Abstract:
A system for implementing an orthogonal frequency division multiplexing scheme and providing an improved range extension. The system includes a transmitter for transmitting data to a receiver. The transmitter includes a symbol mapper for generating a symbol for each of a plurality of subcarriers and a spreading module for spreading out the symbol on each of the plurality of subcarriers by using a direct sequence spread spectrum. The symbol on each of the plurality of subcarriers is spread by multiplying the symbol by predefined length sequences. The receiver includes a de-spreader module for de-spreading the symbols on each of the plurality of subcarriers. The de-spreader module includes a simply correlator receiver for obtaining maximum detection. The correlator produces an output sequence of a same length as an input sequence and the de-spreader module uses a point of maximum correlation on the output sequence to obtain a recovered symbol.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE 
     The present application is a CONTINUATION of U.S. application Ser. No. 12/796,315, filed Jun. 8, 2010, now issued U.S. Pat. No. 8,014,437, which is a CONTINUATION of U.S. application Ser. No. 11/265,134, filed Nov. 3, 2005, now issued U.S. Pat. No. 7,733,939. Said U.S. application Ser. No. 11/265,134 makes reference to, claims priority to and claims benefit from U.S. Provisional Application No. 60/624,196, filed Nov. 3, 2004. The above-identified applications are hereby incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to wireless communication systems and more particularly to an improvement in the range of a wireless LAN device. 
     2. Description of the Related Art 
     A wireless communication device in a communication system communicates directly or indirectly with other wireless communication devices. For direct/point-to-point communications, the participating wireless communication devices tune their receivers and transmitters to the same channel(s) and communicate over those channels. For indirect wireless communications, each wireless communication device communicates directly with an associated base station and/or access point via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or access points communicate with each other directly, via a system controller, the public switch telephone network, the Internet, and/or some other wide area network. 
     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. Typically, the transmitter includes one antenna for transmitting radiofrequency (RF) signals, which are received by one or more antennas of the receiver. When the receiver includes two or more antennas, the receiver selects one of antennas to receive the incoming RF signals. This type of wireless communication between the transmitter and receiver is known as a single-output-single-input (SISO) communication. 
     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 and uses an orthogonal frequency division multiplexing (OFDM) encoding scheme. 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 and also uses the OFDM encoding scheme. 802.11n, a new extension of 802.11, is being developed to address, among other things, 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. 
     Currently, most SISO WLANs are IEEE 802.11 compliant. A current communications system provides a range extension on a SISO system by taking an 802.11a/802.11g signal and cutting the symbol rate. Specifically, the current communications system achieves range extension by dividing a symbol clock by 24, i.e., the inverse of Super-G, which doubles the clock frequency. When the symbol clock is divided, the maximum symbol duration is 96 μsec. and the corresponding rate is 250 kbps. For example, the current communications system takes an 802.11a/802.11g signal that is 16.5 MHz, divides the symbol clock by 24 and cuts the signal to 687.5 kHz. When the bandwidth for a signal is reduced, the integrated thermal noise density of the receiver is also reduced. Therefore, when the bandwidth is reduced by a factor of 24, the thermal noise floor is decreased by 10* log 10(24). This results in a 16 DB “gain” in the sensitivity of the receiver which is equivalent to at least 3 times improvement in the range of a typical wireless system. The cost of this implementation, however, is that the data rate is also decreased by a factor of 24. Furthermore, since legacy systems in the same cell as the current communications system may not detect this very narrow bandwidth, the current communications system does not interoperate with legacy 802.11a/802.11g systems in the same cell. Specifically, a legacy 802.11a/802.11g device may not detect overlapping Base Service Set (BSS) transmissions from the current system and as such the legacy 802.11a/802.11g system will not set its Clear Channel Assessment (CCA) bits appropriately. Therefore, in dense deployments, such as apartment buildings, network chaos is likely to occur when an active BSS in the current communications system overlaps with an active legacy BSS transmission. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a network device implementing an orthogonal frequency division multiplexing scheme and providing an improved range extension. The network device includes receiving means for receiving data and a symbol mapper for generating a symbol for each of a plurality of subcarriers. The network device also includes a spreading module for spreading out the symbol on each of the plurality of subcarriers by using a direct sequence spread spectrum. The symbol on each of the plurality of subcarriers is spread by multiplying the symbol by predefined length sequences. The network device further includes transmitting means for transmitting the data to a receiver. 
     According to another aspect of the invention, there is provided a network device for receiving symbols on a plurality of subcarriers and proving improved range extension. The device includes receiving means for receiving the plurality of subcarriers. The device further includes a de-spreader module for de-spreading the symbols on each of the plurality of subcarriers. The de-spreader module includes a correlator receiver for obtaining maximum detection. The correlator produces an output sequence of a same length as an input sequence and the de-spreader module uses a point of maximum correlation on the output sequence to obtain a recovered symbol. 
     According to another aspect of the invention, there is provided a method implementing an orthogonal frequency division multiplexing scheme for transmitting data to a receiver and providing improved range extension. The method includes the steps of receiving data for processing and generating a symbol for each of a plurality of subcarriers. The method also includes the steps of spreading out the symbol on each of the plurality of subcarriers by using a direct sequence spread spectrum, wherein the symbol on each of the plurality of subcarriers is spread by multiplying the symbol by predefined length sequences; and transmitting the data to a receiver. 
     According to another aspect of the invention, there is provided a method for receiving symbols on a plurality of subcarriers and providing improved range extension. The method includes the steps of receiving the plurality of subcarriers and de-spreading the symbols on each of the plurality of subcarriers. The method also includes the steps of producing an output sequence of a same length as an input sequence and using a point of maximum correlation on the output sequence to obtain a recovered symbol. 
     According to another aspect of the invention, there is provided a system for implementing an orthogonal frequency division multiplexing scheme and providing an improved range extension. The system includes a transmitter for transmitting data to a receiver. The transmitter includes a symbol mapper for generating a symbol for each of a plurality of subcarriers and a spreading module for spreading out the symbol on each of the plurality of subcarriers by using a direct sequence spread spectrum. The symbol on each of the plurality of subcarriers is spread by multiplying the symbol by predefined length sequences. The receiver includes a de-spreader module for de-spreading the symbols on each of the plurality of subcarriers. The de-spreader module includes a simply correlator receiver for obtaining maximum detection. The correlator produces an output sequence of a same length as an input sequence and the de-spreader module uses a point of maximum correlation on the output sequence to obtain a recovered symbol. 
    
    
     
       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 schematic block diagram illustrating a communication system; 
         FIG. 2  illustrates a block diagram of a long-range transmitter used in the present invention; 
         FIG. 3  illustrates a block diagram of a long-range receiver implemented in the inventive system; and 
         FIG. 4  illustrates a long-range frame  400  utilized in the present invention. 
         FIG. 5  illustrates a spreading module of a transmitter that receives, for example, a sub-symbol and outputs L-chips of a set, and a frame that includes, for example, a legacy symbol, L-copies of short training symbols and a proprietary signal according to an embodiment of the present invention. 
     
    
    
     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  is a schematic block diagram illustrating 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. 
     As is known to those skilled in the art, devices implementing both the 802.11a and 802.11g standards use an OFDM encoding scheme 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. Specifically, the 802.11a/802.11g standards specify an OFDM physical layer (PHY) that splits an information signal across 52 separate sub-carriers to provide transmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. Four of the 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 the 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. 
     The present invention uses the OFDM encoding scheme and distributes data over sub-carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” which prevents demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. The present invention reuses most of the data path and implements a more reliable lower rate by applying a Direct Sequence Spread Spectrum (DSSS) to each sub-carrier&#39;s stream of QAM sub-symbols. The assumption in OFDM is that each sub-carrier is a flat fading channel. Thus, the invention uses a simple matched filter receiver per sub-carrier at the receiver with insignificant loss. 
       FIG. 2  illustrates a block diagram of a long-range transmitter  200  used in the present invention. RF transmitter  200  includes a scrambler  202 , a convolutional encoder and puncture module  204 , a QAM symbol mapper  206 , a spreading module  208 , and Inverse Fast Fourier Transfer (IFFT)  210 , a parallel to serial converter  212  and a cyclic prefix insertion module  214 . All the bits in the data portion are scrambled by scrambler  202 . Scrambling is used to randomize the data, which may contain long strings of binary data. The data field is then coded by convolutional encoder  204  with a coding rate of r=½. Symbol mapper  206  modulates the OFDM sub-carriers by using QAM modulation. Specifically, the data enters symbol mapper  206  which generates a QAM symbol for each OFDM sub-carrier. 
     The invention provides spreading gain improvement, wherein after the symbol are mapped to sub-carriers, spreading module  208  spreads out the symbol sequence on each of the parallel flat fading channels by using a direct sequence spread spectrum. Therefore, the symbols on each of the sub-carriers are spread out to the full sub-carrier width. According to the inventive system, each QAM sub-symbol is expanded into a set of L chips. Frank sequences may be used as the spreading code. According to one embodiment, the symbols are spread using a Constant Amplitude Zero Auto Correlation (CAZAC) sequence, wherein when a correlation is performed with itself, a non-zero component is present at only one point in time. The present invention spreads the symbols using length (L) CAZAC sequences, where L equals to 4, 16 or 64 sequences. As such, one symbol from symbol mapper  206  is multiplied by L sequences and when L=4, four symbols are produced by the spreading module  208 , when L=16, 16 symbols are produced by the spreading module  208  and when L=64, 64 symbols are produced by spreading module  208 . 
     A spreading sequence when L=16 and (i) is square root of −1 is presented by the equation:
 
 C   spread,16 =[1 +i,− 1− i,− 1− i,− 1− i, 1+ i, 1− i, 1+ i,− 1+ i, 1+ i, 1+ i,− 1− i, 1+ i, 1+ i, 1+ i,− 1+ i, 1+ i, 1− i] 
 
     When spreading module  208  applies the above spreading sequence, for each sub-carrier, spreading module  208  outputs a Length  16  sequence. IFFT  210  converts the sub-carriers from the frequency domain to the time domain. Parallel to serial converter  212  converts parallel time domain signals to a plurality of serial time signals. Cyclic prefix insertion module  214  introduces the cyclic prefix as a guard interval to each sub-channel. Therefore, orthogonality can be maintained while bandwidth efficiency is maintained. Transmitter  200  then transmits the OFDM symbols to a receiver. 
       FIG. 3  illustrates a block diagram of a long-range receiver  300  used in the present invention. Receiver  300  receives the OFDM sub-carriers and instead of making a decision for each symbol, receiver  300  takes a whole screen of symbols on each of the sub-carriers and runs a correlator on each of the received sub-carriers. Receiver  300  includes cyclic prefix removal module  302 , Fast Fourier Transfer (FFT)  304 , frequency domain module  306 , de-spreader module  308 , QAM symbol demapper  310 , parallel to serial converter  312 , Viterbi decoder  314 , and descrambler  316 . Cyclic prefix removal module  302  removes the cyclic prefix inserted by transmitter  200 . Thereafter, FFT  304  converts the serial time domain signals into frequency signals. Frequency domain module  306  applies a weighting factor on each frequency domain signal. The correlator in de-spreader module  308  despreads the signals that were spread at the transmitter. The invention allows the use of a simple correlator receiver for obtaining maximum detection. The correlator is a matched filter and the path of the filter are the spreading sequence time reversed and complex conjugated. As such, the first element of the sequence becomes the last and the last become the first. In the case of a spreading sequence where L=16, the correlator produces 16 outputs that correspond to the 16 inputs. Thereafter, de-spreader module  308  takes exactly the point of maximum correlation which is exactly the recovered symbol. Processing gain in the inventive system of approximately 10*log 10(L) is achieved by applying the matched filter per subcarrier since the channel decoder processing follows the matched filtering. 
     Symbol demapper  310  then generates the coded bits from each of the sub-carriers in the OFDM sequence. Parallel to serial converter  312  converts the digital time domain signals into a plurality of serial time domain signals. Viterbi Decoder  314  decodes input symbols to produce binary output symbols. Bits in the data portion are descrambled by descrambler  318 . 
     The present invention thus allows for the use of the same bandwidth that is used in legacy systems employing the 802.11a and 802.11g standards. It may also be possible to get a diversity benefit by mappng each of the L chips in a block to a different sub-carrier. Since the equalization is performed before de-spreading, each received chip may be pulled from a different sub-carrier. Although the noise variance on each chip will be different, the present invention provides a frequency diversity benefit. 
     Furthermore, the data path computational complexity when L=4 requires no more than one negation operation per transmitted chip beyond processing implemented in 802.11a/802.11g and no more than one negation operation and one addition per received chip beyond processing implemented in 802.11a/802.11g. When L=16 the data path computational complexity requires no more than two negation operations and two additions per transmitted chip beyond processing implemented in 802.11a/802.11g and no more than two negation operations and three additions per received chip beyond processing implemented in 802.11a/802.11g. Thus, no new multipliers are required. 
     As is known to those skilled in the art, each legacy 802.11a/802.11g system needs to decode a valid SIGNAL field to determine the length of a frame to set its CCA bit. The legacy SIGNAL field specifies the rate and a length value in bytes which matches the length of the actual frame. If additional information is added to the frame, at the end of the frame when the legacy receiver attempts to decode the FCS, it detects an error and discards the frame.  FIG. 4  illustrates a long-range frame  400  utilized in the present invention. According to the present invention, after the legacy preamble and SIGNAL frame  402 , L copies of short training symbols  404  are appended and followed by a proprietary field  406 . The additional copies of short training symbols  404  allow long-range receiver  300  to perform carrier detection in extremely low SNR. Proprietary field  406  includes DSSS-encoded OFDM for long training symbols, SIGNAL and data. The proprietary long training symbols, SIGNAL field and data symbols are transmitted using the inventive DSSS encoding. As such, frame  400  includes information for instructing legacy 802.11a/802.11g receivers to ignore field  406 . According to the invention, a legacy system uses the header in preamble  402  to set its CCA bi,t provided that the actual frame duration does not exceed 5.48 msec and the transmissions from the inventive system are above a sensitivity threshold. The channel utilization in the current invention is exactly the same as the channel utilization in a legacy 802.11a/802.11g system. Furthermore, there is no need to clock DACs, ADC and logic at lower rates. Additionally, there is no requirement for special BSS as the long-range rates are just new rates that can be used in the same BSS with legacy device. Therefore, compatibility is ensured by prepending the legacy preamble and SIGNAL fields. 
     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.