Abstract:
A multi-frequency pilot beacon adapted to a CDMA system using at least two different carrier frequencies, F 1  and F 2 , for supplying PN sequences at these frequencies. The pilot beacon has a pseudorandom noise generator for supplying a pseudorandom noise sequence PN and a frequency conversion mechanism for converting the PN sequence to a first pilot beacon centered at the first CDMA carrier F 1  and a second pilot beacon centered at the second CDMA carrier F 2 . The multi-frequency pilot beacon can be provided at a base station of a given cell to transmit the pilot beacons within that cell and the base station antenna can be used for transmitting the pilot beacons in this embodiment. Alternatively, multi-frequency pilot beacons can be provided wherever necessary within the CDMA system. The multi-frequency pilot beacon is useful in frequency hand-off operations and gathering information about cellular users in the CDMA system

Description:
FIELD OF THE INVENTION 
     This invention relates generally to cellular wireless telephone systems and, in particular, to multi-frequency pilot beacons for hand-off of transceivers operating in Code Division Multiple Access (CDMA) systems. 
     BACKGROUND OF THE INVENTION 
     A cellular communication system is one in which coverage is provided in relatively small areas, commonly referred to as cells, that overlap. These overlapping cells form a grid of radio coverage that extends over a region of interest, e.g., an urban area. 
     In traditional cellular systems each call or radio connection between a mobile transceiver (telephone) and a cellular base station occupies a narrow segment of the frequency spectrum allocated to the provider of the cellular service. Since each call must have its own frequency segment the total number of simultaneous calls which can be handled is limited by the number of segments in the frequency spectrum. 
     When the coverage area is broken up into cells, frequency segments or frequencies can be reused in cells that are far enough apart so that the signals at the same frequency do not interfere with one another. In a typical cellular system the frequency reuse factor (how many cells have to be operating on different frequencies before frequency reuse can occur) is 7. At this reuse factor cells reusing the same frequency are two cells away from each other. This also means, that only a seventh of the allocated frequency spectrum can be used within any given cell. 
     While moving within the cellular grid a mobile transceiver is forced to switch its operating frequency between the channels allocated to the different cells. This process is called “hand-off”. In practice, the base station in one cell hands-off the transceiver call to a base station in another cell by forcing the transceiver to switch frequencies. 
     There are numerous problems with this traditional approach, often resulting in dropped calls and inefficient use of the frequency spectrum. Code Division Multiple Access (CDMA) technology is one of several alternative techniques for supporting cellular wireless communications in such a cellular system. CDMA systems have significant advantages over competing systems for multiple access communications such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and AM modulated systems such as Amplitude Companded Single Sideband (ACSSB) systems. Specifically, CDMA techniques result in a higher spectral efficiency than can be achieved using other multiple access techniques. In other words, more calls can be made in a given frequency band using CDMA than using other technologies. 
     In a prior art CDMA network  10  only one frequency band or carrier frequency, F 1  is used by all cells  12 , as shown in FIG. 1. A base station control  18 , which operates base stations  20  in cells  12  does not issue frequency hand-off commands. That is because a transceiver  14  of a mobile user  16  does not have to hand-off between different frequencies in network  10 . 
     Typically, a CDMA signal  22  in network  10  occupies a 1.25 MHz band (although other implementations can use more or less bandwidth). The band is centered at carrier frequency F 1  and several CDMA signals are superimposed upon each other within the band. As shown in FIG. 2, each CDMA signal  22  is created by multiplying a narrow band (about 10 kHz wide) baseband signal  24  containing the data (e.g., voice data) by a spreading code which increases the resulting bandwidth to 1.25 MHz. On the forward link, from base station  20  to transceiver  14 , CDMA signal  22  is prepared by spreading baseband signal  24  twice; once by a Walsh code and once by a pseudorandom noise sequence PN. 
     Baseband signal  24  is multiplied in a mixer  28  with a Walsh code W i  provided from a Walsh code generator  26  to produce a coded signal  30 . Since individual Walsh codes are orthogonal their inner product satisfies the following condition:            W   i     *     W   j       =     {         0         i   ≠   j             N         i   =   j                                    
     Thus, baseband signal  24  multiplied by Walsh code W i  on the forward link can only be demodulated by a receiver by multiplying it with the same Walsh code, i.e., W i . Multiplication with any other Walsh code will not yield a signal. Hence, the receiver set to use Walsh code W i will reject all signals which are prepared with any Walsh code other than W i . 
     Walsh coded signal  30  is then multiplied with the aid of mixers  32 ,  34  by a short pseudorandom noise sequence PN provided by a PN generator  36 . The PN sequence has a characteristic offset. In this case coded signal  30  is multiplied by an in-phase and a quadrature portion of the PN sequence in accordance with standard modulation techniques. The multiplied signals are converted from digital to analog and filtered by circuits  25 ,  27  and then combined by a combining circuit  38 . The thus created CDMA signal  22  is up-converted by a mixer  23  to carrier frequency F 1  and sent to antenna  40  for transmission. 
     Since the same carrier frequency F 1  is used throughout CDMA network  10  base stations  20  are assigned unique offsets of the PN sequence to distinguish them. For example, base station  20 A uses sequence PN A  which is the PN sequence with an offset A in generating its CDMA signals, base station  20 B uses sequence PN B , and so on. The various sequences PN A , PN B , . . . etc. are generated by shifting the standard PN sequence by varying offset amounts also referred to as PN offsets. The PN sequences are used to multiply each channel including a pilot channel. The pilot channel is defined as the unmodulated Walsh code zero. In other words, the pilot channel requires that generator  26  be set to zero and baseband signal  24  be zero. As a result, only the PN sequence with its given PN offset is transmitted in the pilot channel. 
     Just as in the case of frequency reuse, PN sequences with the same offsets can be reused in cells  12  which are sufficiently far apart to avoid interference, e.g., cells  12 A and  12 B use the same sequence PN A . Transceiver  14  will examine the different PN offsets to thus identify base stations  20  near it. As user  16  moves from one cell  12  to another, transceiver  14  can hand off to a neighboring base station in a soft hand-off process. The carrier frequency remains the same but the PN sequence of the new base station is used. The process is called soft because during the transition from one base station to another transceiver  14  is communicating simultaneously with both base stations. 
     As the number of mobile telephone users increases, more capacity than offered by CDMA network  10  will be required. One way to accomplish this goal is to use more frequency channels within the allocated frequency spectrum by adapting CDMA network  10  to operate at more than one carrier frequency. This means that CDMA network  10  will have to accommodate hard or frequency hand-off between different frequencies used in different cells  12 . 
     The prior art teaches the use of a pilot channel assigned Walsh code zero (0) to carry the PN offset information. The signal corresponding to the PN offset information is referred to as the pilot beacon. Knowledge of the PN offset allows the transceiver to identify with which base station they are communicating. 
     In U.S. Pat. No. 5,848,063 Weaver, Jr. et al. discusses the use of a pilot beacon for handing-off between dissimilar CDMA networks. The hand-off is not necessarily a frequency hand-off (hard hand-off) and the teaching is directed primarily at the hand-off algorithm and uses the measured time delay for the pilot beacon between the base station and the transceiver as a parameter for deciding when to execute a hand-off. U.S. Pat. No. 5,697,055 to Gilhousen et al. also discusses algorithms for determining hand-off between different cellular systems. 
     In U.S. Pat. No. 5,858,661 Weaver, Jr. et al. teach a method for creating areas where certain transceivers cannot communicate with certain base stations. These regions of silence are indicated by the presence of a pilot beacon with a specific PN offset indicating that any mobile transceiver hearing this pilot beacon is within the silence region. 
     In U.S. Pat. Nos. 5,267,261 and 5,101,501 Blakeney, II et al. and Gilhousen et al. teach the details of soft-hand off using pilot channels radiating pilot beacons. Each base station transmits a pilot beacon or pilot tone with a specific PN offset. All pilot beacons are transmitted at the same frequency. 
     In a CDMA system using various frequencies hand-off, in particular hard hand-off or frequency hand off between cells presents a new challenge. None of the prior art teaches how to produce a pilot beacon which can be used for executing such hand-offs in such CDMA networks. 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     Accordingly, it is a primary object of the present invention to provide a pilot beacon for use in a CDMA system using different carrier frequencies. Specifically, the object of the invention is to provide a multi-frequency pilot beacon for transmitting PN offset information to cellular users. 
     It is another object of the invention to adapt a CDMA system to use a multi-frequency pilot beacon for performing hard hand-off operations. 
     Yet another object of the invention is to provide a multi-frequency pilot beacon which is easy to manufacture and integrate in a CDMA system. The multi-frequency pilot beacon can be used in various configurations, including in-building micro-cells. 
     The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below. 
     These objects and advantages are secured by a multi-frequency pilot beacon adapted to a CDMA system using at least two different carrier frequencies, such as a first CDMA carrier frequency F 1  and a second CDMA carrier frequency F 2 . The pilot beacon has a pseudorandom noise generator for supplying a pseudorandom noise sequence PN. It also has a frequency conversion mechanism for converting the PN sequence to a first pilot beacon centered at the first CDMA carrier F 1  and a second pilot beacon centered at the second CDMA carrier F 2 . A transmitting unit transmits the first and second pilot beacons to the transceiver or mobile cellular unit. 
     The multi-frequency pilot beacon can be provided at a base station of a given cell to transmit the pilot beacons within that cell. The base station antenna can be used for transmitting the pilot beacons in this embodiment. Alternatively, multi-frequency pilot beacons can be provided wherever necessary within the CDMA system. In this situation the pilot beacons can be transmitted directly from the pilot beacon unit. 
     The PN sequences (in-phase and quadrature) are preferably digital sequences. The pilot beacon is equipped with a digital-to-analog converter for converting these digital PN sequences to an analog PN sequences. 
     In one embodiment the pilot beacon generates the PN sequences at an intermediate frequency (IF). Additional circuit elements are provided to accommodate this alternative. 
     A CDMA system using the multi-frequency pilot beacon uses the pilot beacons to hand-off the cellular transceiver between carrier frequencies. The carrier frequencies can be used in the same cell or in different, e.g., adjacent cells. Preferably, a hand-off order is issued by a CDMA system controller based on the traffic volume at the carrier frequencies involved. Alternatively, the hand-off order can be based on the location of the transceiver. 
     There are many methods for operating the multi-frequency pilot beacon. The pilot beacons can be transmitted together from the same location or from different locations or even from the base station. The configuration of the specific CDMA network can impose additional requirements on how to use the multi-frequency pilot beacon of the invention. 
    
    
     The particulars of the invention and its various embodiments are described in detail in the detailed description section with reference to the attached drawing figures. 
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic view of a prior art CDMA network. 
     FIG. 2 is a block diagram illustrating the generation of a CDMA signal in a prior art CDMA network. 
     FIG. 3 is a diagram showing a cell in a CDMA network utilizing a multi-frequency pilot beacon according to the invention. 
     FIG. 4 is a diagram illustrating the hand-off between different carrier frequencies in a multi-frequency CDMA network. 
     FIG. 5 is a diagram showing the details of generating pilot beacons at multiple frequencies. 
     FIG. 6 is a block diagram showing a preferred multi-frequency pilot beacon according to the invention. 
     FIG. 7 is a block diagram of another multi-frequency pilot beacon according to the invention. 
     FIG. 8 is a schematic showing the deployment of multi-frequency pilot beacons in another CDMA network. 
     FIG. 9 is a schematic illustrating the use of a multi-frequency pilot beacon in a CDMA network including micro-cells. 
    
    
     DETAILED DESCRIPTION 
     A cell  100  belonging to a CDMA network  102  utilizing a multi-frequency pilot beacon  104  is shown in FIG. 3. A base station  106  of network  102  uses a number N of frequency bands centered at carrier frequencies F 1 , F 2 , . . . FN respectively. For example, as shown in FIG. 4, the total frequency spectrum  112  utilized by network  102  within cell  100  is 15 MHz wide and each frequency band centered around the respective carrier frequencies F 1 , F 2 , . . . FN is 1.25 MHz. This means that N=12; i.e., 12 frequency bands are available within cell  100 . Of course, other frequency ranges yielding different numbers of frequency bands can be utilized as well. 
     CDMA communications or data signals  108  are transmitted between base station  106  and cellular users equipped with transceivers  110 A,  110 B. CDMA signals  108  can be transmitted at any of the carrier frequencies F 1 , F 2 , . . . F 12  available in cell  100 . 
     A base station control  112  is connected to base station  106  as well as base stations of other cells belonging to network  102 . Base station control  112  is responsible for regulating the communications between all transceivers in cell  100  such as transceivers  110 A,  110 B and base station  106 . In particular, control  112  is configured to assign transceivers  110 A,  110 B to handle their calls, i.e., receive and transmit signals  108 , at one of the available carrier frequencies F 1 , F 2 , . . . F 12 . Furthermore, base station control  112  monitors the status of CDMA network  102  and cell  100  and issues frequency or hard hand-off orders to force transceivers  110 A,  110 B to switch the carrier frequencies they are using in communicating with base station  106  depending on the status of network  102  and cell  100 . For example, transceiver  110 A may be originally asked by base station control  112  to handle its call at carrier frequency F 1 . While the call is in progress the traffic volume at carrier frequency F 1  monitored by base station control  112  increases. Thus, base station control  112  issues a hard hand-off order to transceiver  110 A to switch to carrier frequency F 2  and continue the call at F 2 . To aid base station control  112  in making frequency hand-off decisions transceivers  110 A,  110 B, in addition to handling calls at their assigned carrier frequencies, report their status and specifically any signals they receive to base station control  112 . 
     As shown in FIG. 4, a number M of channels is available in each frequency band. In prior art CDMA networks each frequency band contains, in addition to the channels on which calls are handled, a synchronization channel, a paging channel and one pilot channel. The present invention, uses additional pilot offsets at each carrier frequency F 1 , F 2 , . . . FN. In particular, multi-frequency pilot beacon  104  transmits pilot beacons  114  consisting of unmodulated Walsh code zero baseband signal multiplied by standard PN sequence with three different offsets A, B and C in three corresponding pilot channels. For convenience in notation the PN sequences with these three offsets are referred to as PN A , PN B  and PN C  and the corresponding pilot channels are designated A, B, C. 
     Multi-frequency pilot beacon  104  transmits pilot beacons  114  consisting of all three PN sequences PN A , PN B  and PN C  in each frequency band. In other words, each frequency band has three pilot channels in this embodiment. All three pilot channels contain a Walsh code zero baseband signal multiplied by sequences PN A , PN B  or PN C . In the present embodiment first sequence PN A  is used in each frequency band to multiply the baseband data signal to produce CDMA signals  108 . In alternative embodiments, any frequency band may use any one of sequences PN A , PN B  and PN C  for producing CDMA signals  108 . 
     In the present embodiment, sequences PN B  and PN C  in pilot channels B and C are used for frequency hand-off operations. There are many ways of implementing sequences PN B  and PN C  produced by multi-frequency pilot beacon  104  to be used in hand-off operations. 
     For example, transceiver  110 A is enabled to receive pilot channel B and transceiver  110 B is enabled to receive pilot channel C. When communicating with base station  106  in the frequency band at carrier frequency F 1  transceiver  110 A detects sequence PN B  transmitted in pilot channel B in the same frequency band. In reporting its status to base station control  112 , transceiver  110 A informs base station control  112  that it is receiving sequence PN B . Based on this information, and the high traffic volume in frequency band F 1  and low traffic volume in frequency band F 2  base station control  112  sends an order for transceiver  110 A to hard hand-off its call to frequency band F 2 . After this hard hand-off transceiver  110 A uses sequence PN A  of pilot channel A at F 2  to multiply its baseband data to produce CDMA signals  108 . 
     Meanwhile, transceiver  110 B is also communicating with base station  106  in frequency band F 1  and it receives sequence PN C  in pilot channel C. Transceiver  110 B informs base station  106  that it is receiving sequence PN C . Based on this information, and the high traffic volume in frequency band F 1  and low traffic volume in frequency band F 12  base  40  station control  112  sends an order for transceiver  110 B to hard hand-off its call to frequency band F 12 . After this hard hand-off transceiver  110 B uses sequence PN A  of pilot channel A at F 12  to multiply its baseband data to produce CDMA signals  108 . 
     Of course, both transceivers  110 A and  110 B can be enabled to detect pilot channels B and C. Alternatively, transceivers  110 A,  110 B can be enabled to detect pilot channels B and/or C only when operating in a particular subset of frequency bands F 1 , F 2 , . . . F 12 . It should also be noted that the position of multi-frequency pilot beacon  104  in cell  100  can be selected such that pilot channels B and/or C are only detected by transceivers  110 A,  110 B in a particular area of cell  100 . In this case sequence PN A  produced by multi-frequency pilot beacon  104  can be transmitted from base station  106  to ensure that it is detected transceivers  110 A,  110 B at any location within cell  100 . Also, more than one multi-frequency pilot beacon  104  can be used in cell  100 . Preferably, the choice of carrier frequency made by base station control  112  is based on the traffic volumes in the frequency bands used in cell  100 . Alternatively, base station control  112  can issue the hand-off order based on the location of transceivers  110 A,  110 B in cell  100 . For example, carrier frequency F 2  may be determined to be more suitable for communication between transceiver  110 A and base station  106  due to power level considerations or other parameters in addition to or independent of the traffic volume. 
     A person of average skill in the art will recognize that supply of sequences PN B  and PN C  and appropriate enablement of pilot channels B and C of transceivers  110 A,  110 B allows base station control  112  to obtain more information about the status of transceivers  110 A,  110 B. Knowledge of the received pilot offsets enables base station control  112  to make more efficient frequency hand-off decisions and/or to make frequency hand-off decisions based on the status, capabilities, location of transceivers  110 A,  110 B. In fact, any factors which are implicit when transceiver  110 A or  110 B receives pilot channel A and/or B provided by the multi-frequency pilot beacon  104  supply valuable information to base station control  112  for its decision-making process. 
     In this embodiment, all pilot beacons  114  are transmitted in pilot channels A, B, C at each carrier frequency F 1 , F 2 , . . . FN, as shown in FIG.  5 . In other words, 3 pilot channels in each frequency band are used. To accomplish this, multi-frequency pilot beacon  104  has separate multi-frequency beacon units  116 A,  116 B,  116 C for generating PN sequences with offsets PN A , PN B , PN C  at all carrier frequencies F 1 , F 2 , . . . F 12 . A processing unit  118  combines PN sequences with offsets PN A , PN B , PN C  at all carrier frequencies F 1 , F 2 , F 12  and distributes them to the corresponding three pilot channels A, B, and C at each carrier frequency. Processing unit  118  can be connected directly to base station  106  and use the same antenna  120  as base station  106  for transmitting pilot beacons  114 . Alternatively, a separate antenna  122  can be connected to unit  118  for transmitting all or some of the frequency bands F 1 , F 2 , . . . F 12 . The choice will be made by the designer of CDMA system  102  depending on how base station control  112  and transceivers  110 A,  110 B will utilize pilot beacons  114  for frequency hand-off. 
     A preferred multi-frequency beacon unit  130  which can be employed as unit  116 A is shown in FIG.  6 . Unit  130  has an offset circuit  132  for supplying sequence PN A . In accordance with standard modulation techniques, sequence PN A  is translated into in-phase short PN and a quadrature short PN signals. The in-phase and quadrature signals are produced by unit  134  and converted from digital to analog form by digital-to-analog converters  143 ,  145 . It should be noted that digital-to-analog conversion can take place at other locations in unit  130 , as is known in the art. The converted signals are sent to low-pass filters  136 ,  138  respectively for low pass filtering to produce bandlimited signals. These are combined by combining circuit  140  to produce a single sequence PN A . 
     The analog sequence PN A  is sent to N mixers  142 A,  142 B, . . .  142 N. Other sequences, PN B , PN C  etc. can be combined with PN A  before mixing. Mixers  142 A,  142 B, . . .  142 N are also supplied with carrier frequencies F 1 , F 2 , . . . FN of the individual frequency bands. Carrier frequencies F 1 , F 2 , . . . FN can be supplied from any suitable source, e.g., local oscillators or CDMA system oscillators (not shown). Mixers  142 A,  142 B, . . .  142 N perform a standard up-conversion of analog sequence PN A  to carrier frequencies F 1 , F 2 , . . . FN. A person of average skill in the art will realize that the up-conversion process need not be performed in a single up-conversion step using only mixers  142 A,  142 B, . . .  142 N. 
     An Nx 1  power combiner  144  combines sequences PN A  at frequencies F 1 , F 2 , . . . FN. These are then delivered, for example, to processing unit  118  shown in FIG.  5 . When unit  130  is used as pilot beacon unit  116 A in multi-frequency pilot beacon  104  N=12. In this embodiment the number of mixers  142  is twelve and they are supplied with twelve carrier frequencies F 1 , F 2 , . . . F 12 . 
     FIG. 7 shows an alternative embodiment of a multi-frequency pilot beacon unit  150 . Unit  150  is similar to unit  130 , but instead of up-converting signal from baseband, it uses an intermediate frequency (IF) (e.g., 70 MHz) sequence PN A  from circuit  152 . In accordance with standard modulation techniques, sequence PN A  is translated into in-phase short PN and a quadrature short PN signals, both at the intermediate frequency. The in-phase and quadrature signals are produced by unit  154  and sent to digital-to-analog converters  163 A and  163 B which convert the in-phase and quadrature intermediate frequency signals to analog intermediate frequency signals. Low-pass or band pass filters  156 ,  158  filter these signals to produce bandlimited intermediate frequency signals. These filtered analog signals are combined by combining circuit  160  to produce a single intermediate frequency sequence PN A . 
     The intermediate frequency sequence PN A  is sent to N mixers  162 A,  162 B, . . .  162 N. Mixers  162 A,  162 B, . . .  162 N are also supplied with fractional carrier frequencies qF 1 , qF 2 , . . . qFN for up-converting intermediate frequency sequence PN A  to carrier frequencies F 1 , F 2 , . . . FN. The value of q is calculated based on the relationship that qF 1 =F 1 −IF. Fractional carrier frequencies qF 1 , qF 2 , . . . qFN can be derived from any suitable source, e.g., local oscillators or CDMA system oscillators (not shown), as is known in the art. 
     A person of average skill in the art will recognize that the above embodiment of the multi-frequency pilot beacon is adapted for use in CDMA network  102  using  12  frequency bands containing the same number of channels (M). However, the number of frequency bands N can vary as required, and not all channels may be utilized in any particular frequency band. These parameters can also vary from cell to cell. Hence, in some cells it may not be required to transmit all the pilot beacons. Thus, the multi-frequency pilot beacon should be adapted to the particular CDMA system. 
     FIG. 8 illustrates four cells  204 A,  204 B,  204 C and  204 D belonging to another CDMA system  200 . System  200  uses multi-frequency pilot beacons  202 A,  202 B,  202 C and  202 D connected to corresponding base stations  206 A,  206 B,  206 C with the exception of beacon  206 D which is located away from base station  206 D. Beacons  202 A,  202 B and  202 C use antennas  208 A,  208 B and  208 C of corresponding base stations  206 A,  206 B and  206 C for transmitting pilot beacons. Beacon  202 D in cell  204 D uses its own antenna  210  rather than base station&#39;s antenna  208 D for transmitting pilot beacons. 
     Cell  204 A uses frequency bands centered at F 1 , F 2  and F 3  for CDMA signals. Cell  204 B uses frequency bands centered at F 1 , F 2  and F 4 , cell  204 C operates at F 1 , F 2 , F 3  and F 4  while cell  204 D only operates at F 1 . Pilot beacons  202 A,  202 B,  202 C and  202 D transmit one or more PN sequences at all four frequencies F 1 , F 2 , F 3  and F 4 . Each base station uses different pilot offsets to distinguish them from other nearby base stations. For instance, pilot beacon  202 A may transmit PN A  and pilot beacon  202 B may transmit PN B  at all frequencies. Thus, a base station control  212  can obtain information whether any given cellular transceiver traveling between cells  204 A,  204 B,  204 C and  204 D detects one or more PN sequences at the frequency currently assigned to the user. Based on that information base station control  212  can issue hand-off orders to any desired frequency supported either in the cell in which the transceiver is located or in the cell to which the transceiver is travelling. 
     Alternatively, for reasons of efficiency or other considerations, beacons  202 A,  202 B,  202 C and  202 D need not transmit PN sequences at all the frequencies used in their cells. 
     FIG.  9 . illustrates the application of a multi-frequency pilot beacon  220  in a micro-cell  222  of CDMA system  224 . In this case micro-cell  222  covers the inside of a building structure, e.g., an office building. A base station  226  operating at carrier frequency F 2  supports communications within building  222 . Building  222  is located within a cell  228  with base station  230  which supports communications at carrier frequency F 1  and uses a PN offset sequence PN A . Multi-frequency pilot beacon  220  inside building  222  transmits sequence PN B  at frequencies F 1  and F 2 . 
     While within cell  228 , a user of cellular transceiver  232  communicates by CDMA signals at frequency F 1  which uses sequence PN A . When entering building  222  a user of cellular transceiver  234 , e.g., one authorized to communicate in building  222  via base station  226 , will detect sequence PN B  at F 1 . Thus, when transceiver  234  reports detection of sequence PN B  at F 1 , base station control  236  will know that the user of transceiver  234  has entered building  222  and is authorized to operate at frequency F 2 . Hence, base station control  236  sends a hard hand-off order from frequency F 1  to frequency F 2  with pilot channel PN A  to user  234 . The authority of user  234  to communicate within building  222  can be confirmed, e.g., by the identification number of the cellular transceiver of user  234 . The hand-off is made possible with the aid of multi-frequency pilot beacon  220 . 
     Of course, other CDMA systems can also benefit from the use of multi-frequency pilot beacons. In fact, the information derived from the presence of additional PN sequences generated by a multi-frequency pilot beacon and transmitted in additional pilot channels can be used by base station control units for many functions besides frequency hand-off. For example, the detection by a transceiver of three separate PN sequences can be converted by the base station control into a binary number and utilized in deriving additional information about the location, power level or other status data about the transceiver. 
     It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.