Abstract:
Methods and apparatus for improved beacon signaling in a wireless communication system are described. Information is encoded in the tone position of the beacon tone. The information encoded may include sector type, sector index and slope index, as well as some time index. The information is coded in the tone position so that any few of several beacons can be decoded in order to decode the information. The methods and apparatus described in the invention improve the robustness against frequency selective fading and do not require wireless terminals to establish synchronization for reliable base station detection.

Description:
RELATED APPLICATIONS 
     This application relates to the following U.S. patents, which are hereby incorporated by reference herein: U.S. Pat. No. 6,985,498, filed Aug. 13, 2003, entitled “Beacon Signaling in a Wireless System”; U.S. Pat. No. 6,961,364, filed Aug. 18, 2000, entitled “Base Station Identification in Orthogonal Frequency Division Multiplexing Based Spread Spectrum Multiple Access Systems”; and U.S. Pat. No. 6,954,481, filed on Apr. 18, 2000, entitled “Pilot Use in Orthogonal Frequency Division Multiplexing Based Spread Spectrum Multiple Access System”. 
     FIELD 
     This invention relates to communications system and, more particularly, to methods and apparatus for communicating beacon signals. 
     BACKGROUND 
     Communications systems frequently include a plurality of network nodes which are coupled to access nodes, e.g., base stations, through which end nodes, e.g., wireless terminals, are coupled to the network. 
     In a wireless communication system, quality of service characteristics such as ability to make fast and seamless handoffs, typically require a base station to efficiently transmit the information that allows a wireless terminal to locate the presence of the base station(s) closest to it and obtain some basic information about the base station(s) detected so that it can use the received information to make fast and efficient subsequent access to the base station(s). This information can also be used by service providers for network management (network planning and network monitoring) purposes. In some wireless communication systems signals called beacons are used to communicate base station identification information to the wireless terminals. Usually these beacon signals are not required to carry many information bits. Instead what is typically required for beacons is to be robust to channel impairments and be readily detectable well before the wireless terminal may make a decision to attempt to access the corresponding base station. This feature is very important for the ability to make fast and seamless handoffs. For that purpose beacons are usually transmitted at the relatively high power levels. 
     Frequently, the problem in designing efficient beacon coding scheme arises from the tradeoffs between simplicity and reliability/detection delay. Frequency selective fading causes some of the beacons to go through very bad channels and as a result these received symbols are extremely noisy. In some systems that may cause the wireless terminals to wait longer until the channel conditions improve and the base station can be identified for access attempt. 
     Another problem arises from the fact that beacons are typically required to be detected when a wireless terminal is not yet synchronized to the base station which transmitted the beacon signal. In some systems this problem might cause false base station detection, the effect known as “ghosting”. 
     In view of the above discussion, it should be appreciated that there is a need for new and improved ways of communicating beacon signals that are more robust and/or readily decodable in the presence of different channel impairments, e.g., frequency selective fading. 
     SUMMARY OF THE INVENTION 
     Improved ways of communicating beacon signals that are more robust in the presence of noise, e.g., frequency selective fading, and when the wireless terminal is not in sync with the base station are described. Encoding and modulating methods and apparatus have improved frequency diversity capabilities. Beacons can be decoded more reliably even when the wireless terminals are not synchronized to the base station thus reducing the “ghosting” problem. 
     A beacon signal sequence is used to transport a set of information. A beacon signal sequence comprises a sequence of periodical beacon segments, each of which include a radio resource of a number of tones over a certain number of contiguous symbols, for example OFDM symbols. In a beacon segment, one of the tones, referred to as beacon tone, is transmitted at a much higher power than the average power. The frequency location of the beacon tone may vary from one beacon segment to another. In a given beacon segment, the number of tones available for the beacon signal may be smaller than the total number of tones available for the communications system. Moreover, from one beacon segment to another, the set of tones available for the beacon signal may vary. 
     Information is encoded in the tone position of the beacon tone. The information encoded may include sector type, sector index and slope index, as well as some time index. The information is coded in the tone position so that any few of several beacons can be decoded in order to decode the information. 
     Additional features and benefits of the present invention are discussed in the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a network diagram of an exemplary communications system. 
         FIG. 2  illustrates an exemplary end node. 
         FIG. 3  illustrates an exemplary access node. 
         FIG. 4  illustrates an exemplary OFDM spread spectrum air interface technology. 
         FIG. 5  illustrates the periodicity of beacon segments. 
         FIG. 6  illustrates double length beacon tone-symbol timing diagram. 
         FIG. 7  illustrates a partition of tone-symbols into 3 disjoint subsets of tones used in beacon encoding process. 
         FIG. 8  illustrates a module that determines the tone-symbols used for beacon transmission. 
         FIG. 9  illustrates a beacon tone-symbol index selection when base station sector ID is less than 3 and a beacon tone-symbol index selection when base station sector ID is greater than or equal to 3. 
         FIG. 10  illustrates a method for coding and transmitting communication system parameters on beacon tone. 
         FIG. 11  illustrates a wireless communication device. 
     
    
    
     DETAILED DESCRIPTION 
     The methods and apparatus of the present invention for establishing link and network connections to access nodes used to support communications sessions with one or more end nodes, e.g., wireless terminals, can be used with a wide range of communications systems. For example the invention can be used with systems which support mobile communications devices such as notebook computers equipped with modems, PDAs, and a wide variety of other devices which support wireless interfaces in the interests of device mobility. 
       FIG. 1  shows an exemplary communication system  100  implemented including multiple cells: cell  1   102 , cell M  104 . Note that neighboring cells  102 ,  104  overlap slightly, as indicated by cell boundary region  168 , thereby providing the potential for signal interference between signals being transmitted by base stations in neighboring cells. Each cell  102 ,  104  of exemplary system  100  includes three sectors. Cells which have not be subdivided into multiple sectors (N=1), cells with two sectors (N=2) and cells with more than  3  sectors (N&gt;3) are also possible. Cell  102  includes a first sector, sector  1   110 , a second sector, sector  2   112 , and a third sector, sector  3   114 . Each sector  110 ,  112 ,  114  has two sector boundary regions; each boundary region is shared between two adjacent sectors. Sector boundary regions provide the potential for signal interference between signals being transmitted by base stations in neighboring sectors. Line  116  represents a sector boundary region between sector  1   110  and sector  2   112 ; line  118  represents a sector boundary region between sector  2   112  and sector  3   114 ; line  120  represents a sector boundary region between sector  3   114  and sector  1   110 . Similarly, cell M  104  includes a first sector, sector  1   122 , a second sector, sector  2   124 , and a third sector, sector  3   126 . Line  128  represents a sector boundary region between sector  1   122  and sector  2   124 ; line  130  represents a sector boundary region between sector  2   124  and sector  3   126 ; line  132  represents a boundary region between sector  3   126  and sector  1   122 . Cell  1   102  includes a base station (BS), base station  1   106 , and a plurality of end nodes (ENs) in each sector  110 ,  112 ,  114 . Sector  1   110  includes EN( 1 )  136  and EN(X)  138  coupled to BS  106  via wireless links  140 ,  142 , respectively; sector  2   112  includes EN( 1 ′)  144  and EN(X′)  146  coupled to BS  106  via wireless links  148 ,  150 , respectively; sector  3   126  includes EN( 1 ″)  152  and EN(X″)  154  coupled to BS  106  via wireless links  156 ,  158 , respectively. Similarly, cell M  104  includes base station M  108 , and a plurality of end nodes (ENs) in each sector  122 ,  124 ,  126 . Sector  1   122  includes EN( 1 )  136 ′ and EN(X)  138 ′ coupled to BS M  108  via wireless links  140 ′,  142 ′, respectively; sector  2   124  includes EN( 1 ′)  144 ′ and EN(X′)  146 ′ coupled to BS M  108  via wireless links  148 ′,  150 ′, respectively; sector  3   126  includes EN( 1 ″)  152 ′ and EN(X″)  154 ′ coupled to BS  108  via wireless links  156 ′,  158 ′, respectively. System  100  also includes a network node  160  which is coupled to BS  1   106  and BS M  108  via network links  162 ,  164 , respectively. 
     Network node  160  is also coupled to other network nodes, e.g., other base stations, AAA server nodes, intermediate nodes, routers, etc. and the Internet via network link  166 . Network links  162 ,  164 ,  166  may be, e.g., fiber optic cables. Each end node, e.g. EN  1   136  may be a wireless terminal including a transmitter as well as a receiver. The wireless terminals, e.g., EN( 1 )  136  may move through system  100  and may communicate via wireless links with the base station in the cell in which the EN is currently located. The wireless terminals, (WTs), e.g. EN( 1 )  136 , may communicate with peer nodes, e.g., other WTs in system  100  or outside system  100  via a base station, e.g. BS  106 , and/or network node  160 . WTs, e.g., EN( 1 )  136  may be mobile communications devices such as cell phones, personal data assistants with wireless modems, etc. Each base station performs tone subset allocation using a different method for the strip-symbol periods, from the method employed for allocating tones and determining tone hopping in the rest symbol periods, e.g., non strip-symbol periods. The wireless terminals use the tone subset allocation method along with information received from the base station, e.g., base station slope ID, sector ID information, to determine the tones that they can use to receive data and information at specific strip-symbol periods. The tone subset allocation sequence is constructed to spread the inter-sector and inter-cell interference across each of the tones. 
       FIG. 2  illustrates an exemplary base station  200 . Exemplary base station  200  implements the tone subset allocation sequences, with different tone subset allocation sequences generated for each different sector type of the cell. The base station  200  may be used as any one of the base stations  106 ,  108  of the system  100  of  FIG. 1 . The base station  200  includes a receiver  202 , a transmitter  204 , a processor  206 , e.g., CPU, an input/output interface  208  and memory  210  which are coupled together by a bus  209  over which the various elements  202 ,  204 ,  206 ,  208 , and  210  may interchange data and information. 
     Sectorized antenna  203  coupled to receiver  202  is used for receiving data and other signals, e.g., channel reports, from wireless terminals transmissions from each sector within the base station&#39;s cell. Sectorized antenna  205  coupled to transmitter  204  is used for transmitting data and other signals, e.g., control signals, pilot signal, beacon signals, etc. to wireless terminals  300  (see  FIG. 3 ) within each sector of the base station&#39;s cell. In various embodiments of the invention, base station  200  may employ multiple receivers  202  and multiple transmitters  204 , e.g., an individual receivers  202  for each sector and an individual transmitter  204  for each sector. The processor  206 , may be, e.g., a general purpose central processing unit (CPU). Processor  206  controls operation of the base station  200  under direction of one or more routines  218  stored in memory  210 . I/O interface  208  provides a connection to other network nodes, coupling the BS  200  to other base stations, access routers, AAA server nodes, etc., other networks, and the Internet. Memory  210  includes routines  218  and data/information  220 . 
     Data/information  220  includes data  236 , tone subset allocation sequence information  238  including downlink strip-symbol time information  240  and downlink tone information  242 , and wireless terminal (WT) data/info  244  including a plurality of sets of WT information: WT  1  info  246  and WT N info  260 . Each set of WT info, e.g., WT  1  info  246  includes data  248 , terminal ID  250 , sector ID  252 , uplink channel information  254 , downlink channel information  256 , and mode information  258 . 
     Routines  218  include communications routines  222  and base station control routines  224 . Base station control routines  224  includes a scheduler module  226  and signaling routines  228  including a tone subset allocation routine  230  for the strip-symbol periods, other downlink tone allocation hopping routine  232  for the rest of symbol periods, e.g., non strip-symbol periods, and a beacon routine  234 . 
     Data  236  includes data to be transmitted that will be sent to encoder  214  of transmitter  204  for encoding prior to transmission to WTs, and received data from WTs that has been processed through decoder  212  of receiver  202  following reception. Downlink strip-symbol time information  240  includes the frame synchronization structure information, such as the superslot, beaconslot, and ultraslot structure information and information specifying whether a given symbol period is a strip-symbol period, and if so, the index of the strip-symbol period and whether the strip-symbol is a resetting point to truncate the tone subset allocation sequence used by the base station. Downlink tone information  242  includes information including a carrier frequency assigned to the base station  200 , the number and frequency of tones, and the set of tone subsets to be allocated to the strip-symbol periods, and other cell and sector specific values such as slope, slope index and sector type. 
     Data  248  may include data that WT 1   300  has received from a peer node, data that WT  1   300  desires to be transmitted to a peer node, and downlink channel quality report feedback information. Terminal ID  250  is a base station  200  assigned ID that identifies WT  1   300 . Sector ID  252  includes information identifying the sector in which WT 1   300  is operating. Sector ID  252  can be used, for example, to determine the sector type. Uplink channel information  254  includes information identifying channel segments that have been allocated by scheduler  226  for WT 1   300  to use, e.g., uplink traffic channel segments for data, dedicated uplink control channels for requests, power control, timing control, etc. Each uplink channel assigned to WT 1   300  includes one or more logical tones, each logical tone following an uplink hopping sequence. Downlink channel information  256  includes information identifying channel segments that have been allocated by scheduler  226  to carry data and/or information to WT 1   300 , e.g., downlink traffic channel segments for user data. Each downlink channel assigned to WT 1   300  includes one or more logical tones, each following a downlink hopping sequence. Mode information  258  includes information identifying the state of operation of WT 1   300 , e.g. sleep, hold, on. 
     Communications routines  222  control the base station  200  to perform various communications operations and implement various communications protocols. Base station control routines  224  are used to control the base station  200  to perform basic base station functional tasks, e.g., signal generation, reception, and scheduling, including transmitting signals to wireless terminals using the tone subset allocation sequences of the present invention during the strip-symbol periods. 
     Signaling routine  228  controls the operation of receiver  202  with its decoder  212  and transmitter  204  with its encoder  214 . The signaling routine  228  is responsible controlling the generation of transmitted data  236  and control information. Tone subset allocation routine  230  constructs the tone subset to be used in a strip-symbol period and data/info  220  including downlink strip-symbol time info  240  and sector ID  252 . The downlink tone subset allocation sequences will be different for each sector type in a cell and different for adjacent cells. The WTs  300  receive the signals in the strip-symbol periods in accordance with the downlink tone subset allocation sequences; the base station  200  uses the same downlink tone subset allocation sequences in order to generate the transmitted signals. Other downlink tone allocation hopping routine  232  constructs downlink tone hopping sequences, using information including downlink tone information  242 , and downlink channel information  256 , for the symbol periods other than the strip-symbol periods. The downlink data tone hopping sequences are synchronized across the sectors of a cell. Beacon routine  234  controls the transmission of a beacon signal, e.g., a signal of relatively high power signal concentrated on one or a few tones, which may be used for synchronization purposes, e.g., to synchronize the frame timing structure of the downlink signal and therefore the tone subset allocation sequence with respect to an ultra-slot boundary. 
       FIG. 3  illustrates an exemplary wireless terminal (end node)  300  which can be used as any one of the wireless terminals (end nodes), e.g., EN( 1 )  136 , of the system  100  shown in  FIG. 1 . Wireless terminal  300  implements the tone subset allocation sequences. The wireless terminal  300  includes a receiver  302  including a decoder  312 , a transmitter  304  including an encoder  314 , a processor  306 , and memory  308  which are coupled together by a bus  310  over which the various elements  302 ,  304 ,  306 ,  308  can interchange data and information. An antenna  303  used for receiving signals from a base station  200  is coupled to receiver  302 . An antenna  305  used for transmitting signals, e.g., to base station  200  is coupled to transmitter  304 . 
     The processor  306 , e.g., a CPU controls the operation of the wireless terminal  300  by executing routines  320  and using data/information  322  in memory  308 . 
     Data/information  322  includes user data  334 , user information  336 , and tone subset allocation sequence information  350 . User data  334  may include data, intended for a peer node, which will be routed to encoder  314  for encoding prior to transmission by transmitter  304  to base station  200 , and data received from the base station  200  which has been processed by the decoder  312  in receiver  302 . User information  336  includes uplink channel information  338 , downlink channel information  340 , terminal ID information  342 , base station ID information  344 , sector ID information  346 , and mode information  348 . Uplink channel information  338  includes information identifying uplink channels segments that have been assigned by base station  200  for wireless terminal  300  to use when transmitting to the base station  200 . Uplink channels may include uplink traffic channels, dedicated uplink control channels, e.g., request channels, power control channels and timing control channels. Each uplink channel include one or more logic tones, each logical tone following an uplink tone hopping sequence in accordance with the present invention. The uplink hopping sequences are different between each sector type of a cell and between adjacent cells. Downlink channel information  340  includes information identifying downlink channel segments that have been assigned by base station  200  to WT  300  for use when BS  200  is transmitting data/information to WT  300 . Downlink channels may include downlink traffic channels and assignment channels, each downlink channel including one or more logical tone, each logical tone following a downlink hopping sequence, which is synchronized between each sector of the cell. 
     User info  336  also includes terminal ID information  342 , which is a base station  200  assigned identification, base station ID information  344  which identifies the specific base station  200  that WT has established communications with, and sector ID info  346  which identifies the specific sector of the cell where WT  300  is presently located. Base station ID  344  provides a cell slope value and sector ID info  346  provides a sector index type; the cell slope value and sector index type may be used to derive the uplink tone hopping sequences in accordance with the invention. Mode information  348  also included in user info  336  identifies whether the WT  300  is in sleep mode, hold mode, or on mode. 
     Tone subset allocation sequence information  350  includes downlink strip-symbol time information  352  and downlink tone information  354 . Downlink strip-symbol time information  352  include the frame synchronization structure information, such as the superslot, beaconslot, and ultraslot structure information and information specifying whether a given symbol period is a strip-symbol period, and if so, the index of the strip-symbol period and whether the strip-symbol is a resetting point to truncate the tone subset allocation sequence used by the base station. Downlink tone info  354  includes information including a carrier frequency assigned to the base station  200 , the number and frequency of tones, and the set of tone subsets to be allocated to the strip-symbol periods, and other cell and sector specific values such as slope, slope index and sector type. 
     Routines  320  include communications routines  324  and wireless terminal control routines  326 . Communications routines  324  control the various communications protocols used by WT  300 . Wireless terminal control routines  326  controls basic wireless terminal  300  functionality including the control of the receiver  302  and transmitter  304 . Wireless terminal control routines  326  include the signaling routine  328 . The signaling routine  328  includes a tone subset allocation routine  330  for the strip-symbol periods and an other downlink tone allocation hopping routine  332  for the rest of symbol periods, e.g., non strip-symbol periods. Tone subset allocation routine  330  uses user data/info  322  including downlink channel information  340 , base station ID info  344 , e.g., slope index and sector type, and downlink tone information  354  in order to generate the downlink tone subset allocation sequences in accordance with the present invention and process received data transmitted from base station  200 . Other downlink tone allocation hopping routine  330  constructs downlink tone hopping sequences, using information including downlink tone information  354 , and downlink channel information  340 , for the symbol periods other than the strip-symbol periods. Tone subset allocation routine  330 , when executed by processor  306 , is used to determine when and on which tones the wireless terminal  300  is to receive one or more strip-symbol signals from the base station  200 . The uplink tone allocation hopping routine  330  uses a tone subset allocation function, implemented in accordance with the present invention, along with information received from the base station  200 , to determine the tones in which it should transmit on. 
       FIG. 4  illustrates an exemplary OFDM spread spectrum air interface technology, implemented for each sector of each of the cells ( 102 , 104 ) of  FIG. 1 . In  FIG. 4 , horizontal axis  451  represents frequency. The total amount of available bandwidth for a particular carrier frequency  453 , e.g., for uplink signaling for downlink signaling, is divided into a number, K, of equally spaced tones. In some embodiments, there are 113 equally spaced tones. These tones are indexed from  0  to K- 1 . Exemplary tones: tone  0   455 , tone  1   457 , tone  2   459  and tone K- 1   461  are illustrated in  FIG. 4 . The bandwidth is used simultaneously each of the sectors  110 ,  112 ,  114 ,  122 ,  124 ,  126  comprising the two cells  102 ,  104 . In each sector of each cell, the tones,  0  through K- 1 , are used in each sector of each cell respectively to transmit downlink signals. Since the same bandwidth is used in each sector of both the cells  102 ,  104 , the signals transmitted by different cells and sectors on the frequency tones at the same time may interfere with each other, e.g., in the overlapping coverage areas, e.g. sector boundary areas  116 ,  118 ,  120 ,  128 ,  130 ,  132 , and cell boundary areas  168 . 
     Beacons are transmitted, in an exemplary embodiment, via a dedicated channel known as a broadcast beacon channel.  FIG. 5  illustrates the periodicity of beacon symbols for an exemplary embodiment. In  FIG. 5  exemplary beacon signals  506  are plotted on drawing  501 . Drawing  501  includes a vertical axis  500  indicating post hopping tone index vs. a horizontal axis  508  indicating time. The post hopping tone index shows the physical frequency location of a particular tone. In an exemplary embodiment, the post-hopping tone index covers a range of 0 to 112 corresponding to 113 contiguous tones of an exemplary downlink tone block used by a base station sector transmitter. Those 113 tones are available for the base station transmitter to transport information to the wireless terminals. In some embodiments of the invention the available resource for the broadcast beacon channel are beacon segments, each comprising 113 tones over 2 successive OFDM symbols. Define a tone-symbol to be the air link resource of a single tone for the duration of one OFDM symbol transmission time period and a double tone-symbol to be the air link resource of a single tone for the duration of two successive OFDM symbol transmission time periods. In each such exemplary beacon segment, there are totally 113 double tone-symbols, which are numbered according to the post hopping tone index of axis  500 . The beacon segments are transmitted during the downlink strip channel periodically once every 8 super slots. Each such exemplary super slot comprises 114 consecutive OFDM symbol transmission time periods including two downlink strip channel OFDM symbol transmission time periods. A beacon segment corresponds to the strip channel OFDM symbol time periods of one of the super slots of the beacon slot, e.g., one of the first, second or third super slots of the beacon slot. Drawing  501  includes exemplary beacon segments (beacon segment  0   510 , beacon segment  1   512 , beacon segment  2   514 , . . . , beacon segment  17   516 ) corresponding to exemplary beacon slots (beacon slot  0   518 , beacon slot  1   520 , beacon slot  2   522 , . . . , beacon slot  17   524 ), respectively. The duration of 8 super slots comprises one beacon slot period  502 , e.g.,  912  OFDM symbol transmission time periods. In this exemplary embodiment, each beacon slot includes one beacon segment corresponding to the air link resources used to transmit a beacon signal. In a beacon segment, a base station sector transmitter transmits one double tone-symbol  506 , called double beacon tone-symbol, at a much higher power than the average per tone-symbol power. The tone corresponding to the double beacon tone-symbol is called beacon tone in the beacon segment. In one embodiment, all the other double tone-symbols in the beacon segment are transmitted at much (e.g., at least 10 dB) lower average power than the beacon double tone-symbol. 
     In some embodiments, at least some of the base station sector transmitters use multiple, e.g. 3, tone blocks, and some such base station sector transmitter transmits a single double beacon tone-symbol signal  506  in each of the tone blocks once per beacon slot  502 . For example, consider a base station sector transmitter using three downlink tone blocks with each tone block comprising 113 contiguous tones. During each beacon slot, e.g., exemplary beacon slot  0   518 , the base station sector transmitter transmits: (i) a first double beacon tone-symbol signal during the two OFDM strip symbols of the first super slot of the beacon slot for a first beacon segment, (ii) a second double beacon tone-symbol signal during the two OFDM strip symbols of the second super slot of the beacon slot for a second beacon segment, and (iii) a third double beacon tone-symbol signal during the two OFDM strip symbols of the third super slot of the beacon slot for a third beacon segment. 
     In this exemplary embodiment of  FIG. 5 , one ultra slot  504  comprises eighteen beacon slots (beacon slot  0   518 , beacon slot  1   520 , beacon slot  2   522 , . . . , beacon slot  17   524 ). The beacon tone-symbols signals follow a pattern that corresponds to the beacon segments numbered from  0  to  17  which repeats every ultra slot  504 . Within an ultra slot  504 , the 18 beacon slots are indexed by a time index  0 ,  1 , . . . ,  17 . It should be noted that  FIG. 5  is not drawn to scale; each exemplary beacon signal  506  is one tone wide, each exemplary beacon segment is two OFDM symbol transmission time periods in duration, and each exemplary beacon slot is 912 OFDM symbol transmission time periods in duration. 
     A double length beacon tone-symbol signal is generated differently from regular OFDM tone-symbols. Unlike regular OFDM tone-symbols, a double length beacon tone-symbol signal does not have phase discontinuity at the OFDM symbol boundary. It can be viewed as a regular length OFDM symbol with cyclic extension that spans the subsequent OFDM symbol. This is illustrated in  FIG. 6 . 
     The first part  601  of  FIG. 6  shows two regular OFDM symbols ( 603 ,  605 ). The horizontal axis  603  of  FIG. 6  represents the time. Each of the exemplary regular OFDM symbols ( 605 ,  607 ) has an OFDM symbol transmission time duration  609 . Each regular OFDM symbol ( 605 ,  607 ) comprises two parts, a cyclic prefix (CP) portion and a symbol body portion. For example, exemplary first regular OFDM symbol  605  comprises CP  602  and symbol body  604 ; exemplary second regular OFDM symbol  607  comprises CP  608  and symbol body  610 . Note that typically the symbol bodies ( 604 ,  610 ) are constructed using certain constellation, such as QPSK and as a result, there is a phase discontinuity at the time instance  611  when the first OFDM symbol  605  ends and when the second OFDM symbol  607  starts. 
     The second part  650  of  FIG. 6  shows the construction of a special double OFDM symbol  651  including a first OFDM symbol  655  and a second OFDM symbol  657 . The horizontal axis  653  represents time. Each of the exemplary regular OFDM symbols ( 653 ,  657 ) has an OFDM symbol transmission time duration  607 . The construction method used to generate the double beacon signal, which is of 2 OFDM symbol periods, will now be described. The first part  653  of the special double OFDM symbol  651  is constructed in the same way as a regular OFDM symbol  607 . First part  653  includes a cyclic prefix portion  652  followed by a symbol body portion  654 . The second part  655  of the special double OFDM symbol  651 , which follows immediately the first part  653 , is a cyclic extension (CE) of the first part  653  of the special double OFDM symbol  651 , so that there is no phase discontinuity at the time instance  672  when the first OFDM symbol  655  ends and when the second OFDM symbol  657  starts. 
     The broadcast beacon channel is used to broadcast information that helps a wireless terminal to determine the presence of the base station and to acquire system parameters such as base station slope index, base station sector index, base station sector ID and beacon segment index. 
     Information bits are position encoded with the beacon tone-symbols. In this exemplary embodiment for a given tone block 36 tone-symbols are available to be used for beacon signal transmission out of the 113 tone-symbols of an OFDM symbol of a beacon segment, and the remaining 77 tone-symbols are precluded from being used to carry a beacon signal. In some embodiments, the tones corresponding to those 36 tone-symbols are non-contiguous. Furthermore, the tone-symbols can be equally spaced with tone spacing greater than 1. 
     These 36 tone-symbols, which can be used to carry a beacon signal, are divided into 3 disjoint sets of 12 tone-symbols each (first set S 0 , second set S 1  and third set S 2 ). Drawing  700  of  FIG. 7  shows one exemplary mapping. The 36 tone-symbols  702  are indexed using post-hopping tone indexing as indicated in block  700 . Legend  706  indicates that: (i) tone-symbols  708  corresponding to set S 0  are designated by diagonal line shading; (ii) tone-symbols  710  corresponding to set S 1  are designated by vertical and horizontal line shading; and (iii) tone-symbols  712  corresponding to set S 2  are designated by crosshatch shading. Tone-symbols with post hopping tone index values  4 ,  13 ,  22 ,  31 ,  40 ,  49 ,  58 ,  67 ,  76 ,  85 ,  94 ,  103  are used for S 0 . The members of set S 0  are indexed such that: S 0 [ 0 ] corresponds to tone  4 , S 0 [ 1 ] corresponds to tone  13 , . . . , S 0 [ 12 ] corresponds to tone  103 . Tone-symbols with post hopping tone index values  7 ,  16 ,  25 ,  34 ,  43 ,  52 ,  61 ,  70 ,  79 ,  88 ,  97 ,  106  are used for S 1 . The members of set S 1  are indexed such that: S 1 [ 0 ] corresponds to tone  7 , S 1 [ 1 ] corresponds to tone  16 , . . . , S 1 [ 12 ] corresponds to tone  106 . Tone-symbols with post hopping tone index values  10 ,  19 ,  28 ,  37 ,  46 ,  55 ,  64 ,  73 ,  82 ,  91 ,  100 ,  109  are used for S 2 . The members of set S 2  are indexed such that: S 2 [ 0 ] corresponds to tone  10 , S 2 [ 1 ] corresponds to tone  19 , . . . , S 2 [ 19 ] corresponds to tone  109 . This mapping structures the beacon tone spacing such that tone spacing between any two candidate beacon tone-symbols in a given set of 36 tone-symbols corresponding to a tone block is at least 3 tones. This mapping further structures the beacon tone spacing such that tone spacing between any two candidate beacon tone-symbols in a given set of 12 tone-symbols, e.g., set S 0 , set S 1  or set S 2 , is at least 9 tones. Significant spacing (in tones) between tones in a set, e.g., at least 3 tones, helps reduce or minimize leakage from one beacon tone-symbol signal to another when the wireless terminal is not synchronized to the base station. In other words, significant spacing tends to reduce or eliminate the “ghosting” problem. 
     The exemplary beacon tone position coding method presented below encodes base station identification information using some hopping (rather than static); this technique provides frequency diversity and allows the wireless terminal to determine beacon slot index from the hopping pattern. This can be seen from  FIG. 5  where beacon tone-symbols  506  in the ultra slot period  504  hop across post hopping tones between index  4  and index  109 . 
     Beacon tone position coding will now be described. First we calculate the integer number N=3*slope_index+sector_type_index, where slope_index is an integer number in the range [0;95] and sector_type_index is an integer number in the range [0;2]. The range 0 to 95 can be used, for example, in a communication system having 96 beacon slope indexes (slope_index). The range 0 to 2 can be used, for example, in a communication system having 3 sector types. For example, sector_type_index=sector ID mod  3 , where sector ID=0, 1, . . . , 5. The choice of these parameters is determined by exemplary communication system basic parameters and provides the encoding of 96 distinct slopes each deployed in 3 distinct sector types. Second, number N is used to calculate integer numbers R 1 , R 0  and b 0  as follows: b 0 =N mod  2 ; R 0 =(N−b 0 )/2 mod  12 ; and R 1 =floor(N/24). 
     The post hopping tone index for each beacon segment is determined as follows. First consider the case when exemplary base station sector ID is less than 3. Denote “s” to be the time index variable indexing a beacon slot within an ultra slot. So s=0, 1, . . . 17. In a beacon segment s where s mod  3 =0, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 0 : S 0 [2*floor(s/3)], if b 0 =0, or S 0 [11−2*floor(s/3)], if b 0 =1. In a segment s where s mod  3 =1, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 1 : S 1 [mod(R 0 +2*floor(s/3),12)]. In a segment s where s mod  3 =2, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 2 : S 2 [mod(R 1 +2*floor(s/3),12)]. 
       FIG. 8  illustrates the process of selecting the tone-symbol index for the broadcast beacon channel. Beacon tone index selection module  800  selects the final tone-symbol  816  based on slope index information  812  (that can, for example, take 96 distinct values [0;95]) and sector type index information  808  (that can, for example, take 3 distinct values 0,1,2). Beacon tone index selection module may be or may be a part of beacon routine  234 . The selection procedure includes two steps. In a first step, computation module  802  produces information set  803 , the parameters b 0 , R 0 , R 1  according to the equations above. 
     In a second step given the sector ID information  810  (that can, for example, take 6 distinct values [0;5]) and beacon slot index information  814  (that can, for example, take 18 distinct values [0,17]) as well as computed values b 0 , R 0 , R 1  the Tone Mapping Module  804  chooses the output tone-symbol index  816 . The tone-symbol indexes corresponding to the sets S 0 , S 1  and S 2  are stored, e.g. in a memory  806 . 
       FIG. 9  includes a table  900  illustrating an exemplary double length beacon tone-symbol index selection process for every beacon segment in an ultra slot corresponding to a downlink tone block being used by an exemplary base station sector transmitter. Table  900  includes: a first column  902  identifying beacon segment index within the ultraslot, a second column  904  including information used to identify the double length beacon tone-symbol index when the base station sector ID is less than 3, and a third column  906  including information used to identify the double length beacon tone-symbol index when the base station sector ID is greater than or equal to 3. 
     Column  904  is used if base station sector ID values equal to 0, 1, or 2. Column  904  shows which beacon symbol tone index is chosen for transmission in a given beacon segment identified in column  902  given parameters b 0 , R 0  and R 1 . The presented pattern represented by column  904  repeats for every subsequent ultra slot. 
     Now consider the case where base station sector ID is greater than or equal to 3. In a segment s where s mod  3 =0, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 0 : S 0 [2*floor(s/3)], if b 0 =0, or S-[11−2*floor(s/3)], if b 0 =1. In a segment s where s mod  3 =1, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 2 : S 2 [mod(R 1 +2*floor(s/3),12)]. In a segment s where s mod  3 =2, the post hopping tone index of the double length beacon tone-symbol is set to one of the tones in set S 1 : S 1 [mod(R 0 +2*floor(s/3),12)]. 
       FIG. 9  illustrates the double length beacon tone-symbol index selection process for every beacon segment  902  in an ultra slot when base station sector ID is greater than or equals 3 in column  906 . Column  906  is used for base station sector ID values greater than or equal to 3, e.g., 3, 4, 5. Column  906  shows which beacon symbol tone index is chosen for transmission in a given beacon segment identified in column  902  given parameters b 0 , R 0  and R 1 . The presented pattern represented by column  906  repeats for every subsequent ultra slot. 
       FIG. 9  demonstrates that beacon tones transmitted across the ultra slot hop over the frequency band thus providing frequency diversity and therefore increased robustness to the frequency selective fading. For example, assume the following inputs: slope_index=2; sector_type_index=0; and sector ID=3. Then the following beacon tone-symbols are chosen for transmission in beacon segments  0  through  17 :  4  (beacon segment  0 ), 10 , 34 ,  22 , 28 , 52 ,  40 , 46 , 70 ,  58 , 64 , 88 ,  76 , 82 , 106 ,  94 , 100 , 16  (beacon segment  17 ). 
     Decoding the slope index and sector type from the received beacon signals can be performed by any convenient method. For example, wireless terminal  300  includes downlink tone info  354  in memory  308 . As stated above with respect to  FIG. 3 , downlink tone info  354  includes information including a carrier frequency assigned to the base station  200 , the number and frequency of tones, and the set of tone subsets to be allocated to the strip-symbol periods, and other cell and sector specific values such as slope, slope index and sector type. Since wireless terminal  300  has the slope index and sector type corresponding to various beacon tones wireless terminal  300  can look up the base station identification information such as, for example, slope index and sector type, corresponding to the beacon tones of the received beacon signals. This enables the wireless terminal to at least, for example, begin communications with the acquired base station based on as few as two received beacon tone symbols. 
       FIG. 10  is a flow chart illustrating a method for coding and transmitting communication system parameters on beacon tones. In step  1002 , available tones for communicating in a communication system are identified. In step  1004 , a first beacon subset of the available tones is identified, the first beacon subset consisting of less than all of the available tones. For example, the first beacon subset may be set S 0  described above with respect to  FIGS. 7 ,  8  and  9 . In step  1006 , transmission is performed, on a first beacon tone of the first beacon subset of the available tones, with at least 10 dB greater power than an average per tone transmission power of the available tones during a first time period. 
     In step  1008 , at least a first portion of a plurality of communication system parameters is coded into a first beacon tone index, the first beacon tone index corresponding to the first beacon tone. For example, the first portion of the plurality of communication system parameters may be a slope index. Or, for example, the first portion may be a portion of the slope index and a sector type index. For example, first portion may be coded as described above with respect to  FIGS. 7 ,  8  and  9 . More specifically, the first portion may be b 0 , described above with respect to  FIGS. 7 ,  8  and  9 . The first portion b 0  is derived in part from the slope index and in part from the sector type. The first portion b 0  does not completely determine the beacon tone (or beacon segment index), as can be seen in  FIG. 9 . The beacon segment index  902  chosen depends on the second portion R 0  and a third portion R 1  as well, in the example shown with respect to  FIGS. 7 ,  8  and  9 . 
     In step  1010 , a second beacon subset of the available tones is identified, the second beacon subset consisting of less than all of the available tones. For example, the second beacon subset may be subset S I described above with respect to  FIGS. 7 ,  8  and  9 . In step  1012 , transmission is performed, on a second beacon tone of the second beacon subset of the available tones, with at least 10 dB greater power than an average per tone transmission power of the available tones during a first time period. 
       FIG. 11  is a block diagram illustrating a wireless communication device, such as, for example, base station  200  for coding and transmitting communication system parameters on beacon tones. In module  1102 , available tones for communicating in a communication system are identified. Module  1102  may be, for example, beacon tone index selection module  800 . In module  1104 , a first beacon subset of the available tones is identified, the first beacon subset consisting of less than all of the available tones. Module  1104  may be, for example, beacon tone index selection module  800 . For example, the first beacon subset may be set SO described above with respect to  FIGS. 7 ,  8  and  9 . In module  1106 , transmission is performed, on a first beacon tone of the first beacon subset of the available tones, with at least 10 dB greater power than an average per tone transmission power of the available tones during a first time period. Module  1106  may be communication routines  222  which may use transmitter  204  to transmit. 
     In module  1108 , at least a first portion of a plurality of communication system parameters is coded into a first beacon tone index, the first beacon tone index corresponding to the first beacon tone. For example, the first portion of the plurality of communication system parameters may be a slope index. Or, for example, the first portion may be a portion of the slope index and a sector type index. For example, first portion may be coded as described above with respect to  FIGS. 7 ,  8  and  9 . More specifically, the first portion may be b 0 , described above with respect to  FIGS. 7 ,  8  and  9 . The first portion b 0  is derived in part from the slope index and in part from the sector type. The first portion b 0  does not completely determine the beacon tone (or beacon segment index), as can be seen in  FIG. 9 . The beacon segment index  902  chosen depends on the second portion R 0  and a third portion R 1  as well, in the example shown with respect to  FIGS. 7 ,  8  and  9 . Module  1108  may be, for example, beacon tone index selection module  800 . 
     In module  1110 , a second beacon subset of the available tones is identified, the second beacon subset consisting of less than all of the available tones. For example, the second beacon subset may be subset S 1  described above with respect to  FIGS. 7 ,  8  and  9 . Module  1110  may be, for example, beacon tone index selection module  800 . In module  1112 , transmission is performed, on a second beacon tone of the second beacon subset of the available tones, with at least 10 dB greater power than an average per tone transmission power of the available tones during a first time period. Module  11   12  may be communication routines  222  which may use transmitter  204  to transmit. 
     Messages described in the present patent application are stored in the memory of the nodes which generate and/or receive said messages in addition to the nodes through which said messages are communicated. Accordingly, in addition to being directed to methods and apparatus for generating, transmitting and using novel messages of the present invention, the present invention is also directed to machine readable media, e.g., memory, which stores one or more of the novel messages of the type described and shown in the text and figures of the present application. 
     In various embodiments nodes described herein are implemented using one or more modules to perform the steps corresponding to one or more methods described herein, for example, signal processing, message generation and/or transmission steps. Thus, in some embodiments various features are implemented using modules. Such modules may be implemented using software, hardware or a combination of software and hardware. Many of the above described methods or method steps can be implemented using machine executable instructions, such as software, included in a machine readable medium such as a memory device, e.g., RAM, floppy disk, etc. to control a machine, e.g., general purpose computer with or without additional hardware, to implement all or portions of the above described methods, e.g., in one or more nodes. Accordingly, a machine-readable medium can include machine executable instructions for causing a machine, e.g., processor and associated hardware, to perform one or more of the steps of the above-described method(s). 
     Numerous additional variations on the methods and apparatus of the present invention described above will be apparent to those skilled in the art in view of the above description of the invention. Such variations are to be considered within the scope of the invention. The methods and apparatus of the present invention may be, and in various embodiments are, used with CDMA, orthogonal frequency division multiplexing (OFDM), or various other types of communications techniques which may be used to provide wireless communications links between access nodes and mobile nodes. In some embodiments the access nodes are implemented as base stations which establish communications links with mobile nodes using OFDM and/or CDMA. In various embodiments the mobile nodes are implemented as notebook computers, personal data assistants (PDAs), or other portable devices including receiver/transmitter circuits and logic and/or routines, for implementing the methods of the present invention.