Patent Publication Number: US-7720484-B2

Title: Proxy translator for extending the coverage area of a wireless network

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present invention is related to that disclosed in U.S. patent application Ser. No. 10/934,012, entitled “Proxy Mobile Station Using Assignable Mobile Identifier To Access a Wireless Network,” filed concurrently herewith. Patent application Ser. No. 10/934,012 is assigned to the assignee of the present application. The subject matter disclosed in patent application Ser. No. 10/934,012 is hereby incorporated by reference into the present disclosure as if fully set forth herein. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention generally relates to wireless communications and, more specifically, to a proxy translator for use in a wireless communication network. 
   BACKGROUND OF THE INVENTION 
   Wireless communication systems have become-ubiquitous in society. Consumers use a wide range of devices and networks, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks. Wireless service providers are creating new markets for wireless devices and expanding existing markets by making wireless devices and services cheaper and more reliable. Wireless service providers attract new customers by reducing infrastructure costs and operating costs, by increasing handset battery life, and improving quality of service, and new and better features. 
   Inadequate coverage is a persistent problem in the quality of service of any wireless network. Natural and man-made obstacles frequently create radio frequency (RF) “holes” in the coverage area of a wireless network. Voice and data call connections are frequently dropped when a wireless terminal, such as a cell phone or a similar mobile station, enters an RF hole. Mobile stations that are already in an RF hole may not be able to reliably establish new connections. Typical areas in which RF holes occur include underground tunnels, buildings that have large footprints, tall buildings, and underground shopping malls. 
   Wireless service providers may attempt to improve coverage and to eliminate RF holes by one or more conventional methods. A wireless service provider may use higher transmit power from the base transceiver station (BTS) to maintain communication with a mobile station (MS). However, the result is an increase in the noise floor experienced by other mobile stations in the coverage area of the serving BTS, leading to an overall increase in total BTS transmit power and a reduction in the percentage of transmit power available to support other calls. 
   Alternatively, a wireless service provider may attempt to improve coverage by installing another base transceiver system (BTS) in the area of poor coverage. A disadvantage of this method is the cost associated with acquisition and deployment of a complete base transceiver station. A typical full-service BTS may cost between $500,000 and $750,000. Adding another BTS also requires the added cost of provisioning a dedicated backhaul link to provide network connectivity. Still another disadvantage of adding a new BTS is that these devices are provider specific. Devices from two different vendors cannot be interchanged or operated in a given network. 
   Also, a wireless service provider may attempt to improve coverage by deploying RF repeater transceivers. Unfortunately, a repeater re-broadcasts the entire set of signals for all calls carried by the serving BTS, not just for those mobile stations in the poor coverage area. As a result, the mobile stations in the poor coverage area experience a higher noise floor. Thus, increased transmit power causes higher forward link transmit power per user channel on the serving BTS. In CDMA technology, the RF noise in the operating frequency is an important factor, especially in the forward link (i.e., transmission from BTS to MS). The repeater amplifiers add noise on both forward and reverse links. Thus, using repeaters degrades overall network performance by increasing noise in the environment and reduces the traffic capacity of the network. 
   Therefore, there is a need in the art for improved wireless networks having improved RF coverage. In particular, there is a need for an apparatus that can improve service in an area of poor RF coverage without significantly increasing transmit power in within the cell site or in neighboring cell sites. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and related method that extend the coverage area of a CDMA system into an area of high path loss (i.e., poor coverage) or into an area with high interference, such as multiple pilot signals from neighboring cells. The present invention comprises a proxy translator that mimics the operation of a base transceiver subsystem (BTS) in the forward channel and mimics the operation of mobile station (MS) in the reverse channel. In the forward channel, the actual BTS communicates with a proxy mobile station (PMS), which communicates internally in the proxy translator with a proxy BTS (PBTS). The proxy BTS, in turn, communicates with the actual mobile station. In the reverse channel, the actual mobile station communicates with the proxy BTS, which communicates internally with the proxy MS. The proxy MS, in turn, communicates with the actual BTS. 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless network, a proxy translator capable of retransmitting forward channel signals from a base station of the wireless network to a plurality of mobile stations accessing the wireless network. According to an advantageous embodiment of the present invention, the proxy translator comprises: 1) a plurality of proxy mobile stations capable of receiving forward channel signals transmitted by the base station, wherein a first of the proxy mobile stations receives and down-converts a first forward channel signal associated with a first target mobile station to thereby produce a first proxy signal; and 2) a proxy base transceiver subsystem capable of receiving the first proxy signal from the first proxy mobile station, up-converting the first proxy signal to thereby produce a second forward channel signal, and transmitting the second forward channel signal to the first target mobile station. 
   According to one embodiment of the present invention, a second of the proxy mobile stations receives and down-converts a third forward channel signal associated with a second target mobile station to thereby produce a second proxy signal. 
   According to another embodiment of the present invention, the proxy base transceiver subsystem is further capable of receiving the second proxy signal from the second proxy mobile station, up-converting the second proxy signal to thereby produce a fourth forward channel signal, and transmitting the fourth forward channel signal to the second target mobile station. 
   According to still another embodiment of the present invention, the proxy base transceiver subsystem is further capable of receiving a first reverse channel signal from the first target mobile station, down-converting the first reverse channel signal to thereby produce a third proxy signal. 
   According to yet another embodiment of the present invention, the first proxy mobile station is capable of receiving the third proxy signal from the proxy base transceiver subsystem, up-converting the third proxy signal to thereby produce a second reverse channel signal, and transmitting the second reverse channel signal to the base station. 
   According to a further embodiment of the present invention, the proxy base transceiver subsystem is further capable of receiving a third reverse channel signal from the second target mobile station, down-converting the third reverse channel signal to thereby produce a fourth proxy signal. 
   According to a still further embodiment of the present invention, the second proxy mobile station is capable of receiving the fourth proxy signal from the proxy base transceiver subsystem, up-converting the fourth proxy signal to thereby produce a fourth reverse channel signal, and transmitting the fourth reverse channel signal to the base station. 
   According to a yet further embodiment of the present invention, the proxy translator further comprises a controller capable of assigning the first proxy mobile station to process forward and reverse channel signals associated with the first target mobile station and capable of assigning the second proxy mobile station to process forward and reverse channel signals associated with the second target mobile station. 
   In one embodiment of the present invention, the controller programs the first proxy mobile station with a first Electronic Serial Number associated with the first target mobile station. 
   In another embodiment of the present invention, the controller programs the second proxy mobile station with a second Electronic Serial Number associated with the second target mobile station. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
       FIG. 1  illustrates an exemplary wireless network that implements a plurality of proxy translators according to the principles of the present invention; 
       FIG. 2  illustrates the exemplary proxy translators in  FIG. 1  in greater detail according to an exemplary embodiment of the present invention; 
       FIG. 3  is a message flow diagram illustrating the operation of a proxy translator during a mobile-terminated call operation according to one embodiment of the present invention; 
       FIG. 4  is a message flow diagram illustrating the operation of a proxy translator during a mobile-originated call operation according to another embodiment of the present invention; and 
       FIG. 5  is a message flow diagram  500  illustrating a handoff between two proxy translators according to the principles of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless network. 
     FIG. 1  illustrates exemplary wireless network  100 , implements a plurality of proxy translators according to the principles of the present invention. Wireless network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  communicate with a plurality of mobile stations (MS)  111 - 114  using, for example, the CDMA2000 air interface standard. In an advantageous embodiment of the present invention, mobile stations  111 - 114  are capable of receiving data traffic and/or voice traffic on two or more channels simultaneously. Mobile stations  111 - 114  may be any suitable wireless devices (e.g., conventional cell phones, PCS handsets, personal digital assistant (PDA) handsets, portable computers, telemetry devices) that are capable of communicating with base stations  101 - 103  via wireless links. 
   The present invention is not limited to communicating with mobile devices. The present invention also encompasses other types of wireless access terminals, including fixed wireless terminals. For the sake of simplicity, only mobile stations are shown and discussed hereafter. However, it should be understood that the use of the term “mobile station” in the claims and in the description below is intended to encompass both truly mobile devices (e.g., cell phones, wireless laptops) and stationary wireless terminals (e.g., a machine monitor with wireless capability). 
   Dotted lines show the approximate boundaries of cell sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions. 
   As is well known in the art, each of cell sites  121 - 123  is comprised of a plurality of sectors, where a directional antenna coupled to the base station illuminates each sector. The embodiment of  FIG. 1  illustrates the base station in the center of the cell. Alternate embodiments may position the directional antennas in corners of the sectors. The system of the present invention is not limited to any particular cell site configuration. 
   In one embodiment of the present invention, each of BS  101 , BS  102  and BS  103  comprises a base station controller (BSC) and one or more base transceiver subsystem(s) (BTS). Base station controllers and base transceiver subsystems are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver subsystems, for specified cells within a wireless communications network. A base transceiver subsystem comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces and RF transmitters and RF receivers. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver subsystems in each of cells  121 ,  122  and  123  and the base station controller associated with each base transceiver subsystem are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
   BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public switched telephone network (PSTN) (not shown) via communication line  131  and mobile switching center (MSC)  140 . BS  101 , BS  102  and BS  103  also transfer data signals, such as packet data, with the Internet (not shown) via communication line  131  and packet data server node (PDSN)  150 . Packet control function (PCF) unit  190  controls the flow of data packets between base stations  101 - 103  and PDSN  150 . PCF unit  190  may be implemented as part of PDSN  150 , as part of MSC  140 , or as a stand-alone device that communicates with PDSN  150 , as shown in  FIG. 1 . Line  131  also provides the connection path for control signals transmitted between MSC  140  and BS  101 , BS  102  and BS  103  that establish connections for voice and data circuits between MSC  140  and BS  101 , BS  102  and BS  103 . 
   Communication line  131  may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network packet data backbone connection, or any other type of data connection. Line  131  links each vocoder in the BSC with switch elements in MSC  140 . The connections on line  131  may transmit analog voice signals or digital voice signals in pulse code modulated (PCM) format, Internet Protocol (IP) format, asynchronous transfer mode (ATM) format, or the like. 
   MSC  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the PSTN or Internet. MSC  140  is well known to those skilled in the art. In some embodiments of the present invention, communications line  131  may be several different data links where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
   In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 . MS  113  is located in cell site  122  and is in communication with BS  102 . MS  114  is located in cell site  123  and is in communication with BS  103 . MS  112  is also located close to the edge of cell site  123  and is moving in the direction of cell site  123 , as indicated by the direction arrow proximate MS  112 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a hand-off will occur. 
   Natural and man-made obstacles create radio frequency (RF) holes in the coverage area of wireless network  100 . By way of example, RF hole  165  (indicated by dotted line) exists in cell site  121 . If MS  111  or MS  112  enters RF hole  165 , an existing voice call or data call connection may be dropped. Also, MS  111  or MS  112  may not be able to reliably establish new call connections. 
   Accordingly, to eliminate RF holes, such as RF hole  165 , and to extend coverage area, wireless network  100  further comprises proxy translator (PT)  161  and proxy translator (PT)  162 . PT  161  is disposed near the outer boundary of cell site  121  and extends the range of BS  101  to reach mobile stations that are in the vicinity of PT  161 , but just outside the coverage area of cell site  121 . Deploying PT  161  in this manner may be necessary if it would be prohibitively expensive to add a new cell site next to cell site  121 . PT  162  is disposed near the edge of RF hole  165  and improves coverage within RF hole  165 . Advantageously, PT  161  increases the strength of forward and reverse channel signals only in the vicinity of the outer edge of cell site  121  and PT  162  increases the strength of forward and reverse channel signals only in the vicinity of RF hole  165 . Thus, the amount of increased signal interference in adjacent cell sites  122  and  123  is minimal or non-existent. 
     FIG. 2  illustrates exemplary proxy translators  161  and  162  in greater detail according to an exemplary embodiment of the present invention. Since proxy translator (PT)  161  and proxy translator (PT)  162  are substantially identical, it is unnecessary and redundant to explain the operation of each PT separately. Therefore, the explanation of the present invention that follows will generally be limited to discussion of PT  162 . 
   PT  162  comprises proxy mobile station pool  210 , proxy base transceiver subsystem (BTS)  220 , and proxy translator controller  230 , and antenna  241 - 243 . Proxy translator controller  230  directs the overall operation of PT  162 . Proxy mobile station pool  210  comprises N proxy mobile stations, including exemplary proxy mobile stations  211 - 214 . Proxy mobile station (MS)  211 , proxy mobile station (MS)  212 , proxy mobile station (MS)  213 , and proxy mobile station (MS)  214  are arbitrarily labeled Proxy MS  1 , Proxy MS  2 , Proxy MS  3 , and Proxy MS n, respectively. 
   Proxy mobile stations  211 - 214  communicate with base station  101  via antenna  241 . Proxy BTS  220  communicates with mobile stations  251  and  252  via main antenna  242  and, optionally, via a receive diversity antenna  243 . Mobile station (MS)  251  and mobile station (MS)  252  are disposed in or near RF hole  165  (or, in the case of PT  161 , near or beyond the outer boundary of cell site  121 ). According to an advantageous embodiment of the present invention, each one of the proxy mobile stations in proxy mobile station pool  210  comprises a programmable (or configurable) RF transceiver and associated signal processing circuits that are capable of performing all of the functions of a conventional wireless mobile station, such as, for example, a CDMA2000 compatible cell phone or similar wireless terminal. 
   Proxy mobile stations  211 - 214  in proxy translator  162  are located at the edge of the region of good coverage and operate in conjunction with BS  101  to provide extended coverage in RF hole  165 . Each one of proxy mobile stations  211 - 214  is capable of serving as a proxy or substitute for MS  251  or MS  252  or other mobile stations in RF hole  165 . Each one of proxy mobile stations  211 - 214  communicates with BS  101  on the CDMA air interface on behalf of one of MS  251  or MS  252  or other mobile stations in RF hole  165 . Proxy BTS  220 , also placed at the edge of the region of good coverage, communicates with MS  251  or MS  252  or other mobile stations in RF hole  165  as a proxy or substitute for BS  101 . A selected one of proxy mobile stations  211 - 214  receives overhead channels from BS  101  and transfers the overhead channel signals to proxy BTS  220 , which transmits the overhead signals to MS  251 , MS  252 , and other mobile stations in RF hole  165 . 
   Proxy translator controller  230  assigns a proxy mobile station (e.g., proxy MS  211 ) as a substitute for each mobile station (e.g., MS  251 ) in RF hole  165 . During a call, the assigned proxy mobile station  211  communicates with BS  101  while proxy BTS  220  communicates with mobile station  251 . A data link between proxy BTS  220  and proxy MS  211  transfers the user data and control signals on the forward and reverse links. This mimics the operation of MS  251  if MS  251  were capable of direct communication with BS  101 . This enables wireless network  100  to treat MS  251  as a normal mobile station. Proxy translator controller  230  dynamically assigns and de-assigns the mobile station identifier (MS_ID) of MS  251  to proxy MS  211  as MS  251  enters and exits RF hole  165 . Thus, from the perspective of BS  101 , proxy MS  211  appears to be the same device as MS  251 . 
   The mobile station identifier (MS_ID) of MS  251  (or any other mobile station) is a fixed length (e.g., 32 bits, 64 bits) value or variable length value that uniquely describes the mobile station (or other wireless terminal). The MS_ID may comprise a conventional identifier, such as an Electronic Serial Number (ESN), a User Identity Module (UIM) ID, a Subscriber Identity Module (SIM) ID, or a Mobile Equipment Identifier (MEID), among others. 
   In the forward channel, proxy BTS  220  multiplexes and transmits only the user data information received from active (or assigned) proxy mobile stations in proxy mobile station pool  210 . Thus, only forward channel signals directed to MS  251 , MS  252  or other mobile stations in RF hole  165  are retransmitted. This results in much less transmit power in the forward channel compared to conventional repeaters, which retransmit all forward channels signals, including forward channels signals for mobile stations that are not in or near RF hole  165 . In the reverse channel, proxy BTS  220  receives reverse channel signals from MS  252 , MS  252  and other mobile station in or near RF hole  165 . The reverse channel signals are demodulated and transferred to the appropriate one of proxy mobile stations  211 - 214  for retransmission to BS  101 . 
   According to an exemplary embodiment of the present invention, control messages and user data traffic in the forward and reverse channels are transferred between proxy BTS  220  and proxy mobile stations  211 - 214  as proxy signals. For the purposes of this disclosure, the term “proxy signals” may include baseband signals, intermediate frequency (IF) signals, radio frequency (RF) signals, and any other form of processed signals that may be derived from the actual signals received by antennas  241 - 243 . For example, proxy BTS  220  may down-convert a reverse channel RF signal received from antenna  242  to a baseband signal, an IF signal, or another RF signal that is transferred to proxy mobile station  211 . Proxy MS  211  then up-converts the proxy signal from proxy BTS  220  for retransmission to BS  101 . Similarly, proxy MS  211  may convert a forward channel RF signal received from antenna  241  to a baseband signal, a IF signal, or another RF signal that is transferred to proxy BTS  220 . Proxy BTS  220  then up-converts the proxy signal from proxy MS  211  for retransmission to mobile stations  251  and  252 . 
   Proxy translator  162  transmits on the same frequency and pseudo-random noise (PN) offset as BS  101 . Proxy translator  162  is designed to utilize high front-to-back isolation of the antenna system such that forward channel signals re-transmitted to MS  251  and MS  252  do not interfere with the forward channel signals received from BS  101 . Because proxy translator  162  transmits on the same PN offset as BS  101 , a handoff operation to another base station remains the same as that of BS  101  under normal configuration. 
   If MS  251  is initially off and is activated when it is already in RF hole  165 , MS  251  initially detects the control signals (pilot, access, etc.) that are transmitted by proxy translator  162 . Since these control signals are the same as the control signals transmitted by BS  101 , MS  251  accesses proxy translator  162  in the same manner that MS  251  would access BS  101 . Thus, MS  251  transmits the mobile station identifier (e.g., ESN) associated with MS  251  as part of the normal process of accessing a base station under, for example, the CDMA2000 protocol). When proxy translator  162  receives the mobile station identifier from MS  251 , proxy translator  162  programs proxy mobile station  211  to use the same mobile station identifier to access BS  101 . Thus, proxy mobile station  211  appears the same to BS  101  as MS  251  would appear. 
   However, MS  251  may not always be activated after MS  251  is already in RF hole  165 . In many situations, MS  251  may roam into RF hole  165  after MS  251  has already accessed BS  101 . In this situation proxy translator  162  must obtain the mobile station identifier from MS  251  by some other means. In one embodiment of the present invention, when proxy translator  162  detects the present of MS  251 , proxy translator  162  may use a new special-purpose control channel message that prompts MS  251  to re-transmit its mobile station identifier. As an example of implementation for the CDMA2000 family of standards, changes may be made to existing CDMA2000 protocol messages, including base station-assigned messages such as the Extended Channel Assignment Message (ECAM), the Universal Handoff Direction Message (UHDM), and the In-Traffic System Parameters Message (ITSPM). The changes may be made to fields carrying the identifier information, either as a part of upper layer signaling or as a part of LAC addressing. To prevent the mobile station identifier information from being misused by hackers, the mobile station identifier may be encrypted before it is sent over the air. 
   In an alternate embodiment of the present invention, proxy translator may use a special-purpose traffic channel message that prompts MS  251  to re-transmit its mobile station identifier. By way of example, U.S. patent application Ser. No. 10/672,607, filed Sep. 26, 2003, entitled “System and Method for Providing Mobile Station Registration in a Traffic Channel in a Wireless Communication System” discloses a wireless network that uses a traffic channel to register a mobile station and to obtain an ESN from the mobile station. U.S. patent application Ser. No. 10/672,607, which is assigned to the assignee of the present application, is hereby incorporated by reference into the present disclosure as if fully set forth herein. 
     FIG. 3  depicts message flow diagram  300 , which illustrates the operation of proxy translator  162  during a mobile-terminated call operation according to an exemplary embodiment of the present invention. In  FIG. 3 , it is assumed that MS  251  has roamed into, or was activated within, the coverage area of proxy translator  162  and has already accessed wireless network  100  via PT  162 . Initially, BS  101  transmits Page message  301  in the forward channel to proxy mobile station  211  via the air interface. Proxy mobile station  211  then transmits Page message  302  as a proxy signal via wireline  350  to proxy BTS  220 . Finally, proxy BTS  220  transmits Page message  303  via the air interface to mobile station  251 . 
   Mobile station  251  responds in the reverse channel by transmitting Page Response message  304  via the air interface to proxy BTS  220 . Proxy BTS then transmits Page Response message  305  as a proxy signal via wireline  350  to proxy mobile station  211 . Finally, proxy mobile station  211  transmits Page Response message  306  via the air interface to base station  101 . Next, BS  101  transmits Channel Assignment message  307  in the forward channel to proxy mobile station  211  via the air interface. Proxy mobile station  211  then transmits Channel Assignment message  308  as a proxy signal via wireline  350  to proxy BTS  220 . Proxy BTS  220  then transmits Channel Assignment message  309  via the air interface to mobile station  251 . 
   BS  101  transmits Null Frames  310  in the forward channel to proxy mobile station  211  via the air interface. Proxy mobile station  211  then transmits Null Frames  311  as a proxy signal via wireline  350  to proxy BTS  220 . Proxy BTS  220  then transmits Null Frames  312  via the air interface to mobile station  251 . Mobile station  251  responds in the reverse channel by transmitting Preambles  313  via the air interface to proxy BTS  220 . Proxy BTS then transmits Preambles  314  as a proxy signal via wireline  350  to proxy mobile station  211 . Finally, proxy mobile station  211  transmits Preambles  315  via the air interface to base station  101 . 
   The message flow in  FIG. 3  continues for the duration of the call session, as conventional CDMA2000 messages are transmitted between MS  101  and MS  251  using PT  162  as an intermediary. It is not necessary to illustrate the remainder of the call session, however. 
     FIG. 4  depicts message flow diagram  400 , which illustrates the operation of proxy translator  162  during a mobile-originated call operation according to an exemplary embodiment of the present invention. In  FIG. 4 , it is assumed that MS  251  has roamed into, or was activated within, the coverage area of proxy translator  162  and has already accessed wireless network  100  via PT  162 . Initially, mobile station  251  transmits in the reverse channel by transmitting Origination message  401  via the air interface to proxy BTS  220 . Proxy BTS then transmits Origination message  402  as a proxy signal via wireline  350  to proxy mobile station  211 . Finally, proxy mobile station  211  transmits Origination message  403  via the air interface to base station  101 . 
   Next, BS  101  transmits Channel Assignment message  404  in the forward channel to proxy mobile station  211  via the air interface. Proxy mobile station  211  then transmits Channel Assignment message  405  as a proxy signal via wireline  350  to proxy BTS  220 . Proxy BTS  220  then transmits Channel Assignment message  406  via the air interface to mobile station  251 . 
   BS  101  transmits Null Frames  407  in the forward channel to proxy mobile station  211  via the air interface. Proxy mobile station  211  then transmits Null Frames  408  as a proxy signal via wireline  350  to proxy BTS  220 . Proxy BTS  220  then transmits Null Frames  409  via the air interface to mobile station  251 . Mobile station  251  responds in the reverse channel by transmitting Preambles  410  via the air interface to proxy BTS  220 . Proxy BTS then transmits Preambles  411  as a proxy signal via wireline  350  to proxy mobile station  211 . Finally, proxy mobile station  211  transmits Preambles  412  via the air interface to base station  101 . 
   The message flow in  FIG. 4  continues for the duration of the call session, as conventional CDMA2000 messages are transmitted between MS  101  and MS  251  using PT  162  as an intermediary. It is not necessary to illustrate the remainder of the call session, however. 
   Advantageously, the present invention does not retransmit all of the received RF signals from BS  101 . Instead, proxy BTS  220  and proxy MS  211  only retransmit signals for mobile stations within or near RF hole  165 . With a lower noise floor in the poor coverage area, the performance of the entire cell coverage by BS  101  is improved. Also, proxy translator  162  is able to communicate over the air with an existing base station and does not need any other network connections in order to function. 
   Since proxy mobile stations  211 - 214  imitate (or “spoof”) the control and traffic signals of actual mobile stations and proxy BTS  220  imitates the control and traffic signals of BS  101 , it is possible to daisy chain two or more proxy translators. For example, if proxy translator  161  is close enough to proxy translator  162 , proxy mobile stations  211 - 214  of proxy translator  161  may communicate with proxy BTS  220  of proxy translator  162 . Thus, at one end, proxy mobile stations  211 - 214  of proxy translator  162  would communicate with BS  101 . At the other end, proxy BTS  220  of proxy translator  161  would communicate with mobile stations  251  and  252 . In the middle, proxy mobile stations  211 - 214  of proxy translator  161  would communicate with proxy BTS  220  of proxy translator  162 . 
   Even if two proxy translators are not set up in a daisy chain configuration, it still is possible for two proxy translators to interoperate. For example, it is possible to perform a handoff of a mobile station from a first proxy translator to a second proxy translator. Such a handoff is described below with respect to  FIG. 5 . 
     FIG. 5  depicts message flow diagram  500 , which illustrates a handoff operation between two proxy translators according to the principles of the present invention. For example, if proxy translator  161  is close enough to proxy translator  162 , mobile station  251  could be handed off from proxy translator  162  to proxy translator  162 , or vice versa. In  FIG. 5 , it is assumed that MS  251  has roamed into, or was activated within, the coverage area of proxy translator  162  and has already accessed wireless network  100  via PT  162 . 
   In the operation depicted in  FIG. 5 , MS  251  is handed off from a first base transceiver subsystem (BTS 1 ) associated with BS  101  to a second base transceiver subsystem (BTS 2 ) associated with BS  101 . PT  162  communicates with BTS 1  and transmits the forward and reverse traffic channel signals and the control channel signals (including the pilot signal) associated with PT  162 . Similarly, PT  161  communicates with BTS 2  and transmits the forward and reverse traffic channel signals and the control channel signals (including the pilot signal) associated with PT  161 . 
   During routine operation, mobile station  251  transmits Pilot Strength Measurement message (PSMM)  501   a  to proxy BTS  220   a  in PT  162 . PSMM  501   a  reports the strengths of the pilot signals from PT  162  and PT  161 , as seen by MS  251 . Proxy BTS  220   a  transmits PSMM  501   b  to proxy mobile station  211   a , which transmits PSMM  501   c  to BTS 1 . Since the pilot signal from PT  161  is stronger than the pilot signal from PT  162 , BTS 1  transmits Handoff (HO) Request message  502  to the base station controller (BSC) of BS  101 . 
   In response to Handoff Request message  502 , the BSC transmits Assignment Request and Confirmation (ARC) message  503   a  to BTS 1 . BTS  1  then transmits ARC message  503   b  to proxy mobile station  211   a , which transmits ARC message  503   c  to proxy BTS  220   a . The BSC also transmits Handoff Direction message (HDM)  504   a  to BTS 1 . BTS  1  then transmits HDM  504   b  to proxy mobile station  211   a , which transmits HDM  504   c  to proxy BTS  220   a . Proxy BTS  220   a  then transmits HDM  504   d  to MS  251 . 
   In response, MS  251  transmits Handoff Completion message (HCM)  505   a  to proxy BTS  220   b  of PT  161 . Proxy BTS  220   b  transmits HCM  505   b  to proxy mobile station  211   b , which transmits HCM  505   c  to BTS 2  in BS  101 . Finally, BTS 2  transmits HCM  505   d  to the BSC, thereby completing the handoff of MS  251  from PT  162  to PT  161 . 
   Those skilled in the art will understand that the handoff procedure described in  FIG. 5  is not limited to handoffs between base transceiver subsystems associated with the same base station. 
   Those skilled in the art will be able to adapt the message flow in  FIG. 5  to provide an alternate embodiment which performs a handoff from a first BTS associated with a first base station to a second BTS associated with a second base station via the mobile switching center (MSC) of a wireless network. Such an alternate embodiment will still fall within the scope of the present invention. 
   Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.