limited in this respect. 
   In embodiments of the present invention controller  350  may use the following exemplary two-stage algorithm to adjust AGC  330 :
         If the energy value is under a predetermined value;   Stage one may be: Set AGC  330  level based on past energy measurements;   Provide a coarse frequency estimation, a timing estimation, a signal to noise ratio (SNR) estimation, a channel quality estimation, and AGC estimation for transmission received by a first antenna e.g. antenna  310 ;   Stage two may be: Repeat previous stage estimations for all other antennas e.g. antenna  315 ; and   select antenna based on a score of the channel quality estimation, and set the AGC  330  accordingly.
 
In some embodiments of the present invention, the first stage may be done in 2 microsecond and the algorithm may be performed at no more then 8 microsecond.
       

   Additionally or alternatively, although the scope of the present invention is not limited in this respect, controller  350  may use the following exemplary three-stage algorithm to adjust AGC  330 :
         Stay with the antenna in which the high energy value was measured;   Lower the AGC gain;   Estimate the AGC level needed to adjust the AGC, and set the AGC accordingly;   Provide a coarse frequency estimation, a timing estimation, an SNR estimation, a channel quality estimation, and an AGC estimation; and   Set the AGC accordingly.       

   Although the scope of the present invention is not limited in this respect, an exemplary timing estimation will now be described. The timing estimation may be channel dependant. Timing estimation may be done by use of cyclic correlation with a short training sequence  211  to estimate the impulse response of the channel. Based on the impulse response of the channel, the timing may be determined such that inter-symbol interference is minimal. In addition, the timing estimation may output the start of the 3.2 microsecond interval (not shown) that may be fed into a fast Foriur transform module (FFT). 
   Although the scope of the present invention is not limited in this respect, an exemplary method of coarse frequency estimation will now be described. Transmission  200  may be sampled and a stream of 64 consecutive samples may be used for the estimation. The coarse frequency estimation may be done by calculating the frequency error according to below equation: 
           freq_err   ≈       phase   [       ∑   1   32     ⁢       (       sig   ⁡     (   n   )       -   DC     )     ·       (       sig   (     n   +   32     )     -   DC     )     *         ]     ⁢           ⁢   where                   D   ⁢           ⁢   C     =       1   32     ⁢       ∑   1   32     ⁢       sig   ⁡     (   n   )       .               
wherein;
         DC is the direct current (DC) level; and   phase is the phase of the transmission.
 
The result of the calculation e.g. freq_err may be added and or deleted from the previous frequency.
       
   Although the scope of the present invention is not limited in this respect, an exemplary method of the AGC estimation may be done by transferring the estimated energy level to dB relative to a desired value. The result may be added to the current AGC state. 
   While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.