Abstract:
Whitespace devices can use unused television frequencies for transmission and reception of WiFi OFDM signals. Three contiguous bands, such as former channels 2, 3, and 4, may be bonded together to define a whitespace band. In order to fit a WiFi OFDM signal into this whitespace band, a whitespace device compresses the bandwidth of each WiFi OFDM signal using a specific spectrum mask. Very low transmission power is needed for the modified WiFi OFDM signals, eliminating the need for high power amplifiers and most of the WiFi OFDM designs such as PHY and MAC can be reused with minor modifications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/245,209, filed Oct. 3, 2008, titled “System and Method for Data Distribution in VHF/UHF Bands,” which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     This invention relates generally to distribution of data signals in a home environment, and more particularly to a system and method for data distribution in the VHF/UHF band. 
     WiFi (Wireless Fidelity) is the trade name for the global set of 802.11 standards drafted for wireless Local Area Networks (LAN); any standard Wi-Fi device will work anywhere in the world. WiFi is one of the most popular wireless technologies; it is widely available in public hotspots, homes, and campuses worldwide, being supported by nearly every modern personal computer, laptop, most advanced game consoles, printers, and many other consumer devices. Routers which incorporate a Digital Subscriber Line (DSL) modem or a cable modem and a Wi-Fi access point, often set up in homes and other premises, provide Internet access and internetworking to all devices connected (wirelessly or by cable) to them. 
     Wi-Fi uses both single carrier direct-sequence spread spectrum radio technology (812.11b) and multi-carrier Orthogonal Frequency Division multiplexing (OFDM) radio technology (e.g. 802.11a, g, j, n). The Institute for Electronic and Electrical Engineers (IEEE) has established a set of standards for Wireless Local Area Network (WLAN) computer communication, collectively known as the IEEE 802.11 standard that are applicable to Wi-Fi signals. 
     The 802.11a standard uses OFDM radio technology in the 5 GHz U-NII band, which offers 8 non-overlapping channels and provides data rates of up to 54 Mbps. Another standard that uses OFDM is 802.11g, which attempts to combine the best features of the 802.11a and 802.11b standards. It uses enables data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, and the 2.4 GHz frequency for greater range. The 802.11j standard is an amendment designed especially for Japanese markets. It allows WLAN operation in the 4.9 to 5 GHz band to conform to Japanese rules for radio operation for indoor, outdoor, and mobile applications. Finally, the 802.11n standard is a proposed amendment which improves upon the previous 802.11 standards by adding multiple-input multiple-output (MIMO) and many other newer features. Though there are already many products on the market based on the latest draft of this proposal, the 802.11n standard will not be finalized until December 2009. 
     In the U.S., 802.11a and 802.11g devices may be operated without a license. The 802.11a standard uses 20 MHz channels and operates in three unlicensed bands, known as the Unlicensed National Information Infrastructure (U-NII) bands; four 20 MHz channels are specified in each of these bands. The lower U-NII band, extending from 5.15 to 5.25 GHz, accommodates four channels with a 40 mW power limit; the middle U-NII band, extending from 5.25 to 5.35 GHz accommodates four channels with a 200 mW power limit; and the upper U-NII band, extending from 5.725 to 5.825 GHz, accommodates four 20 MHz channels with an 800 mW power limit. 
     An 802.11a signal uses OFDM modulation with 52 subcarriers, which include 48 data subcarriers and four pilot subcarriers; the subcarriers can be modulated using BPSK, QPSK, 16 QAM, or 64 QAM. The total symbol duration is 4 μs, and includes a useful symbol duration of 3.2 μs and a guard interval of 0.8 μs, with a peak data rate of 54 Mbps. Subcarriers are spaced apart by 312.5 kHz so that the signal actually occupies a bandwidth of 16.25 MHz in theory. 
     In the United States, there are roughly 210 television (TV) broadcast regions and 1700 TV broadcasting stations. Currently, each TV station is assigned around eight radio frequency (RF) channels for NTSC broadcast, each channel occupying 6 MHz in the VHF/UHF spectrum. The Federal Communications Commission (FCC) has mandated that all full-power television broadcasts will use the Advanced Television Systems Committee (ATSC) standards for digital TV by no later than Feb. 17, 2009. All NTSC television transmissions will be terminated by that date. Following the NTSC TV switch-off, the FCC will allocate channels 2 through 51 to digital TV; channels 52 through 69 that occupy the lower half of the 700 MHz band have been already reallocated through auction to various advanced commercial wireless services for consumers. 
     The ATSC standard mandates a bandwidth of 6 MHz for each TV channel, use of Trellis Eight-Vestigial Side Band (8-VSB) modulation, and Reed-Solomon encoding. The TV receiver has some basic requirements to property decode the ATSC signal and provide good quality TV pictures. These requirements include that the TV Signal to Noise Ratio (SNR) is no less than 15.2 dB, a thermal noise floor of −106.2 dBm (dBm is the abbreviation for the power ratio measurement units), and a sensitivity of between −81 and −84 dBm etc. 
     As each TV station operating in a certain geographic region/area uses only a limited number of channels from the TV band, some digital channels remain unused in the respective area: this locally available spectrum is called “whitespace.” 
     It is expected that the FCC will allow the whitespace bands to be used only by devices that do not interfere with existing TV broadcast, wireless microphones, or Global Positioning System (GPS) systems deployed in that area. Consequently, the signals radiated by any whitespace devices/equipment operating in the ATSC spectrum must follow the FCC regulations so that the quality of the primary TV service will not be degraded by the signals using the nearby whitespace. Thus, the new whitespace devices should be designed so as to not affect the TV tuner sensitivity (−81˜−84 dB) and the TV receiver performance at SNR=15.2 dB. 
     A known solution for distributing multimedia content within a home is wireless high definition TV (HDTV). However, wireless HDTV requires a very high data rate (greater than 1 Gbps) and the 60 GHz band is not suitable for transmission over distances greater than 10 m. In addition, the quality of such a wireless link is not satisfactory and the cost is high. 
     Another known solution for distributing data and video within a home is WiFi. However, the WiFi band has uncontrollable interferences and the quality cannot be guaranteed. 
     Thus, there is a need to provide an inexpensive and efficient way to broadcast multimedia content within a confined environment, using wireless solutions. There is also a need to recycle the spectrum that is not used in a certain geographical area. 
     SUMMARY 
     Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts are provided by the entire disclosure. 
     It is an object of the invention to provide a method and system for wireless distribution of data and/or video within a home using OFDM technology, and in particular using WiFi OFDM signals. While the specification describes WiFi OFDM variants of the invention; it is to be understood that the invention applies to other technologies and is not restricted to WiFi OFDM signals. 
     Another object of the invention to provide a method and system for confining (retrofitting) WiFi OFDM signals into the whitespace that will become available once digital TV signal is phased in. As discussed in the Background section of this specification, the bandwidth used by the WiFi OFDM signals in the 5 GHz band is 20 MHz, and is therefore, slightly greater than the bandwidth of three consecutive TV channels in North America, which is 3×6=18 MHz. Also, a standard WiFi OFDM signal cannot fit within the whitespace band of three consecutive TV channels directly, due to high shoulders of its signal spectrum, which will severely interfere with the adjacent TV channels. The solution disclosed here confines WiFi OFDM signals into the whitespace, both in terms of bandwidth and emitted power, without causing interference with the existing TV broadcast. 
     It is to be noted that the invention described herein is equally applicable to whitespace of various widths. The particular example of retrofitting a WiFi OFDM signal within an 18 MHz bands is a practical solution for North America, that result in minimal changes to the existing WiFi device. However, the invention is not restricted to a whitespace of 18 MHz; applying the technique described here, narrower whitespace bands may be used. As well, as other countries that have a different TV channel width, whitespace freed by two TV channels may be used according to the invention. For example, a TV channel in Japan occupies 8 MHz, so that there is no need to use the whitespace freed by three TV channels; two could be enough. As well in the width of a TV channel in European countries is 7 MHz; in this case the WiFi OFDM signal can be used with the more relaxed embodiment of this invention, or less than three TV channels may be used by modifying the spectrum mask according to the invention in an appropriate way. 
     Still another object of the invention is to provide wireless distribution of data and/or video with minimal changes to the hardware of the existing WiFi devices. 
     In various exemplary embodiments, a method of transmitting user data over a local area network (LAN) within a VHF/UHF band may comprise identifying in the VHF/UHF band a whitespace band B WS  available in an area of operation of the LAN; generating a baseband WiFi OFDM signal from user data; reconfiguring the baseband WiFi OFDM signal into a modified WiFi OFDM signal using a transmit spectrum mask adapted to confine the bandwidth of the modified WiFi OFDM signal into the whitespace band B WS , and to attenuate the modified WiFi OFDM signal at the edges of the whitespace band B WS  for maintaining a performance of any neighboring TV channel unchanged; and transmitting the modified WiFi OFDM signal over the whitespace band. 
     Advantageously, the invention provides a solution for reusing the whitespace available in a respective area, at low cost and with a better performance than the current solutions. These advantages result from use of the lower part of the spectrum (VHF/UHF rather than 5 GHz), which results for example in a simplified design of the RF part of the existing devices. This is because at lower frequencies, the distances at which signals may be transmitted are greater that in the higher frequency bands; a direct result is that the transmitter design may use only a preamplifier rather than a power amplifier as in the current designs, resulting in a cost decrease. 
     The foregoing objects and advantages of the invention are illustrative of those that can be achieved by the various exemplary embodiments and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the various exemplary embodiments will be apparent from the description herein or can be learned from practicing the various exemplary embodiments, both as embodied herein or as modified in view of any variation that may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel methods, arrangements, combinations, and improvements herein shown and described in various exemplary embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is next described with reference to the following drawings, where like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1A  shows the eight WiFi carriers in the lower and middle U-NII bands; 
         FIG. 1B  shows the four WiFi carriers in the upper U-NII band; 
         FIG. 2  depicts the transmit spectrum for a WiFi OFDM signal; 
         FIG. 3  illustrates the U.S. ATSC broadcast band; 
         FIG. 4  is a flowchart of the method according to an embodiment of the invention; 
         FIG. 5  depicts an emission mask for a WiFi OFDM signal according to an embodiment of the present invention; 
         FIG. 6  depicts an alternative emission mask for an exemplary WiFi OFDM signal according to another embodiment of the present invention; 
         FIG. 7A  shows an exemplary transmitter according to an embodiment of the present invention; 
         FIG. 7B  shows the RF units of a conventional WiFi transmitter that are replaced in the transmitter shown in  FIG. 7A ; 
         FIG. 8A  shows an exemplary receiver according to an embodiment of the present invention; and 
         FIG. 8B  shows the RF units of a conventional WiFi receiver that are replaced in the receiver shown in  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments. It is again noted that WiFi and North America ATSC standard are used by way of example. Other OFDM signals may be retrofitted in the whitespace freed by transition to digital TV in other parts of the world. Also, use of whitespace provided by three consecutive TV channels is described here as the preferred embodiment of the invention: whitespace of different width may also be used for transmitting OFDM signals in the VI-IF/UHF spectrum. 
       FIGS. 1A and 1B  show the 802.11a carriers in the 5 GHz band, where  FIG. 1A  shows eight WiFi carriers C 1 -C 8  in the lower and middle U-NII bands, and  FIG. 2  shows the four WiFi carriers C 9 -C 12  in the upper U-NII band. Each central frequency is spaced 20 MHz relative to neighboring carriers. In  FIG. 1A , for a lower band edge of 5.15 GHz and an upper band edge of 5.35 GHz, the total bandwidth is 200 MHz. The first central frequency C 1  is 30 MHz above lower edge of the lower U-NII band and the eighth central frequency C 8 , is 30 MHz below the upper edge of the middle U-NII band. In the upper U-NII band shown in  FIG. 1B , the total bandwidth is 100 MHz, extending between a lower band edge of 5.725 GHz and an upper band edge of 5.825 GHz. The first carrier C 9  is 20 MHz above the lower edge of the upper U-NII band and the fourth carrier C 12  is 20 MHz below the upper edge of the band. 
     Besides the central frequency of each channel, the 802.11 standard also specifies a spectral mask defining the permitted distribution of power across each channel.  FIG. 2  depicts the transmit spectrum mask  20  according to the 802.11a standard and the power spectrum  25  of a typical WiFi OFDM signal. As shown in  FIG. 2 , the mask has a maximum plateau  21  at 9 MHz around the central frequency F C . Then, the signal is attenuated by about 20 dBr (“dBr” stands for “relative”) from its peak energy in the range of 9-11 MHz from the central frequency F C , shown at  22 ,  22 ′, so that in practice the channels are effectively 22 MHz wide. A smaller rate of power decay creates a skirt  23 ,  23 ′ over the range 11-20 MHz away from F C , where the power level only drops from −20 dBr to −28 dBr. The mask then declines below −40 dBr, shown at  24 ,  24 ′ at frequencies more than 30 MHz away from f. As seen in  FIG. 2 , the wide skirt  24 ,  24 ′ of a standard WiFi OFDM signal extends well outside of the 20 MHz range. It is however assumed that the signal on any channel is sufficiently attenuated outside the 20 MHz bandwidth to minimally interfere with a transmitter on any other channel. 
       FIG. 3  illustrates the U.S. digital television broadcast band after Feb. 17, 2009. The ATSC television signals will be broadcast in the VHF (very high frequency) band and/or the lower part of the UHF (ultra high frequency) band. As seen in  FIG. 3 , the digital TV channels are grouped into five bands denoted with T 1 -T 5 . The band T 1  occupied by channels 2-4 has 18 MHz, extending from 54 MHz to 72 MHz. The band T 2  occupied by channels 5-6 has 12 MHz between 76 MHz to 88 MHz, the band T 3  occupied by channels 7-13 has 42 MHz, between 174 MHz and 216 MHz. Further, the band T 4  carrying channels 14-36 occupies 138 MHz, extending from 470 MHz to 608 MHz and the band T 5  occupied by channels 38-51 has 84 MHz, from 614 MHz to 698 MHz. Thus, this group of 49 channels covers a total spectrum of 294 MHz (18+12+42+138+84). 
     Since channels 2, 3, and 4 will be reserved for some specific applications, after this reservation, the commercial ATSC TV channels will encompass 274 MHz, ranging from 76 MHz to 698 MHz, as shown in gray on the lower part of  FIG. 3 . 
     One embodiment of the present invention includes analyzing bandwidth allocation in the VHF/UHF band, detecting a frequency band denoted generally with B WS  that is unused, and transmitting data and video over a WiFi OFDM signal in this unused bandwidth. In the case that the available whitespace is 18 MHz (e.g. the bandwidth not used by three consecutive RF channels based on the North America TV standards), one embodiment of the invention reconfigures the WiFi OFDM signal in order to retrofit a 20 MHz WiFi signal into the 18 MHz band of these three consecutive TV channels. 
     In addition, band T 1  occupied by channels 2, 3, and 4 may also became available as whitespace for use by other applications. T 1  has traditionally been set aside for set top boxes or Video Cassette Recorders (VCRs), Digital Versatile Discs (DVDs), etc. However, T 1  may stay free most of the time, once non-radio frequency (RF) means of TV signal transport, such as the High Definition Multimedia Interface (HDMI), become prevalent. 
       FIG. 4  shows a flowchart depicting the steps of the method according to one embodiment of the invention. In step  100 , the whitespace is detected using, for example, a wavelet analyzer as described in U.S. patent application Ser. No. 12/078,979, titled “A System and Method for Utilizing Spectral Resources in Wireless Communications,” filed Apr. 9, 2008, which is incorporated herein by reference. The wavelet analyzer is operable to monitor the wireless signals present within the frequency and time domains of a communication spectrum (here, the VHF/UHF spectrum) with a view to automatically and continuously identify bandwidth that is not used currently (whitespace) in the area of interest. It is to be noted that other means for identifying idle bandwidth suitable for the transmission of WiFi OFDM signals may also be used, without departing from the scope of this invention. 
     In step  110 , it is established if a whitespace bandwidth corresponding to three consecutive TV channels is available. As shown by branch “No” of the decision block  110 , the search for identifying whitespace extending over three consecutive TV channels continues until successful; it is to be noted that since the number of TV channels broadcast in each geographical area is limited (currently there are 8 TV channels per station), the likelihood to find such whitespace is quite high. 
     As one illustrative example, assume that three free consecutive channels are identified as shown by branch “Yes” of decision block  110 ; for example, these are channels C 8 , C 9  and C 10  from band T 3  (see  FIG. 3 ). In this case, step  130  is performed next since these channels are not TV channels C 2 -C 4 , as established at decision block  120 . These free TV channels occupy 18 MHz, and as discussed above, a WiFi OFDM signal normally requires a 20 MHz bandwidth and has a wide skirt that extends well beyond this range. According to this embodiment of the invention, standard signals are modified so as to retrofit them into the 18 MHz band, as shown by step  130 . The modified WiFi OFDM signal is also formatted so as to be consistent with all FCC requirements regarding interference with neighboring TV channels. 
     Next, the modified WiFi OFDM signal is adapted for transmission in the whitespace identified in step  100 . This means that the baseband WiFi OFDM signal is modulated on subcarriers selected in the whitespace, as shown in step  140 , and then transmitted over the whitespace band in step  150 . Details on how the WiFi OFDM signal is modified and adapted for transmission in this whitespace band will be described in further detail in connection with  FIG. 5 , which provides a novel emission mask for retrofitting a 20 MHz standard WiFi OFDM signal into an 18 MHz band. 
     If the free channels identified in step  100  are TV channels C 2 -C 4 , as shown by branch “Yes” of decision block  120 , step  140  and  150  are performed, whereby the WiFi OFDM signal is adapted for transmission in the whitespace otherwise occupied by C 2 -C 4 . Details on how the WiFi OFDM signal is adapted for transmission in this whitespace band will be described in further detail in connection with  FIG. 6 . 
       FIG. 5  depicts a novel emission mask  500  designed for a WiFi OFDM signal  550  according to an embodiment of the present invention.  FIG. 5  shows the sub-channels of the WiFi OFDM signal centered about the channel frequency denoted with F C . As discussed previously, the WiFi OFDM signal uses 52 subcarriers (and 12 null subcarriers). In this embodiment, three consecutive idle TV channels are selected for transmission of the WiFi OFDM signal; these channels could be, as in the above example, channels C 8 , C 9 , and C 10  of band T 3  from  FIG. 3 . The selection is made based on the assumption that channels C 8 -C 10  are not used locally for transmission of ATSC TV signals. It is to be noted that these channels are in the middle of the T 3  band, and as such, neighboring ATSC TV channels C 7  and C 11  may be active. Consequently, the emission mask for this case must take into account the presence of adjacent channels C 7  and C 11 , and be designed such that the WiFi OFDM signal does not detrimentally affect the quality of the adjacent TV channels. 
     As shown in  FIG. 5 , the emission mask  500  according to this embodiment has a somewhat different format relative to standard WiFi mask  20  shown in  FIG. 2 . As in the case of the spectrum mask  20 , the signal plateau  510  for the maximum level extends 9 MHz on both sides of the central frequency F C . However, the attenuation slope of the power curve shown by the skirts  520 ,  520 ′ is very high; the power level drops dramatically in a space of only 500 kHz, declining to −36 dBr at 9.5 MHz away from the central frequency F C . Power level continues to decline thereafter, as shown by slopes  530 ,  530 ′ reaching −99 dBr at 15 MHz away from F C . 
     The WiFi OFDM signal  550  of embodiment shown in  FIG. 5  has an upper guard band  554  of 2.5 MHz, protecting the adjacent TV channel at the higher end of the 18 MHz whitespace band, and a lower guard band  552  of 2.5 MHz, protecting the adjacent TV channel at the lower end of the 18 MHz whitespace band. These guard bands  552 ,  554  are obtained with a proper implementation of the filters  706  (see  FIG. 7A ) which guarantees that any interference with adjacent channels meets the FCC interference regulations for TV usage in the whitespace band.  FIG. 5  also shows at  540  an ideal signal spectrum; it is to be noted that in practice, filters  706  may be designed to shape the signal spectrum between mask  500  and the ideal spectrum  550 . 
     The WiFi OFDM signal  550  is modified to match mask  500 . In order to provide the upper and lower guard bands  554 ,  552 , the spectrum actually occupied by the modified WiFi OFDM signal  450  between subcarriers  1  and  52  is only 13 MHz instead of the 16.25 MHz that would have been occupied by a standard WiFi OFDM signal. This results in a subcarrier spacing of 250 kHz (13 MHz/52 subcarriers), which is lower that the subcarrier spacing of the standard WiFi OFDM signals of 312.5 kHz. 
     In this example, the useful symbol duration is lengthened from 3.2 μs of the standard WiFi OFDM signal to 4 μs and the guard interval between subcarriers is proportionately increased from 0.8 us to 1.0 μs. The peak data rate is lower than for standard WiFi OFDM signals, dropping to 43.2 Mbps instead of the standard 54 Mbps, due to the increase in symbol duration from 4 μs to 5 μs. This may require the system timers to be reset. However, the decrease in peak data rate is not likely to impact the overall system throughput much, since the modified WiFi OFDM signal uses a lower frequency band (VHF/UHF) and therefore can better cope with the environmental channel statistics. 
       FIG. 6  depicts an emission mask  600  used for an exemplary WiFi OFDM signal  600  according to another embodiment of the present invention, suited for use in the TV band denoted with T 1  in  FIG. 3 . As band T 1  is used by only three digital TV channels, C 2 -C 4 , the design requirements for the WiFi OFDM signal in this band are more relaxed; there are no TV channels to interfere with to the right or left of this band. Emission mask  600  according to this embodiment is similar to the mask  500  shown in  FIG. 5 , but is translated at a different central frequency F C .  FIG. 6  also shows at  640  an ideal signal spectrum according to this embodiment of the invention; it is to be noted that in practice, filters  706  (see  FIG. 7A ) may be designed to shape the signal spectrum between mask  600  and the ideal signal spectrum  650 . 
       FIG. 6  provides the specific value of the frequencies from the spectral mask, because the position of the channels 2-4 in the spectrum is known. Since the requirements in band T 1  are more relaxed, in this embodiment the parameters of the WiFi OFDM signal  650  differ from the parameters of the WiFi OFDM signal  550 . Thus, the bandwidth of signal  650  is 16.25 MHz, the same as in the case of the standard signal, but it ranges from 54.875 MHz to 71.125 MHz. The subcarrier spacing in this embodiment is 312.5 kHz (16.25 MHz/52 subcarriers), again the same as in the case of the standard signal. The symbol duration and guaranteed data rate are also consistent with the 802.11a and 802.11g standards, at 4 μs (3.2 μs for the useful symbol duration and 0.8 μs for the guard interval duration) and 54 Mbps, respectively. 
       FIG. 7A  depicts an exemplary OFDM transmitter  700  according to one embodiment of the invention. As shown in  FIG. 7A , OFDM transmitter  700  comprises a plurality of baseband blocks which may be similar to the blocks used by a conventional WiFi transmitter, such as a FEC encoder  701 , an interleaver  702 , a constellation mapping block  703 , an OFDM symbol construction block  704 , and an inverse Fourier transform block  705 . The part of the transmitter  700  denoted with  750  is, however, different from that of the corresponding part of a conventional transmitter shown in  FIG. 7B . 
     A first difference is the design of the baseband filters  706  from the filters  711  shown in  FIG. 7B . As a preliminary matter, filters  706  are illustrated as one distinct unit only to provide a clear explanation of the frequency characteristics. As known in the art, signal filtering and shaping may be a multistage process rather than a one stage process. Also, filter  706  is not necessarily connected after the DAC  707 . Alternatively, the DAC  707  may itself include filters that contribute to signal shaping. 
     Filters  706  shape the WiFi OFDM signal according to the masks  500  or  600 , shown in  FIG. 5  or  FIG. 6 , respectively. The differences between the transmit spectrum mask  20  used for conventional WiFi OFDM signals and the transmit spectrum mask  500  or  600  used for the modified WiFi OFDM signal of the invention were discussed previously. 
     Another difference is that transmitter  700  uses a low power amplifier or a preamplifier  708  that amplifies the symbols before modulation in the mixer (multiplier)  709 . While conventional WiFi systems require a high power amplifier  714 , as shown in  FIG. 7   a , the present invention may use a less costly preamplifier, as little power is needed to broadcast for a short distance (within a house) in the VHF/UHF band. Using an ultralow power design to cover a home environment, power may be no more than, for example, 200 mV/m. 
     Another difference appears in the structure of the mixers  709  of the transmitter  700 , as opposed to the mixer  713  of a conventional transmitter. Transmitter  700  uses subcarriers in the VHF/UHF band, as discussed in connection with  FIGS. 5 and 6 , rather than in the 2.4 GHz or 5 GHz bands used for the standard WiFi OFDM signals. Therefore, mixer  709  should heterodyne the baseband signals to the center of the whitespace band in the VHF/UHF band. For the example where channels C 8 , C 9  and C 10  are bonded together to form the white-space band as in  FIG. 5 , mixer  709  should be designed to mix a VHF frequency corresponding to the central frequency of the band occupied by these channels. For the case when the channels 2, 3, and 4 are bonded together to form the whitespace band as in  FIG. 6 , the desired frequency range extends from roughly 54 MHz to 72 MHz. Mixer  709  should be designed to mix a VHF frequency of approximately 63 MHz, corresponding to the central frequency of the band T 1  in this example. 
     Also, a VHF/UHF antenna  710  is used for transmitting the WiFi OFDM signals over short distances by transmitter  700 , rater than an antenna  715  used by the conventional WiFi OFDM signals that are transmitted in the 2.4 GHz or 5 GHz bands over longer distances. 
     In the example of  FIG. 7A , the size of the Fast Fourier Transform remains unchanged at 64, as the number of subcarriers used by the modified WiFi OFDM signal is still 64, namely 48 data subcarriers, 4 pilots and 12 null subcarriers. Among these, the twelve null subcarriers (e.g.  0 ,  27 - 37 ) may be used for guard bands. The four pilot subcarriers may, for example, be subcarriers  7 ,  21 ,  43 , and  57 . 
       FIG. 8A  shows an exemplary OFDM receiver  800  according to one embodiment of the present invention. As shown in  FIG. 8A , receiver  800  comprises a plurality of baseband units that are similar to the units used by conventional WiFi receivers. The RF part of the receiver, i.e. the antenna  801  and the RF receiving unit  802  differ from the corresponding units used by the conventional WiFi systems shown in  FIG. 8B . 
     Thus, VHF antenna  801  is adapted to receive incoming signals in the VHF band that are broadcast over relatively short distances. The receiving unit  802  includes a Low Pass Filter (LPF)  811  that removes high frequency noise and passes the signals in the VHF band. An analog-to-digital converter (ADC)  812  of receiving unit  802  converts the received analog signal into a sequence of bits, a synchronizer  813  converts the sequence of decoding bits into a sequence of frames of bits, each of the sequence of frames having M decoding bits. In contrast, the receiving unit  820  for standard WiFi OFDM signals, shown in  FIG. 8B  uses a WiFi RF filter  821  suitable for the respective 2.4/5 GHz bands. As well, ADC  822  and synchronizer  823  of receiving unit  820  are designed for recovering the baseband signals from the standard WiFi band, and not the VHF band allocated to the digital TV channels. 
     The baseband units used by the receiver  800  operate to perform the reverse operation on the baseband signals provided by the receiving unit  802 . Thus, Fast Fourier Transforming (FFT) unit  803  decodes the bits in the sequence of frames to generate a sequence of symbol frames, each of the frames having at least N time domain decoded symbols. Channel estimation and equalization unit  804  and demapper  805  process the sequence of decoding symbol frames to generate a sequence of frames of N interleaved sub-channel bits, and deinterleaver  806  processes each of the frames of N interleaved sub-channel bits to generate a stream of N recovered bits. The FEC decoder  807  performs error correction and descrambler  808  recovers the bits of the original signals. 
     From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous system and method for data distribution in VHF/UHF band. The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. One skilled in the art will readily recognize from such discussion that various changes, modifications, and variations may be made therein without departing from the spirit and scope of the invention. Accordingly, disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.