Abstract:
A network device for generating an expanded long training sequence with a minimal peak-to-average ratio. The network device includes a signal generating circuit for generating the expanded long training sequence. The network device also includes an Inverse Fourier Transform for processing the expanded long training sequence from the signal generating circuit and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The expanded long training sequence and the optimal expanded long training sequence are stored on more than 52 sub-carriers.

Description:
This application claims benefit under 35 U.S.C §119(e) of provisional application No. 60/591,104, filed on Jul. 27, 2004 and provisional application No. 60/634,102 filed on Dec. 8, 2004, the contents of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to wireless communication systems and more particularly to long training sequences of minimum peak-to-average power ratio which may be used by legacy systems. 
   2. Description of the Related Art 
   Each wireless communication device participating in wireless communications includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver. As is known to those skilled in the art, the transmitter typically includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. 
   The receiver is typically coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives, via the antenna, inbound RF signals and amplifies the inbound RF signals. The intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signal into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with a particular wireless communication standard. 
   Different wireless devices in a wireless communication system may be compliant with different standards or different variations of the same standard. For example, 802.11a an extension of the 802.11 standard, provides up to 54 Mbps in the 5 GHz band. 802.11b, another extension of the 802.11 standard, provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11g, another extension of the 802.11 standard, provides 20+ Mbps in the 2.4 GHz band. 802.11n, a new extension of 802.11, is being developed to address, among other thins, higher throughput and compatibility issues. An 802.11a compliant communications device may reside in the same WLAN as a device that is compliant with another 802.11 standard. When devices that are compliant with multiple versions of the 802.11 standard are in the same WLAN, the devices that are compliant with older versions are considered to be legacy devices. To ensure backward compatibility with legacy devices, specific mechanisms must be employed to insure that the legacy devices know when a device that is compliant with a newer version of the standard is using a wireless channel to avoid a collision. New implementations of wireless communication protocol enable higher speed throughput, while also enabling legacy devices which might be only compliant with 802.11a or 802.11g to communicate in systems which are operating at higher speeds. 
   Devices implementing both the 802.11a and 802.11g standards use an orthogonal frequency division multiplexing (OFDM) encoding scheme. OFDM is a frequency division multiplexing modulation technique for transmitting large amounts of digital data over a radio wave. OFDM works by spreading a single data stream over a band of sub-carriers, each of which is transmitted in parallel. In 802.11a and 802.11g compliant devices, only 52 of the 64 active sub-carriers are used. Four of the active sub-carriers are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission. The remaining 48 sub-carriers provide separate wireless pathways for sending information in a parallel fashion. The 52 sub-carriers are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16 Quadrature Amplitude Modulation (QAM), or 64 QAM. Therefore, 802.11a and 802.11g compliant devices use sub-carriers −26 to +26, with the 0-index sub-carrier set to 0 and 0-index sub-carrier being the carrier frequency. As such, only part of the 20 Mhz bandwidth supported by 802.11a and 802.11g is use. 
   In 802.11a/802.11g, each data packet starts with a preamble which includes a short training sequence followed by a long training sequence. The short and long training sequences are used for synchronization between the sender and the receiver. The long training sequence of 802.11a and 802.11g is defined such that each of sub-carriers −26 to +26 has one BPSK consellation point, either +1 or −1. 
   There exists a need to create a long training sequence of minimum peak-to-average ratio that uses more sub-carriers without interfering with adjacent channels. The inventive long trains sequence with a minimum peak-to-average power ratio should be usable by legacy devices in order to estimate channel impulse response and to estimate carrier frequency offset between a transmitter and a receiver. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, there is provided a network device for generating an expanded long training sequence with a minimal peak-to-average ratio. The network device includes a signal generating circuit for generating the expanded long training sequence. The network device also includes an Inverse Fourier Transform for processing the expanded long training sequence from the signal generating circuit and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The expanded long training sequence and the optimal expanded long training sequence are stored on more than 52 sub-carriers. 
   According to another aspect of the invention, there is provided a network device for generating an expanded long training sequence with a minimal peak-to-average ratio. The network device includes a signal generating circuit for generating the expanded long training sequence. The network device also includes an Inverse Fourier Transform for processing the expanded long training sequence from the signal generating circuit and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The expanded long training sequence and the optimal expanded long training sequence are stored on more than 56 sub-carriers. 
   According to another aspect of the invention, there is provided a network device for generating an expanded long training sequence with a minimal peak-to-average ratio. The network device includes a signal generating circuit for generating the expanded long training sequence. The network device also includes an Inverse Fourier Transform for processing the expanded long training sequence from the signal generating circuit and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The expanded long training sequence and the optimal expanded long training sequence are stored on more than 63 sub-carriers. 
   According to another aspect of the invention, there is provided a method for generating an expanded long training sequence with a minimal peak-to-average ratio. The method includes the steps of generating the expanded long training sequence and producing an optimal expanded long training sequence with a minimal peak-to-average ratio. The method also includes the step of storing the expanded long training sequence and the optimal expanded long training sequence on more than 52 sub-carriers 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention, wherein: 
       FIG. 1  illustrates a communication system that includes a plurality of base stations, a plurality of wireless communication devices and a network hardware component; 
       FIG. 2  illustrates a schematic block diagram of a processor that is configured to generate an expanded long training sequence; 
       FIG. 3  is a schematic block diagram of a processor that is configured to process an expanded long training sequence; 
       FIG. 4  illustrates the long training sequence that is used in 56 active sub-carriers; and 
       FIG. 5  illustrates the long training sequence that is used in 63 active sub-carriers. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 1  illustrates a communication system  10  that includes a plurality of base stations and/or access points  12 - 16 , a plurality of wireless communication devices  18 - 32  and a network hardware component  34 . Wireless communication devices  18 - 32  may be laptop computers  18  and  26 , personal digital assistant hosts  20  and  30 , personal computer  24  and  32  and/or cellular telephone  22  and  28 . Base stations or access points  12 - 16  are operably coupled to network hardware  34  via local area network connections  36 ,  38  and  40 . Network hardware  34 , for example a router, a switch, a bridge, a modem, or a system controller, provides a wide area network connection for communication system  10 . Each of base stations or access points  12 - 16  has an associated antenna or antenna array to communicate with the wireless communication devices in its area. Typically, the wireless communication devices register with a particular base station or access point  12 - 14  to receive services from communication system  10 . Each wireless communication device includes a built-in radio or is coupled to an associated radio. The radio includes at least one radio frequency (RF) transmitter and at least one RF receiver. 
   The present invention provides an expanded long training sequence of minimum peak-to-average power ratio and thereby decreases power back-off. The inventive expanded long training sequence may be used by 802.11a or 802.11g devices for estimating the channel impulse response and by a receiver for estimating the carrier frequency offset between the transmitter clock and receiver clock. The inventive expanded long training sequence is usable by 802.11a or 802.11g systems only if the values at sub-carriers −26 to +26 are identical to those of the current long training sequence used in 802.11a and 802.11g systems. As such, the invention utilized the same +1 or −1 binary phase shift key (BPSK) encoding for each new sub-carrier and the long training sequence of 802.11a or 802.11g systems is maintained in the present invention. 
   In a first embodiment of the invention, the expanded long training sequence is implemented in 56 active sub-carriers including sub-carriers −28 to +28. In another embodiment, an expanded long training sequence is implemented using 63 active sub-carriers, i.e., all of the active sub-carriers (−32 to +31) except the 0-index sub-carrier which is set to 0. In both embodiments of the invention, orthogonality is not affected, since a 64-point orthogonal transform is used to generate the time-domain sequence. Additionally, the output of an autocorrelator for computing the carrier frequency offset is not affected by the extra sub-carriers. 
     FIG. 2  illustrates a schematic block diagram of a processor that is configured to generate an expanded long training sequence. Processor  200  includes a symbol mapper  202 , a frequency domain window  204 , a signal generating circuit  205 , an inverse fast Fourier transform (IFFT) module  206 , a serial to parallel module  208 , a digital transmit filter and/or time domain window module  210 , and digital to analog converters (D/A)  212 . For an expanded long training sequence, symbol mapper  202  generates symbols from the coded bits for each of the 64 subcarriers of an OFDM sequence. Frequency domain window  204  applies a weighting factor on each subcarrier. Signal generating circuit  205  generates the expanded long training sequence and if 56 active sub-carriers are being used, signal generating circuit generates the expanded long training sequence and stores the expanded long training sequence in sub-carriers −28 to +28. If 63 active sub-carriers are being used, signal generating circuit generates the expanded long training sequence and stores the expanded long training sequence in sub-carriers −32 to +32 i.e., all of the active sub-carriers (−32 to +31) except the 0-index sub-carrier which is set to 0. The inventive long training sequence is inputted into an Inverse Fourier Transform  206 . The invention uses the same +1 or −1 BPSK encoding for each new sub-carrier. Inverse Fourier Transform  206  may be an inverse Fast Fourier Transform (IFFT) or Inverse Discrete Fourier Transform (IFDT). Inverse Fourier Transform  206  processes the long training sequence from signal generating circuit  205  and thereafter produces an optimal expanded long training sequence with a minimal peak-to-average power ratio. The optimal expanded long training sequence may be used in either 56 active sub-carriers or 63 active subscribers. Serial to parallel module  208  converts the serial time domain signals into parallel time domain signals that are subsequently filtered and converted to analog signals via the D/A. 
     FIG. 3  is a schematic block diagram of a processor that is configured to process an expanded long training sequence. Processor  300  includes a symbol demapper  302 , a frequency domain window  304 , a fast Fourier transform (FFT) module  306 , a parallel to serial module  308 , a digital receiver filter and/or time domain window module  310 , and analog to digital converters (A/D)  312 . A/D converters  312  convert the sequence into digital signals that are filtered via digital receiver filter  310 . Parallel to serial module  308  converts the digital time domain signals into a plurality of serial time domain signals. FFT module  306  converts the serial time domain signals into frequency domain signals. Frequency domain window  304  applies a weighting factor on each frequency domain signal. Symbol demapper  302  generates the coded bits from each of the 64 subcarriers of an OFDM sequence received from the frequency domain window. 
     FIG. 4  illustrates the long training sequence with a minimum peak-to-average power ratio that is used in 56 active sub-carriers. Out of the 16 possibilities for the four new sub-carrier positions, the sequence illustrated in  FIG. 4  has the minimum peak-to-average power ratio, i.e., a peak-to-average power ratio of 3.6 dB. 
     FIG. 5  illustrates the long training sequence with a minimum peak-to-average power ratio that is used in 63 active sub-carriers. Out of the 2048 possibilities for the eleven new sub-carrier positions, the sequence illustrated in  FIG. 5  has the minimum peak-to-average power ratio, i.e., a peak-to-average power ratio of 3.6 dB. 
   It should be appreciated by one skilled in art, that the present invention may be utilized in any device that implements the OFDM encoding scheme. The foregoing description has been directed to specific embodiments of this invention. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.