Patent Publication Number: US-8995292-B2

Title: Systems and etiquette for home gateways using white space

Description:
RELATED APPLICATIONS  
     This patent application is a continuation of U.S. patent application Ser. No. 12/274,141, entitled “SYSTEMS AND ETIQUETTE FOR HOME GATEWAYS USING WHITE SPACE,” filed Nov. 19, 2008, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
     This invention relates generally to distribution of data signals over white space, and more particularly to a system and etiquette for using the white space freed within a specified TV market by conversion from analog to digital TV (DTV) systems. 
     BACKGROUND  
     The beginning of the 21st century viewed a significant development of user wireless terminals for both fixed and mobile environments. For example, the modern PCs (personal computers) and notebooks (laptops) are enabled with wireless connections to allow for some mobility and reduce the wiring between the components of the PC. Home LAN convergence rapidly integrates home routers, wireless access points and DSL modems to connect home computers to a subscribed Internet service. In response to the growing demand for broadband connection to user premises, a worldwide trend became apparent, namely a move from analog to digital TV Digital (DTV). DTV has several advantages over analog TV. Thus, DTV is more flexible and efficient than analog television; it enables special services such as multiplexing (more than one program on the same channel), electronic program guides and additional languages, spoken or subtitled. Another significant advantage of the DTV is that the digital channels take up less bandwidth and provide a better performance. This means that digital broadcasters can provide high definition (HDTV) services, with higher-quality images and sound. The sale of non-television services may also provide an additional revenue source. 
     In the US, the FCC mandated transition to digital TV (DTV). Conversion to DTV results in important bandwidth becoming free in the VHF (very high frequency) and/or the lower part of the UHF (ultra high frequency) spectrum, used currently by TV broadcasters. As each TV station operating in a certain geographic region/area (TV market) uses only a limited number of channels from the TV band, some digital channels remain unused by broadcasters in the respective area: this locally available spectrum is called “white space”. 
     For example, in the United States there are roughly 210 TV broadcast markets 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. These developments open the way to providing a variety of new, dedicated services to individual/family subscribers. The FCC intends to allocate channels 2 through 51 to digital TV; channels 52through 69 that occupy the lower half of the 700 MHz band have already been reallocated through auction to various advanced commercial wireless services for consumers. When transition DTV ends in early 2009, every one of the 210 TV markets in the US will have 15 to 40 unassigned (vacant) channels reserved for broadcasting, but not in use. Vacant TV channels are perfectly suited for other unlicensed wireless Internet services. Access to vacant TV channels facilitates a market for low-cost, high-capacity, mobile wireless broadband networks, including the emerging in-building networks. Using this white space, the wireless broadband industry could deliver Internet access to every household for as little as $10 a month by some estimates. 
     Currently, most flat panel HDTV sets still use cablings to connect to set top box or other devices such as DVDs, BlueRay devices or VCRs. The cabling requirements limit the flexibility of HDTV installation and the appearance. So, there is a market demand to find solutions that eliminate or minimize use of cables. Some companies proposed wireless HDTV solutions in the 60 GHz spectrum, whereby delivering raw data wirelessly to the HDMI port, which is a standard HDTV interface. Although there is some progress in this area, the performance and cost of this solution still poses design and operational challenges. 
     On Oct. 15, 2008, the FCC&#39;s engineering office released a report that spells out the rules the devices must meet in order to use the white space, the main requirement being not to interfere with the primary services active in the respective area. Thus, the signals radiated by any white space device operating in the ATSC spectrum must follow the FCC regulations so that the quality of the primary services such as TV broadcast, wireless microphones, or other emerging services already deployed or which will be deployed in that area will not be degraded by these new services. The new white space devices should be designed so as to not affect the TV tuner sensitivity and the TV receiver performance specified by the Advanced Television Systems Committee (ATSC) standards. The specification uses the term “white space etiquette” for this set of regulations that must be accounted for when designing and using white space devices. For conformity with these requirements, FCC requires both fixed and portable devices to include geolocation capabilities and use an FCC database of TV signals and location of venues such as stadiums and churches that use wireless microphones. These database and geolocation capabilities would, in theory, prevent interference with broadcast TV stations and wireless microphones and ensure compliance with FCC rules. 
     The mobile industry is considering using the white space by developing standards on technologies convergence into an architecture that is comfortable, easy to use and attractively priced. For example, the IEEE 802.22 Working Group, formed in 2004, received the mandate to develop a standard for Wireless Regional Area Networks (WRAN). The mission for this technology is to provide rural area broadband services to single-family residential, multi-dwelling units, small office/home office, small businesses, etc. The standard will be used by license-exempt devices in the spectrum allocated to the TV broadcast service on a non-interfering basis. The 802.22 draft specifies that the network should operate in a point to multipoint configuration, where an outdoor installed access point (AP) will control the medium access for all the customer premise equipment attached to it, while avoiding interference with the broadcast services. One key feature of the WRAN APs is the capability to perform a distributed spectrum sensing, where the customer premises equipment will be sensing the spectrum and will be sending periodic reports to the serving AP informing it about what they sensed. Based on the information gathered, the AP will evaluate whether a change is necessary in the channel utilized, or conversely, if it should stay transmitting and receiving over the same one. OFDMA is proposed as the modulation scheme for transmission in uplink and downlink. Channel bonding is also specified for cases when higher bit rates and distances are needed. 
     However, the 802.22 draft proposes use of the white space only in chunks of 6 MHz (the width of a TV channel specified by the ATSC standard) and does not allow for finer granularity. Also, it requires high power transmissions in both base stations and terminals so as to cover an area with a 30 km radius. This results in inefficient use of bandwidth (which is a valuable resource) for the case when only a portion of a 6 MHz channel is actually needed. For instance, when a wireless microphone application occupies a few hundreds kHz bandwidth from a 6 MHz piece of spectrum, the reminder of the 6 MHz may be wasted. As well, the bandwidth resources may be inefficiently utilized when the broadband market is in populated areas such as homes or shopping malls etc. 
     Therefore, there is a need to provide an inexpensive and efficient way to broadcast multimedia content within a specified area using wireless solutions. There is also a need to set overall rules (etiquette) on how to use the white space spectrum that is reserved but not used by the primary services in a certain area, without affecting operation of the existent services. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide methods and systems for identifying and using the white space available in a specified location for delivering broadband services over a home network. The present invention provides efficient ways to detect the white space and also efficient ways to utilize and share the white space. 
     Another object of the invention is to provide methods and systems for delivering broadband services to users over a home network, without degrading the quality of the primary services, such as TV broadcast, digital microphone and other services active in the respective area. 
     Accordingly, the invention provides a system for redistributing data signals to wireless enabled devices located within a specified area, comprising: a spectrum detector for identifying a white space channel including one or more pieces of spectrum that are not currently used in the specified area; a data distribution unit adapted to receive, process and distribute to devices it serves, information signals received from a variety of sources over the white space channel; and a control channel processor for providing a shared signalling channel for enabling signalling between the wireless enabled device and the data distribution unit. 
     The invention also provides a method for redistributing data signals to wireless enabled devices located within a specified area, comprising: a) establishing a shared signalling channel over a piece of spectrum that is not currently used in the specified area; b) on receipt of a service request from a device transmitted over the shared signalling channel, establishing a white space channel using one or more pieces of spectrum that are not currently used in the specified area; c) distributing an information signal over the white space channel to the device; and d) controlling transfer of the information signal to the device by exchanging control messages over the shared signalling channel. 
     Still further, is directed to a method for redistributing data signals to wireless enabled devices located within a specified area served by a gateway, comprising: a) on receipt of service requests from one or more devices, identifying a corresponding number of white space channels within the spectrum that is not currently used in the specified area; b) reserving the white space allocated to the white space channels identified in the specified area; c) broadcasting a white space allocation message for advertising to all neighbouring gateways the current channel allocation in the specified area; and d) distributing the information signals over the white space channels to the respective devices. 
     Advantageously, the invention provides an efficient way of identifying pieces of white space in a specified area and using this white space for delivering secondary services to subscribers present in that area. By complying with the etiquette proposed by the invention for white space use, the scanning and detection operation becomes simple, and therefore practical and cost effective. The systems and methods of the invention are also providing ways to using the identified white space in such a way so as to not interfere with the services already active in the respective area. 
     Another advantage of the method and systems of the invention is that it allows for a simpler, cost effective design of white space devices. This invention also provides a practical way for cognitive white space device implementation. 
    
    
     
       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 ATSC broadcast band; 
         FIG. 1B  illustrates the four bands where white space spectrum may become available; 
         FIG. 2  shows a block diagram of a system for distributing multimedia data to subscribers at a specified location according to an embodiment of the invention; 
         FIG. 3A  illustrates use of wavelets for identifying white space according to an embodiment of the invention; 
         FIG. 3B  shows examples of white space allocation per users; 
         FIG. 4A  illustrates the spectrum used by a signalling channel according to an embodiment of the invention; 
         FIG. 4B  shows an example of a TDD frame that may be used for the signalling channel; 
         FIG. 5A  illustrates an embodiment of a common spectrum reservation OFDM symbol used by the gateways to advise the spectrum reservation status to neighbouring gateways; 
         FIG. 5B  shows the common spectrum reservation OFDM symbol in the frequency domain; 
         FIG. 5C  illustrates an example of how pieces of white space can be identified using bin indexing, according to an embodiment of the invention; 
         FIG. 6A  is a flowchart of the method according to an embodiment of the invention, showing the steps performed by the gateway; and 
         FIG. 6B  is a flowchart of the method according to an embodiment of the invention, showing the steps performed by the device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION  
     In this specification, the term “primary services” is used for DTV broadcast, wireless microphone and any other applications that are entitled by regulations to use a specified portion of the spectrum. As indicated above, each TV station operating in a certain geographic region/area uses only a limited number of channels from the spectrum allocated to the DTV, such that some pieces of spectrum (contiguous or not) remain unused in the respective area: this locally available spectrum is called “white space”. Thus, the white space differs from TV market to TV market; the term “specified area” or “location” is used to designate a particular area such as single or multi-dwelling units, small office/home office, small businesses, multi-tenant buildings, public and private campuses, etc located in a certain TV market. The term “home network” refers to a wireless network serving such an area. 
     The present invention provides methods and systems for distribution of data signals in a specified area, and more particularly to a system and etiquette for using any available white space. In particular, the description refers to the white space freed within such an area by the conversion from analog to DTV broadcast, and uses as an example the North America ATSC standards; however, the methods and systems described here are also applicable to other jurisdictions, standards and/or parts of the spectrum. For example, a DTV channel in Japan occupies 8 MHz and the width of a DTV channel in European countries is 7 MHz. The invention is not restricted to identifying unused pieces of spectrum 6 MHz wide (the width of an ATSC channel); applying the techniques described here, narrower or larger pieces of unused spectrum may be considered. For example, if a portion of a 6 MHz piece of white space is occupied by a wireless microphone, the remaining part of that 6 MHz piece of spectrum can also be used according to this invention. Still further, the invention is described in connection with local wireless TV broadcast, but same principles are applicable to other types of signal distribution in the white space, and the white space need not necessarily be in the VHF/UHF bands. 
     Turning to the North America example, the ATSC standards mandate 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 properly decode the ATSC signal and provide good quality TV pictures. These requirements include a Signal to Noise Ratio (SNR) no less than 15 dB or equivalently, a bit error rate (BER) no worse than 3×10 −6 , a thermal noise floor of −106.2 dBm (dBm is the abbreviation for the power ratio measurement units), and a sensitivity between −81 and −84 dBm, etc. Any secondary service contending for the white space available in a certain area must comply to a certain etiquette so as to not interfere with the primary services (TV broadcast, wireless microphone, etc). 
     Referring now to the drawings,  FIG. 1A  illustrates the US digital television broadcast spectrum after Feb. 17, 2009, showing five bands denoted with T 1 -T 5 . Band T 1  occupied by ATSC channels  2 - 4  has 18 MHz, extending from 54 MHz to 72 MHz. Band T 2  occupied by channels 5-6 has 12 MHz between 76 MHz to 88 MHz, band T 3  occupied by channels  7 - 13  has 42 MHz between 174 MHz and 216 MHz. Further, band T 4  carrying channels  14 - 36  occupies 138 MHz, extending from 470 MHz to 608 MHz and band T 5  occupied by channels  38 - 51  has 84 MHz, from 614 MHz to 698 MHz. Thus, this group of 49 ATSC 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 on  FIG. 1B  by bands T 2 -T 5 . 
       FIG. 2  shows a block diagram of a system for distributing multimedia data to subscribers at a specified location according to an embodiment of the invention. The system includes a gateway  10 , which is preferably a home gateway, which broadcasts TV signals inside a home to a device  9 . Device  9  is a wireless enabled data device; while  FIG. 2  shows a computer, it is to be understood that device  9  can equally be a digital TV set provided with a set-top box, or any other device that enables a user to select, access and view multimedia content (e.g. TV programming, movies, games, etc). It is also to be noted that this invention enables use of conventional TV sets, with no changes required to the tuner. In general terms, gateway  10  comprises a data distribution unit  1 , a spectrum detector  2  a control channel processor  3  and a backhaul interface  20 . This Figure also shows the components of the data distribution unit  1 , namely, a baseband processor  6  and a distributor  7 . 
     Spectrum detector  2  is used to sense the white space available in the respective area (see above definition) over a part of the wireless communication spectrum (licensed or not) and provide this information to distributor unit  1 . The part of the wireless communication spectrum over which the white space is sensed is preferably preset when the system is installed, to a certain part or parts of the spectrum that are known to be underutilized in a certain region. For example, it may include the spectrum freed by transition from analog to digital TV, or parts of the unlicensed spectrum (e.g. 2.4 GHz and 5 GHz bands used by WiFi). As another example, it can include the entire VHF/UHF spectrum. Spectrum detector  2  senses the wireless signals present in the scanned spectrum, using an antenna  4 . 
     The control channel processor  3  is used for enabling the devices to communicate with the distribution unit  1  over a shared signalling channel, denoted with  5  on  FIG. 2 . Signalling channel  5  enables establishment of a master-slave relationship between the gateway and the devices it serves. By using this signalling channel, the gateway controls operation of the devices in conformity with the white space etiquette. In turn, the shared signalling channel is used by all devices served by distribution unit for connection set-up (as a rendezvous channel), for communicating to unit  1  information such as access requests, bandwidth requests, etc, as known to persons skilled in the art. Antenna  4  is used for transmitting the control information to the devices over the signalling channel  5  and for receiving the service requests and status information from the devices over channel  5 . In the embodiment shown in  FIG. 2 , the channel processor  3  and spectrum detector  2  share antenna  4 , but embodiments with different antennae for these operations are also possible. 
     Data distribution unit  1  receives the user signals from various sources and transmits these to the device  9  over the white space identified by the spectrum detector  2 ; antenna unit  13  is used for transmitting the data to the devices. As indicated above, parameters of the information signal transmitted by the unit  1  are designed so as to not interfere with the primary services present in the respective area. In general terms, the power of the information signal is very low, the carrier frequency and the band of the signals are selected in parts of the spectrum that are not currently used by other services, and the transmission is synchronized to the ATSC broadcast. 
     The operation of the spectrum detector  2  is described in connection with  FIGS. 3A and 3B . As indicated above, spectrum detector  2  scans selected part of the wireless communication spectrum. As an example, the parts of interest could be the bands T 2 -T 5  allocated to DTV. The searched spectrum may be further defied in each TV market based on the tables (available) with the regional allocation of TV channels. Since each TV market uses a maximum of 12 TV channels (i.e. 72 MHz), it is very likely that more than 200 MHz of white space will be available in any particular location. If a look-up table with the locally allocated TV channels is provided at the gateway, the gateway may exclude the spectrum occupied by these channels from the scan; if not, gateway  10  scans the entire TV spectrum. 
     The spectrum analyzing operation involves scanning a spectrum of interest for sensing (detecting) any signals occupying that spectrum, analyzing the sensed signals and identifying the pieces of spectrum that are not currently occupied; in other words, detecting pieces of white space. The term “current spectrum occupancy” refers here to the spectrum that is currently allocated to various primary services, or used by any active secondary service. Spectrum analyzing is repeated at regular intervals, as the spectrum occupancy varies arbitrarily with wireless devices being connected/disconnected to/from the gateway, so that the spectrum detector  2  enables the gateway to know at all times of the spectrum occupancy at the respective location. 
     Spectrum detector  2  may be a spectrum analyzer of any kind. For example, a preferred embodiment of the invention uses a wavelet spectrum analyzer as described in the co-pending patent application U.S. patent application Ser. No. 12/078,979 identified above.  FIG. 3A  illustrates use of wavelets for identifying white space according to an embodiment of the invention;  FIG. 3B  shows examples of white space allocations per users. A wavelet spectrum analyzer/detector uses a frequency and time map, which is divided into frequency-time cells, as shown in  FIG. 3A  at  31  and  32 . Using wavelet signal analysis, signal energy within each of the frequency-time cells is measured in order to identify frequency-time cells with little or no detectable signal activity. The frequency-time cells identified as unused provide an opportunity for use by the secondary services. 
     Another feature of the wavelet spectrum detector  2  is that the size of the frequency-time cells can be adjusted for finer or coarser granularity, in both time and frequency. For example, the time-frequency cells  31 ,  32  have different time and frequency dimensions: in  FIG. 3A , cell  31  may provide a piece of free spectrum with a bandwidth Δf=f B −f 2 , for a duration of time ΔT=t 2 −t 1 ; cell  32  may provide another a piece of free spectrum with a bandwidth Δf=f 2 −f 1 , for a duration of time ΔT=t 4 −t 3 . As the frequency-time cells identified as usable (free) change in time and frequency, the selection of communication channels by the gateway  10  is “dynamic”. As the frequency-time cells identified as usable (free) change also in size, detection of white space and selection of the communication channels by the gateway is also “adaptive”. By identifying “smaller” frequency-time cells with no detectable signal activity, the home network may take advantage of instances within a communication spectrum where a certain spectrum can be shared in both frequency and time domains. It also enables identifying more than one piece of spectrum for use by a new service, when only one piece of spectrum is insufficient for the respective service. 
       FIG. 3B  shows examples of white space allocations per user. For example, in order to obtain 6 MHz for broadcasting a TV channel to a user A, spectrum analyzer  2  scans the spectrum and identifies a 2 MHz piece of white space in T 2 , denoted with A 1  and 4 MHz of white space in T 3 , denoted with A 2  (note that the width of the identified spectrum is exaggerated in  FIG. 3B  for better illustrating the example). As another example, if 1 MHz is available in band T 3 , shown by B 1 , 3 MHz are available in T 4  shown as B 2 , and 2 MHz are available in T 5  shown as B 3 , a user B will receive a TV broadcast from the gateway on a white space channel including all three pieces of spectrum B 1 , B 2  and B 3 . Still another example, not shown, is when the piece of spectrum identified by the gateway for a certain receiver has 6 MHz in which case the white space channel is made of one piece of spectrum only. 
     The dynamic and adaptive attributes of the wireless spectrum allocation method and system of the invention make it quite different from the conventional wireless spectrum allocation schemes. First, conventional wireless communications RF channels are usually pre-designed in that the transmission channels are allocated either in frequency in (FDMA systems), in time (TDMA systems), both in frequency and time (OFDMA), or in codes (CDMA systems). But, it is well known that wireless devices use a piece of spectrum and release it when communication is completed, so that spectrum utilization is not ideal when the allocation is pre-designed, as some pieces of spectrum may be idle during some periods of times, while some devices cannot gain access to this idle spectrum. Furthermore, in conventional systems, for enabling multiple users to share a piece of spectrum, the bandwidth and waveforms need to be pre-designed so as to keep the channels well separated to reduce the interference. For example the GSM systems, which can accommodate simultaneously more than 8 calls within a 200 kHz channel, use a Gaussian-like waveform. WCDMA systems have a 5 MHz channel bandwidth and a chip rate of 3.84 Mcps and use a radio resource controller (RRC) with a roll off factor 0.22, WiFi OFDM systems have a bandwidth 20 MHz and use rectangular pulses, etc. 
     To summarize, the channels established between the gateway  10  and devices  9  according to the invention change dynamically in time, frequency and size. To distinguish these channels from the conventional ones, the specification uses the term “TV channel” for the DTV channels and the term “white space channel” for a logical channel formed by one or more pieces of spectrum identified by gateway  10  and allocated to a certain device for a respective secondary service: it can include a piece of white space spectrum or a plurality of pieces bonded together, the white space of spectrum, the pieces being consecutive or not. 
     The signalling channel, used for enabling proper operation of the devices served by gateway  10 , is described in connection with  FIGS. 4A and 4B . The gateway starts transmitting this channel from the moment it is turned on; this enables the devices  9  to look for the gateway, connect to it, request and receive services and monitor the channel for further instructions.  FIG. 4A  illustrates selection of the shared signalling channel according to an embodiment of the invention, and  FIG. 4B  shows an example of a TDD frame that may be used for the shared signalling channel. 
     Gateway  10  selects the shared signalling channel (or signalling channel or rendezvous channel)  5  in the white space. As seen in  FIG. 4A , the signalling channel  5  is selected in a fixed relationship with the left edge of a respective unoccupied DTV channel, as shown by Δ on  FIG. 4A  and may occupy for example 300 kHz. Once the gateway is synchronized to the DTV broadcast, synchronization is maintained for all data transmitted to the devices it serves. For example, the signalling channel  5  of the invention may be set at for 500 kHz from the ATSC pilot  41  (the ATSC pilot is designed to be transmitted at 309.44 kHz from the start of a TV channel). It is noted here that this is just an example, and other arrangements can be equally used. Thus, the carrier frequency for the signaling channel may be in a pre-selected relationship with a frequency mark defined by the ATSC specification, such as for example the beginning of an ATSC channel reserved but not used in the spectrum portion of interest. It is also to be noted that the ATSC signal  40  in  FIG. 4A  is shown only for illustrating how the gateway is synchronized to the ATSC broadcast; no ATSC signal is in fact present in the spectrum where the control channel is established. To summarize, according to the invention, is a narrow (e.g. 300 kHz), low power channel established over the air between the gateway antenna  4  and device&#39;s antenna  4 ′. The signalling channel  5  is selected into a specified part of the white space spectrum, so that the device does not need to scan large portions of the spectrum; this will reduce complexity of the device scanner. 
     Channel  5  may be a TDD channel, shared by the devices that are on. Since the spectrum occupied by this channel is small, the shared signalling channel may be transmitted using a small portion of spectrum in TV channels  2 ,  3  or  4 , or any 6 MHz TV channel that is not in use. Alternatively, the signalling channel  43  may be transmitted within the guard band between the ATSC channels, while maintaining the fixed relationship with the ATSC pilot  41 . 
     The width of the signalling channel may be set based on the number of devices served by the gateway  10 . For example a 300 kHz wide channel may be shared by eight devices. As the white space otherwise occupied by an active TV channel is much larger (6 MHz), the reminder of white space may be used by the gateway for other services. 
       FIG. 4B  shows an example of an embodiment of the frame  43  of such a bidirectional shared signalling channel. In this example, as the channel is 300 kHz wide, the resulting speed is roughly 300/600 bits/ms for a BPSK or QPSK modulation. The frame  43  has 10×1 ms slots, for a frame duration of 10 ms. Of the 10 slots, 8 slots are allocated to the devices, and the first and the last are reserved for general control data. The information in the header of a frame identifies this as the signalling channel. As seen in the lower part of  FIG. 4B , out of two consecutive frames, frame  44  is broadcast by the gateway  10  to all devices D 1 -D 8 ; each device receives and processes the data from the slot that is addressed to it. The next frame denoted with  45  carries data transmitted by the devices to the gateway, in sequential order. It is noted that this is just an example of the signalling channel and that numerous other embodiment of this channel may be equally used. 
     When gateway  10  receives a connection request from a device  9 , it selects a white space channel on which to broadcast to device  9 . The white space channels are allocated based on the available spectrum sensed by the detector  2 , and takes into account the bandwidth necessary for the requested service. Once the white space channel is selected, the gateway informs device  9  on the parameters of this channel (frequency, time, size) over the signalling channel, and starts broadcasting data to the device/s. 
     Returning now to  FIG. 2 , backhaul interface  20  comprises a plurality of interfaces  21 - 26  that receive user signals from various sources. The interfaces include the respective demodulators and error correctors that decode and process the source signals of various formats into a baseband signal that carries the user data. For example, interface  20  may include a Quadrature Phase-Shift Keying/Forward Error Correction (QPSK/FEC) decoder  21 , an Orthogonal Frequency-Division Multiplexing/FEC (OFDM/FEC) decoder  22 , a Quadrature Amplitude Modulation/FEC (QAM/FEC) decoder  23 , a Digital Subscriber Line (xDSL) unit  24 , a Fiber To The Home (FTTH) unit  25 , and a Digital Versatile Disc (DVD) unit  26 . Interfaces  21 - 26  are known to the persons skilled in the art and therefore are not described in further details here. It is however to be noted that interfaces  21 - 26  preferably do not perform any MPEG decoding to keep the data rate below 30 Mbps and reduce the cost. 
     A particular signal source  21 - 26  is selectively coupled to the baseband (BB) processor  6 . For the example of a North American type gateway, BB processor  6  converts the signals to ATSC standard signals to ensure compatibility of the signals broadcast by the gateway with ATSC equipment at the user end. The signal is then provided to distributor  7  that up-converts the BB signal received from processor  6  on the respective white-space channel/s detected by spectrum detector  2 , and transmits the signal to the devices  9 . 
     Distributor  7  is shown with four branches, each for a certain carrier frequency f 1 -f 4  in the embodiment of  FIG. 2 , illustrating baseband filters  27 , mixers  28 , filters  29  and amplifiers  30 . The components of distributor  7  are merely one possible structure. Distributor  7  may be replaced by a suitably designed equivalent, as will be apparent to those of skill in the art. The number of branches used for distribution of the signal to device  9  is also a design parameter; a maximum of four branches is preferred, but the invention is not limited to this number. 
     At any time, it is possible that less than four branches of the distributor are used for transmission to a device  9 . For the example of user B of  FIG. 3B , since the white space channel allocated to device  9  includes pieces of white space B 1 , B 2  and B 3 , distributor  7  uses only three of the four branches. In this case, let&#39;s say that f 1  is the carrier frequency in B 1 , f 2  is the carrier frequency in B 2 , and f 3  is the carrier frequency in B 3 . The baseband signal, let&#39;s call it S, is de-multiplexed by processor  6  into three component signals s 1 , s 2  and s 3  according to the available bandwidth B 1 , B 2  and B 3 , and each component is transmitted along one of the branches. The components are first filtered by filters  27 , then mixed with the respective carrier f 1 , f 2  and f 3  in modulators  28  and further shaped by filters  29 , to obtain a respective RF component S 1 , S 2  and S 3 . Next, S 1 , S 2  and S 3 , amplified as shown at  30 , are combined in a RF combiner  39  for transmission, after being preferably OFDMA modulated. 
     For the example shown in  FIG. 3B  where the white space is provided by two pieces of spectrum A 1  and A 2 , if A 1  and A 2  have the same bandwidth, a transmitter diversity scheme can be achieved by transmitting the same signal via both branches, in which case two antennae are provided by antenna unit  13 ; the receiver may thus obtain a higher gain by manipulating the amplitude, phase and symbol pattern of the two received signals. As another example, when the gateway identifies 6 MHz of white space in one piece, only one branch of the distributor  7  is used for broadcasting an ATSC channel. 
     As indicated above, the present invention also provides for a common spectrum reservation message, which is shown in  FIG. 5A . This message informs the neighbouring gateways of the spectrum reservations made by each gateway  10 . In this way, the white space spectrum already reserved in the respective area is known at all times to all gateways operating in the respective area. Each home gateway transmits the reservation message at predefined time intervals using a common OFDM symbol. The common spectrum reservation OFDM symbol carries spectrum usage information and further refines the available spectrum by detecting incumbent systems and devices. 
     As an example, the common spectrum reservation OFDM symbol uses a sub-carrier spacing Δf, and has a size N for the Fourier transform (DFT). In a preferred embodiment, Δf=200 kHz and N=1370. The length of the cyclic prefix (CP) is selected to compensate for the delay spread among the neighbouring gateways (to allow multipath to settle before the main data arrives).  FIG. 5B  shows how the common spectrum reservation OFDM symbol is designed for the above example where Δf=200 kHz, N=1370, and the spectrum under consideration is the VHF/UHF band, between 76 and 698 MHz. As seen, “zero&#39;s” are transmitted in the intervals between bands T 2 -T 3 ; T 3 -T 4 ; T 4 -T 5 ; while a transmitted zero does not modulate anything, it raises the noise level so that it still can be detected (i.e. still some decoding is possible). 
       FIG. 5C  illustrates an example of how the spectrum of interest is divided into bins, for enabling the gateways to identify and indicate any piece of spectrum using a bin index. Bins in one of bands T 2 , T 3 , T 4  or T 5  can be identified using the first 2 bits of the index (for four bands). For example, the bins in T 2  could be identified by a 00 value for the bin index, the bins in T 3  by a 01 value, in T 4  by a 10 value, and in T 5  by a 11 value. As the width of the bands T 2 -T 5  is different, each of the bands T 2  to T 5  will need to use a different number of bins. Thus, based on a Δf=200 kHz (granularity of the pieces of spectrum to be allocated to the secondary services) band T 2  requires 6 bits to identify 59 bins, band T 3  requires 8 bits to identify 309 bins, band T 4  requires 10 bits to identify 689 bins and band T 5  also requires 10 bits to identify 959 bins. In order to obtain the 6 MHz needed for broadcasting a TV channel, and using the above exemplary granularity of Δf=200 kHz, the gateway must identify 30 bins, which results in an index of 2 bits (for identifying T 2 -T 5 )+30×10 bits (for identifying the bins in the largest of T 2 -T 5 )=302 bits for the worst case scenario. 
     The bin indexing method may be used for identifying pieces of white space in other transmission bands than VHF/UHF. As well, other ways of identifying the white space may also be envisaged. 
     Devices  9 , shown generically in  FIG. 2  as a computer with an antenna  4 ′, are equipped with a scanner that detects the shared signalling channel upon turn-on and look to it. As indicated above, use of the shared signalling channel enables the devices to receive control information from the control channel processor  3  and to transmit to the control channel processor  3  service requests and status information. Device  9  also receives the high rate data channel from the gateway, over the air interface established between the gateway antenna  13  and device&#39;s antenna  4 ′. No changes to the device tuner are needed when the data is transmitted in the TV spectrum and MPEG format. 
       FIG. 6A  is a flowchart of the method according to the invention illustrating the steps performed by a gateway  10 .  FIG. 6B  is a flowchart of the method according to the invention illustrating the steps performed by a device  9  for establishing communication with the gateway over white space identified by gateway  10 .  FIGS. 6A and 6B  are described with reference to block diagram of gateway  10  shown in  FIG. 2 . 
     As shown in  FIG. 6A , on turn-on, spectrum detector  2  of gateway  10  starts to scan the selected portion/s (band/s) of the wireless communication spectrum, which is, for example, the TV spectrum in bands T 2 -T 5  (see  FIG. 1B ). This operation is shown by steps  61 ,  62 , and  63 , and involves sensing the signals present in the VHF/UHF spectrum, analyzing the sensed signals and detecting the pieces of spectrum that are not currently occupied. The scanning, analyzing and detection operation is repeated at regular intervals, as the spectrum occupancy varies arbitrarily with wireless devices being connected/disconnected to/from the gateway, so that the gateway is aware at all times of the spectrum occupancy at the respective location. At step  61 , if a look-up table with the locally allocated channels is available at the gateway, as shown by branch “YES” of decision block  61 , the gateway excludes the spectrum occupied by these channels from the scan, as shown at step  62 . If not, as shown by branch “NO” of decision block  61 , gateway  10  scans the entire TV spectrum, step  63 . 
     Next, the gateway selects the shared signalling channel  5  needed to control the devices through signalling information between the gateway and the devices. This is shown in step  64 . As indicated before, the signalling channel enables devices  9  to look for the gateway, for connecting to it, requesting services and receiving instructions. 
     If gateway  10  receives a connection request from a device  9 , as shown in step  65 , it will identify and select a white space channel on which to broadcast to device  9 , step  66 . The white space channels are allocated based on the available spectrum sensed in steps  61 - 63 , and this operation takes into account the bandwidth necessary for the requested service (in the exemplary case of an ATSC channel, the bandwidth necessary is 6 MHz, and can be available as a single piece or made from a plurality of white space pieces). Once the white space channel is selected, the gateway informs device  9  on the parameters of this channel over the signalling channel, step  67 . In this way, the devices are informed of the frequency, start time, current bandwidth allocation, duration etc. This information is preferably in the form of bit maps, and enables the gateway and devices to transmit starting from the designated time and frequency grids, rather than arbitrarily. 
     Next, in step  68 , the gateway starts broadcasting data to the device/s, as shown, using the frequency-time grid received from the gateway. As indicated above, since the grids are synchronized with the ATSC broadcast the interference is reduced to conform to the FCC requirements. This is further possible since the power of the signals transmitted by the gateway is very small. 
     At the same time, gateway  10  reserves the respective white channel, step  66 , and provides the neighbouring gateways with the information about the white space it reserved, step  69 . As indicated above, this is accomplished using the common spectrum reservation OFDM symbol. The reservation stays active during the entire life of the connection. It is to be noted that step  69  is repeated at regular intervals of time, to update the neighbouring gateways with the current white space reservation information as devices at the location served by gateway  10  change their state between active, idle and inactive. 
       FIG. 6B  is a flowchart of the method according to an embodiment of the invention, showing the steps performed by a device  9 . In step  70  the device, which has been powered up, scans the spectrum for any gateway broadcast and detects the signalling channel  5 . Once channel  5  is detected, device  9  sends to the gateway a request for a connection and service over this channel, step  71 . Once the connection has been established, the device requests a certain service, step  72 . If, for example, a user wishes to connect to TV channel  7 , it sends to the gateway a join signal that specifies TV channel  7 . The gateway has the current spectrum occupancy information, as provided by the spectrum detector  2 . Using this data, the gateway allocates to device  9  a white-space channel available for broadcasting channel  7  to the device, and transmits instructions to the device as how and where to receive the requested multimedia media content. In step  73 , the device receives these instructions from the gateway  10  and in step  74  it receives the signal and processes it to extract the multimedia content.