Abstract:
A method and system for producing a plurality of pilot beacon signals with different carrier frequencies is disclosed. One original pilot beacon signal is first obtained, and a splitter is used to make copies of the pilot beacon signal. Each of the copies are fed into a mixer for generating a plurality of intermediate signals with different frequencies. A bandpass filter is subsequently used to select a final one among the intermediate signals. Thus, a set of pilot beacon signals with different frequencies are successfully obtained.

Description:
CROSS REFERENCE 
     This application claims the benefit of provisional application assigned U.S. Ser. No. 60/118,272 filed on Feb. 2, 1999. 
    
    
     BACKGROUND 
     The present invention relates generally to wireless communication networks, and more particularly, to a system and method for generating a plurality of pilot beacons to facilitate a precise hard handoff. 
     The success of code division multiple access (CDMA) wireless communication in the recent years has brought quality service to many mobile users. With the advancement of the technology and the demand of the market, it is foreseeable that more private CDMA networks will be installed in addition to existing public CDMA networks such as the Sprint PCS network by Sprint, Inc. Major corporations or institutes may have the desire to establish private CDMA networks on their campuses to provide high quality and more economical wireless service to their employees or guests. However, such campuses are often heavily populated with mobile units such as cellular phones. To worsen the situation, the size of the campus is relatively small so that it may reside entirely in one cell of a larger network. 
     One major feature of CDMA technology that differentiates it from other wireless communication technologies is its use and reuse of specific frequencies. Although it is a technical edge over other communication technologies such as time division multiple access, it makes the campus scenario described above an undesirable “hot spot” since a comparatively large number of users may have to be serviced by one sector of the cell using the same frequency. This possible “traffic jam on the communication highway” reduces the quality of the wireless services provided to the mobile units on the campus. 
     As a result, it may be desirable for a private CDMA network on a campus to use a carrier frequency different from the surrounding networks. And to further improve the quality of the services on the campus, numerous micro cells may be used, each of which having a comparatively small radius (typically less than one hundred meters). In contrast, a “macro cell” for a standard CDMA network surrounding the campus may have a radius around 3 to 5 kilometers. Consequently the signal interferences between the surrounding network and the private network on the campus, as well as the signal interferences among the micro cells in the private network, are greatly reduced because different carrier frequencies are employed. However, for providing continuous wireless service to a mobile unit (MU) when it enters or leaves the campus, a hard handoff must occur between the private network and the outside network. 
     There are several difficulties associated with the above described hard handoff. For one, an active wireless service such as an ongoing telephone call should not be interrupted or dropped due to the change of the carrier frequency when carrying out such a hard handoff. In addition, the service provided should be properly billed, potentially to different service providers, according to the use of the service of the different networks. Thus, for example, when a mobile user exits the campus, he should be able to continue his conversation on the MU, even though the MU needs to switch from the private network to the surrounding network. Such a transition must be performed in a relatively small border area between the campus and outside, and vice versa when the mobile user enters the campus. 
     Unfortunately, various problems exist for precisely performing the handoff at the border area. First, soft handoffs do not work well in this situation. It is known in the art that the soft handoff for CDMA technology has been studied and developed greatly. But in order to take advantage of a soft handoff, only one carrier frequency can be involved. Soft handoffs are thus most useful for switching the wireless service between two Base Transmission Stations (BTS) using the same carrier frequency. For the situation discussed above, since at least two carrier frequencies are involved, the soft handoff scheme would be hard to deploy. For example, in order to use the soft handoff scheme, as it is known in the art, the only alternative is to configure a tier of cells for providing a soft handoff zone between the campus and the surrounding network. This would dramatically increase the cost of installing such a private network since additional, expensive infrastructure equipment must be installed. Moreover, significant redesign of the surrounding network is also required to accommodate this change, which adds further costs to the private network. Therefore, a soft handoff is economically infeasible and a hard handoff design is needed. 
     There is still a problem associated with using a conventional hard handoff scheme, such as a round trip delay handoff, in a small area. As it is known in the art, the round trip delay handoff can detect the motion of a MU only when it moves for more than 250 meters. This does not work for the campus scenario mentioned above because the campuses themselves may have a radius of less than 500 meters. The handoffs thus can not be guaranteed to happen at or near the entrance or the exit of the campus since the border area between the campus and the surrounding network is probably only about 25 meters in length. 
     Therefore, what is needed is an economical and reliable system and method to direct a precision hard handoff to accommodate the carrier frequency change between a relatively small private CDMA network and its surrounding networks. 
     SUMMARY 
     In response to the above described problems and difficulties, a technical advance is provided by a unique method and system for providing a precision hard handoff between two networks. In one embodiment, the two networks include a first wireless communication system for serving a defined area, such as a campus, at a first frequency and a second wireless communication system adjacent to the first system and operating at a second frequency. A first pilot beacon distributor is positioned at a transition point of the campus for directionally transmitting a first pilot beacon signal into the campus. When a mobile unit operating on the second frequency crosses the transition point into the campus, it receives the first pilot beacon signal and it shifts operation to the first frequency. 
     In some embodiments, a second pilot beacon distributor is also positioned at the transition point for directionally transmitting a second pilot beacon signal away from the campus. As a result, when a mobile unit operating on the first frequency crosses the transition point out of the campus, it receives the second pilot beacon signal and it shifts operation to the second frequency. 
     In another embodiment, a plurality of pilot beacon distributors are installed at each entrance or exit of a campus and are connected by fiber optical cables to a radio frequency (RF) beacon source which may remain at a central location on the campus. Only one beacon source is needed for each carrier frequency covering the entrance or exit area. The pilot beacon distributors have directional beacon antennas that transmit the pilot beacons to help trigger a hard handoff for any MU moving in or out of the campus. 
     In some embodiments, a first pilot beacon distributor is used to direct hard handoff from a carrier frequency for the private network on the campus, e.g., F 1 , to a target carrier frequency, e.g., F 2 , of the outside network for outgoing traffic, and a second pilot beacon distributor is also installed close to the first pilot beacon distributor for steering incoming traffic to hard handoff from F 2  to F 1 . 
     Some embodiments may provide a directional non-beacon antenna that does not transmit the pilot beacon. The directional non-beacon antenna may be coupled with the above mentioned directional beacon antenna in the pilot beacon distributors. The directional beacon antenna may have a large front-to-back ratio, while the non-beacon antenna may not have a significant front-to-back ratio. 
     These embodiments prevent a ping-pong phenomenon between the first and the second pilot beacon distributors. For example, a ping-pong phenomenon may occur when both pilot beacon distributors detect the motion of a MU and both decide to trigger a handoff to its target carrier frequency. In this example, the MU will bounce between the two frequencies (F 1  and F 2 ) without actually being able to settle on a right one. 
     These embodiments also prevent call drops from occurring if the signal strength of a target carrier frequency is too weak at the locations where the handoff is desired. For example, a handoff may become unreliable since the target network can not effectively “take over” the service. 
     These embodiments are also specifically designed to maintain a proper signal-to-noise ratio (SNR). If the SNR under the current carrier frequency changes too fast, it could cause a call drop as well and the handoff can never be completed. 
     In order to accommodate complex CDMA networks where multiple frequencies are used both for the surrounding network and the private network on the campus, a multiple-beacon generator may also be used. Instead of installing a pilot beacon unit for each frequency, the multiple-beacon generator can produce copies of a pilot beacon at different carrier frequencies. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a campus having a private CDMA network with a first carrier frequency and a surrounding CDMA network having a second carrier frequency, with pilot beacon distributors installed at the entrance and exit of the campus. 
     FIG. 2 shows a portion of the hardware configuration in FIG. 1 for facilitating an active hard handoff using one of the pilot beacon distributors. 
     FIG. 3 is a flow diagram showing steps to complete a precise, active hard handoff in accordance with one embodiment of the present invention. 
     FIG. 4 is a flow chart for an idle hard handoff in accordance with one embodiment of the present invention. 
     FIG. 5 demonstrates a possible ping-pong phenomenon associated with the hardware configuration of FIG.  1 . 
     FIG. 6 shows one embodiment of the present invention using a directional non-beacon antenna in a back lobe direction of the beacon antenna of the pilot beacon distributors to enhance the system performance in accordance with one embodiment of the present invention. 
     FIG. 7 is an overview of hardware arrangement of a plurality of pilot beacon distributors and a pilot beacon source. 
     FIG. 8 illustrates one embodiment of the present invention for generating a plurality of pilot beacons using frequency division. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Techniques and requirements that are only specific to certain embodiments should not be imported into other embodiments. Also, specific examples of components, signals and network technologies are described below to help clarify the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. For illustration purposes, drawings may not be to scale. 
     Referring now to FIG. 1, a campus  10  having a private CDMA network is surrounded by an existing CDMA network  12 . Two entrances (or exits)  16  are assumed to be the only ways through which a MU can move in or out of the campus  10 . For the sake of illustration, it is also assumed that the private CDMA network has a carrier frequency F 1 , and the surrounding CDMA network  12  has another carrier frequency F 2 . Two pilot beacon distributors are installed at or near each of the entrances  16 , with one inbound pilot beacon distributor  18  directing handoff to the frequency F 1  and one outbound pilot beacon distributor  20  directing handoff to the frequency F 2 . Further, the pilot beacon distributors contain beacon antennas for transmitting pilot beacons. Directional antennas are purposefully used for beacon antennas instead of omni-directional antennas since they can be perceived by the MUs coming from only one direction. With this configuration, once a MU  22  passes through an area covered by an outbound pilot beacon distributor  20 , its wireless service is expected to be switched to the frequency F 2  of the surrounding network  12 . And similarly, if the MU  22  passes through the area covered by the inbound pilot beacon distributor  18 , it is to be switched to the frequency F 1  of the private network on the campus  10 . 
     The hard handoff is thus expected to happen in the entrance areas  16  so that a mobile user can enjoy the benefit of the private network, which may provide a different wireless service to the MU for a discounted fee or even free of cost. It is also desired that the triggering event occurring at or around the entrances  16  is performed with precision. That is, the precision handoff at the entrances  16  accommodates proper billing and desired services provided by both the private network on the campus  10  and the surrounding network  12 . 
     FIG. 2 shows a portion of the hardware configuration for facilitating an active hard handoff, i.e., a hard handoff when a MU carries an active conversation, using a pilot beacon distributor. FIG. 3 is a flow diagram showing steps taken by one or more components of FIG. 2 to complete the active hard handoff. As is known in the art, each established down link using CDMA technology has four communication channels—pilot, synch, paging, and traffic. The traffic channel carries the regular data information such as those for a telephone conversation. A pseudo-random noise (PN), which is encoded and transmitted with the pilot signal in the pilot channel by every BTS, has a phase shift from the same PN transmitted by a neighboring BTS. While a MU is in use, it is aware of the existence of its neighboring BTSs by sorting the PN information of any received pilot signals. If a critical parameter E c /I 0  (where E c  is the energy of the expected signal and I 0  is the total energy of everything received) of a pilot signal transmitted by a neighboring BTS is found to be higher than a predetermined threshold value, the MU perceives that the neighboring BTS is a possible candidate for handoff. 
     When the MU  22  is active under the coverage of the BTS 1   24 , which uses the carrier frequency F 1 , and moves into the coverage area of the outbound pilot beacon distributor  20  while leaving the campus  10 , the MU receives a new pilot signal transmitted by the pilot beacon distributor  20 . The new pilot signal has a carrier frequency of F 1  but with a different PN, as illustrated in step  30  of FIG.  3 . If the MU  22  calculates the parameter E c /I 0  and finds that the value of the parameter E c /I 0  has not exceeded the threshold value, in step  32 , as known in the art according to the IS-95 standard, no action will be taken. 
     If the parameter E c /I 0  is greater than the threshold value, and since the new pilot signal uses the same frequency F 1 , the MU  22  believes that a soft handoff should be the appropriate way to switch over the service. Thus, in step  34 , it sends a Pilot Strength Measurement Message (PSMM) to BTS 1 , which is its current home base, asking for permission to complete a soft handoff. Once BTS 1  receives the message in step  36 , it searches for the identification of the PN of the new signal in a pilot signal database. 
     Since a distinctive PN phase shift can be encoded for the pilot beacon, BTS 1  can quickly realize that the new pilot signal the MU  22  has just received is not initiated from a standard BTS, but it is a pilot beacon instead. Further, information relating to that particular PN in the database also informs BTS 1  that the MU  22  needs a hard handoff to shift the carrier frequency from F 1  to F 2 , which is used by the BTS 2   26 . Therefore, in step  38  of FIG. 3, BTS 1   24  sends an Extended Handoff Direction Message (EHDM) to the MU  22  instructing it to prepare for a hard handoff to F 2  under the coverage of BTS 2   26 . Once the MU  22  decodes the message, it reaches for BTS 2  and carries out the handoff in step  40 . Similar hard handoff mechanisms are expected when the MU  22  moves from the outside network to the campus  10  through the coverage area of the inbound pilot beacon distributor  18 . 
     FIG. 4 is a flow chart illustrating the steps involved for an idle hard handoff. When the MU  22  is powered on but idle, i.e., no active conversation is conducted, an idle hard handoff mechanism must take place during the transition from the campus network to the surrounding network (or vice versa). It is known in the art that an idle MU recognizes one BTS as its home base although it is not using the home base BTS to service any active conversation. The pilot signal of this home base BTS is thus closely monitored. Further, the MU may constantly search for new pilot signals. In order to steer the transition, in step  42 , a Global Service Redirection (GSR) parameter is enabled in the paging channels of the two pilot beacons transmitted by the pilot beacon distributors  18  and  20 . The GSR setting, if read by the MU  22 , can force a transition to a particular carrier frequency. 
     In step  44 , when the MU  22  moves close to the pilot beacon distributor  20 , it should “see” a new pilot signal immediately because of its pilot searching function. As shown in step  46 , according to the IS-95 standard, unless the parameter E c /I 0  of the new found pilot signal is 3 dB stronger than the incumbent one, the MU  22  will not take any further actions. If the 3 dB condition is met, the MU  22  replaces the incumbent pilot signal that it is monitoring with the new pilot signal. By doing so, the MU  22  takes on the new BTS (the pilot beacon distributor  20 ) as its home base for the time being. Then, in step  48 , the MU  22  reads the synch and paging channels of the new link, and immediately finds a GSR setting in the paging channel. The GSR setting instructs the MU to further handoff to the target carrier frequency F 2 . The MU  22  is to follow the instruction of the GSR, in step  49 , and complete the hard handoff to F 2 . Similar idle hard handoff steps are taken using a pilot beacon transmitted by the pilot beacon distributor  18  when the MU  22  moves in the opposite direction. 
     For implementing certain embodiments of the present invention, beacon antennas in the pilot beacon distributors must be set low enough to avoid “overshooting” the pilot beacon signals. An antenna of about 5 feet in height, and with a low power consumption of about 0.01 mW is suitable to cover a regular two lane entrance to a parking lot of the campus. 
     FIG. 5 presents a demonstration of a possible ping-pong phenomenon associated with some embodiments of the hardware configuration of FIG.  1 . In FIG. 5, a plot of the parameter E c /I 0  is superimposed on a drawing of the entrance of the campus. The ping-pong phenomenon occurs due to the hardware configuration of two pilot beacon distributors at the entrance area. In FIG. 5, a first curve  50  represents the level of the parameter E c /I 0  for the pilot beacon of the outbound pilot beacon distributor  20 , and similarly a second curve  52  for the inbound pilot beacon distributor  18 . Two horizontal phantom lines  54  and  55  show the levels of the signal strength that trigger a hard handoff, and two vertical phantom lines  56  and  58  delineate an area between them that is susceptible to the ping-pong phenomenon. When the MU  22  is leaving the campus and passes line  56 , according to the disclosure explained above, the pilot beacon distributor  20  will help to trigger a hard handoff to the surrounding network since the level of the parameter E c /I 0  of the pilot signal has surpassed the threshold level as indicated by phantom line  55  in curve  50 . 
     For the same reason, the MU  22  will be instructed to hard handoff in the area left of line  58 , as shown in the figure, when it comes into the campus from outside. This leaves a small zone of uncertainty  60  between lines  56  and  58  where both pilot signals from the outbound pilot beacon distributor  20  and the inbound pilot beacon distributor  18  are above their corresponding thresholds. Thus the MU  22  is likely to “bounce” between these two handoffs without knowing which one to settle on. While it is bouncing, a ping-pong phenomenon occurs, and an active call can be dropped. 
     Another problem associated with some embodiments is that a hard handoff may fail in the entrance area  16  if the SNR of the current carrier frequency drops too fast. Referring to FIG. 2 again, since a hard handoff “drops” an old carrier frequency before it “grabs” a new one, the MU  22  would be in limbo. This is especially true if the current carrier frequency of BTS 1  hades after the MU has sent the PSMM to BTS 1   24  but prior to receiving the EHDM from BTS 1   24 . Consequently, the EHDM from BTS 1  can never be received, and the handoff can never occur. For instance, it is likely that the MU has passed the outbound pilot beacon distributor  20  on its way out while still awaiting the EHDM message from the BTS 1 . Thus, referring back to FIG. 5, it is quintessential to provide a strong coverage of the current carrier frequency F 1  in the area right of line  56 , as shown in the figure, and even to the right of the outbound pilot beacon distributor  20  to help outgoing traffic. This is also true to provide an enhanced coverage of F 2  in the entrance area left of line  58  and even left of the inbound pilot beacon distributor  18 . 
     Another problem associate with some embodiments is that the target carrier may fail to provide a strong signal coverage in the border area. If the MU can not “grab” the target carrier frequency when it is so instructed, then the call can also be dropped since a service gap is created during the handoff. 
     One way to solve the ping-pong phenomenon is to use directional beacon antennas having a very large front-to-back signal ratio so that the signal strength of the pilot beacon drops very fast in the back lope direction of the pilot beacon distributors. Also, the beacon antennas for the inbound pilot beacon distributor  18  and the outbound pilot beacon distributor  20  can be set far apart so that the area of uncertainty  60  created by the overlap of the coverages from both of them does not exist. However, this approach may be impractical in cases where the entrance area  16  is relatively small. 
     FIG. 6 shows one embodiment of the present invention using a directional antenna in a back lobe of the beacon antenna in a pilot beacon distributor to improve the performance of the disclosed system. This configuration not only eliminates the ping-pong phenomenon, but also provides solutions to the other two performance problems mentioned above, i.e., the fast SNR change and the weak target carrier frequency problems. 
     In the pilot beacon distributor,  18  or  20 , a directional beacon antenna having a high front-to-back signal ratio is chosen first. Another directional non-beacon antenna that does not have a high front-to-back signal ratio is also installed in a back lope direction of the beacon antenna. The coupling of these two antennas effectively reshapes the zone of uncertainty  60 . As shown in FIG. 6, for example, a directional antenna  62  is installed to cover the opposite direction, or the back lobe direction of the beacon antenna in the pilot beacon distributor  20 . The antenna  62  is a standard non-beacon antenna that uses F 1  as the carrier frequency, and it does not have a high front-to-back signal ratio. 
     As mentioned early, the parameter E c /I 0  represents the relative strength of the expected signal over the energy of overall signals received. A general mathematical formula as follows may clearly represent the relationship of the expected signal and the overall energy of signals received, and thus helps to understand the change done to curves  50  and  52  in FIG. 5 to form those in FIG.  6 :          (     Ec   /   Io     )     =       Signal   exp         Signal   exp     +     Signal   other                                
     Logically, for a pilot beacon transmitted by the pilot beacon distributor  20 ,            (     Ec   /   Io     )     Beacon     =         Signal   B       Signal   Total       =       Signal   B         Signal   B     +     Signal   NB                                  
     where Signal B  is the signal strength of the pilot beacon, Signal Total  is the total energy of all received signals, and Signal NB  is the signal strength of all non-beacon signals. And similarly for a non-beacon signal transmitted by the non-beacon antenna  62 ,            (     Ec   /   Io     )     NonBeacon     =       Signal   NB         Signal   NB     +     Signal   B                                
     In the back lobe of the pilot beacon distributor  20 , Signal NB  significantly increases due to the transmission of the non-beacon antenna  62 , which in turn increases the value of the denominator of the fraction and thus reduces the total ratio of the parameter E c /I 0  for the pilot beacon in this direction. In a graphical representation, curve  50  and  52  drop sharply between the phantom lines  56  and  58 . The intersection point of line  56  and curve  52  along with that of lines  58  and curve  50  represent two threshold points above which a handoff is triggered. By installing such a directional non-beacon antenna in the back lope direction of each beacon antenna in the pilot beacon distributors, the zone of uncertainty  60  no longer exists since it becomes an area that none of the pilot beacon distributors will trigger a hard handoff. As a result, the sharpness of the falling edge of curve  52  and the rising edge of curve  50  predominantly determines the precision of the handoff. 
     Another advantage of this configuration is that the location of lines  56  and  58  can be adjusted based on a careful selection of the directional non-beacon antennas  62  and  64 . 
     Another advantage of this arrangement is that by installing the directional antennas  62  and  64  for transmitting regular non-beacon signals, they could be used to boost the signal strength of their corresponding carrier frequencies. Thus, the non-beacon antenna  62  can be configured to use F 1  as its frequency. As a result, it can help the private network on the campus to fully support the wireless service to the entrance area until a hard handoff is triggered. Similarly, the non-beacon antenna  64  can use F 2  as its carrier frequency to boost the signals transmitted by the surrounding network too. For instance, when the coverage of the carrier frequency F 1  is strengthened in the entrance area, an exiting MU  22  is unlikely to have its wireless service dropped since it has ample time to receive the EHDM from its current BTS. Since the non-beacon antenna  62  does not have a high front-to-back ratio, its coverage extends to the right of the outbound pilot beacon distributor  20 , and thus the MU can be passing beyond the outbound pilot beacon distributor  20  and still receive the EHDM. 
     Still another advantage is that this arrangement eliminates the problem when a weak coverage of the target frequency abandons the MU and renders the handoff incomplete. The non-beacon antenna  62 , at the carrier frequency F 1 , also serves to enhance the coverage area of the target frequency F 1  for any MU entering the campus through the entrance. Likewise, the non-beacon antenna  64  at the carrier frequency F 2  boosts the signal strength of the surrounding network around the entrance for any outgoing MU to handoff properly. 
     FIG. 7 shows a layout of a fiber optical pilot beacon distribution system according to one embodiment of the present invention. For the campus  10  having a private network, a RF beacon source  70  distributes pilot beacons to the pilot beacon distributors  20  at every entrance of the campus through a plurality of fiber optical cables  72 . Existing fiber distribution systems that are commercially available, such as BriteCell manufactured by the Allen Telecom Company, can be used for the implementation of the present invention. The beacon source  70  can also be a beacon hub connecting to a beacon source  73  located outside the campus. The beacon source  73  can be installed among a plurality of private networks so that they can share the beacons of the same frequency through the beacon hub. This beacon sharing scheme further reduces the cost for installing the private network. 
     In some of the private networks, multiple carrier frequencies may be employed. It is also very likely that multiple carrier frequencies may be used by the surrounding networks. In order to carry out the precision handoff, a pilot beacon for each frequency is needed. One solution is to use commercially available pilot beacon units (PBU). However, if a private network has N numbers of carrier frequencies and the surrounding networks have M numbers of carrier frequencies, and the campus has K numbers of entrances, then the total number of PBUs that are necessary is (N+M)×K, which could add significant cost to the private network design. Unfortunately there has been no other alternative that is more reliable than using the pilot beacon to trigger the precision handoffs. 
     As it is also known in the art, various frequency hopping schemes are proposed to provide multiple pilot beacons. But the cost of such a scheme still remains expensive. What is needed is a simple and economical multiple-beacon generator. 
     FIG. 8 shows a design of an economical multiple-beacon generator for generating a plurality of pilot beacons. As mentioned above, in a complex communication network, multiple carrier frequencies may be employed both for the private network and the surrounding networks. Since a pilot beacon is needed for each carrier frequency at each location, this limitation can greatly and rapidly increase the cost of the entire fiber optical pilot beacon distribution system when the networks expand to have more carrier frequencies. In order to control the cost and accommodate the interaction between a multiple-frequency private network and a multiple-frequency surrounding network, installation of excessive numbers of PBUs must be avoided. It is noticed that when a plurality of pilot beacons are deployed, all of them carry identical information being transmitted at different frequencies. Naturally, as long as multiple copies of a pilot beacon are produced for different carrier frequencies, they can satisfy the needs of the system in accordance with the teachings of the present invention. 
     A method to generate multiple pilot beacons at different frequencies from a pilot beacon is disclosed below. In this disclosure, only one PBU is needed no matter how many carrier frequencies are involved and how many entrances exit. An original pilot beacon at one frequency F at the pilot beacon distributor  20  is fed into a splitter  76  which clones N copies of the pilot beacon S 0 , S 1  . . . Sn. Although these copies have a signal level 10×Log (N) dB lower than the original pilot beacon, the signal level is not a concern. Each of these clones, except S 0 , is further fed into a mixer  78 . The mixer  78  creates intermediate signals at slightly different frequencies around the original frequency F. Subsequently, a band pass filter  80  is used to eliminate unwanted image frequencies to render a resultant pilot beacon at a distinct carrier frequency. Therefore, the resultant set of pilot beacons F 1  . . . Fn, in conjunction with S 0  which is now F 0 , have different frequencies that will not interfere with each other. 
     It is understood that the application of the above described fiber optical pilot beacon distribution system is not limited to hard handoffs between a private network on a campus and its surrounding networks. It can be applied wherever a precise hard handoff is desired. 
     While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention, as set forth in the following claims.