Patent Publication Number: US-2007123258-A1

Title: Mobile terminal autonomous handoff for unidirectional mobile communication

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method and a mobile receiver for autonomous handoff control at the mobile receiver in support of unidirectional communication between servers and the mobile receiver.  
      2. Description of the Related Art  
      Mobile communication networks use Radio Base Stations (RBS) as servers to provide bidirectional communication with mobile terminals. Bidirectional communication provides support for traditional voice services and for recent advances such as packet data services or videoconferencing. A small number of unidirectional services are available. For example, broadcast Short Message Service (SMS) can be used to send a short alphanumeric message towards an unlimited number of mobile terminals. However, the broadcast SMS can only be sent on a broadcast channel with a typical maximum length in the order of one or two hundred alphanumeric characters, depending on the air interface standard in use. Hence, unidirectional service capabilities of mobile communication networks are severely limited.  
      Reference is now made to  FIG. 1 , which shows a mobile terminal  100  and two proximally located servers, or RBSs  110  and  120 , as known in the art of mobile cellular communications. Both RBSs transmit on at least two channels, at least one channel per RBS being used for broadcast of parameters, respectively referenced by numerals  112  and  122 , and at least one but preferably a plurality of channels per RBS being used as downlink traffic channels, also called forward traffic channels, respectively referenced by numerals  114  and  124 . Depending on the air interface in use, the at least two channels per RBS may operate using distinct frequencies, distinct timeslots or distinct channel codes, or combinations of frequencies, timeslots and codes. Those of ordinary skills in the art will appreciate that the hereinabove description of  FIG. 1 , which mentions at least one channel for broadcast and at least one channel for downlink traffic, relate to a case of omni directional transmission by the RBSs wherein one RBS has one coverage area or cell. As it is well known in the art, RBSs may comprise directional antennas to provide sectorized coverage wherein each sector comprise a coverage area for at least one broadcast channel and at least one downlink traffic channel. Hence, a single RBS may provide one or several coverage areas or cells.  
      To support voice or data traffic, the RBS  110  or  120  allocates a traffic channel pair to a given mobile terminal  100 , said traffic channel pair comprising a downlink traffic channel and an uplink traffic channel. While it is receiving on the downlink traffic channel  114  or  124 , the mobile terminal  100  also transmits towards the RBS on an uplink traffic channel  118  or  128 , also called reverse link traffic channel. The uplink traffic channel  118  or  128  is of a same nature as that of the downlink traffic channel  114  or  124 . Generally, the uplink and downlink traffic channels are separated by a fixed frequency separation. In some air interfaces supporting Time Division Duplex (TDD), the uplink and downlink traffic channels share a same frequency, but occupy distinct timeslots. Bidirectional communication is achieved by allocating to the mobile terminal  100  a downlink traffic channel  114  or  124  paired with an uplink traffic channel  118  or  128 .  
      In the case of Time Division Multiple Access (TDMA) RBSs, each cell comprises a control channel  112  or  122  on a first timeslot of a first radio frequency, the control channel being used for broadcasting parameters. Each cell further comprises at least one downlink traffic channel  114  or  124 , which is either found on a second timeslot of the same first radio frequency, or on a timeslot of another radio frequency. Each downlink traffic channel  114  or  124  is paired with an uplink traffic channel  118  or  128 . Two or more TDMA cells comprise distinct frequency sets, in order to avoid interference. The mobile terminal  100  is capable of tuning on any frequency supported by the two cells and is further capable of selecting any timeslot on any such frequency. The mobile terminal  100  is capable of decoding parameters broadcasted on the control channels  112  and  122  as well as decoding a content of the downlink traffic channels  114  and  124 . The parameters broadcasted on the control channel  112  or  122  of each cell comprise a list of measurement channels of the neighboring cells. The control channels  112  and  122  can send information dedicated to a given mobile terminal, comprising for example a traffic channel designation used to assign the downlink traffic channel  114  or  124 , paired with the uplink traffic channel  118  or  128 , to the mobile terminal  100  at the beginning of a call. The content of the uplink and downlink traffic channels typically comprise voice or data, but the downlink traffic channels  114  and  124  also comprises a list of measurement channels of the neighboring cells. A measurement channel of a cell can be any radio frequency continuously transmitted on that cell. The measurement channel is typically, but not necessarily, the frequency that carries the control channel  112  or  122  for that cell.  
      In the case of Code Division Multiple Access (CDMA) RBSs, each cell comprises a pilot channel  116  or  126 , at least one paging channel  112  or  122  and at least one downlink traffic channel  114  or  124 . Each downlink traffic channel  114  or  124  is paired with an uplink traffic channel  118  or  128 . The pilot channels  116  and  126  are used to help the mobile terminal  100  in quickly locating the cell. The paging channels  114  and  124  are used for broadcasting parameters. In a cell, on a same frequency, the pilot channel  116  or  126 , one or several paging channels  112  or  122  and one or more downlink traffic channels  114  or  124  paired with uplink traffic channels  114  or  124  are distinguished by use of channelization codes (not shown), such as for instance Walsh codes or mobile specific pseudo random noise sequences. Downlink and uplink transmission for two or more CDMA cells is typically on a same frequency pair, but cells that share the same frequency pair are separated by distinct cell specific pseudo random sequence codes (not shown), in order to avoid interference. The mobile terminal  100  is capable of tuning on any frequency supported by the two cells and is further capable of selecting any channel based on the channel&#39;s channelization code and pseudo random sequence, on any such frequency. The mobile terminal  100  is capable of decoding parameters broadcasted on the paging channels  112  and  122  as well as decoding a content of any downlink traffic channel  114  and  124 . The parameters broadcasted on the paging channel  112  or  122  of each cell comprise a list of pilot channels  116  and  126  of the neighboring cells. The paging channels  112  and  122  can also send information dedicated to a given mobile terminal, comprising for example a traffic channel designation used to assign the downlink traffic channel  114  or  124  paired with the uplink traffic channel  118  or  128  to the mobile terminal  100  at the beginning of a call. The content of the uplink and downlink traffic channels typically comprises voice or data, but the downlink traffic channels  114  and  124  also comprises a list of pilot channels  116  and  126  of the neighboring cells. In CDMA, the pilot channels  116  and  126  of the neighboring cells are effectively used as measurement channels for those cells.  
      Whether communication between RBSs and the mobile terminal  100  is effectuated by use of TDMA or CDMA, the mobile terminal  100  has a capability of evaluating the signal strength of downlink channels in the cell it is currently selecting as well as the signal strength of downlink channels in the neighboring cells. When the mobile terminal  100  is busy, receiving a downlink traffic channel  114  or  124  paired with an uplink traffic channel  118  or  128  on which it is transmitting, it conventionally provides signal strength measurements to the mobile communication network. The mobile terminal  100  transmits its signal strength measurements on the reverse traffic channel  118  or  128 . The signal strength measurements are used, in a concept known as Mobile Assisted Hand Off (MAHO), as a means to assist cell reselection for the mobile terminal  100  by the mobile communication network. A network node of the mobile communication network, in control of the RBSs, takes any handoff decision and allocates a traffic channel pair in a new cell to the mobile terminal  100 , based in part on the MAHO measurements.  
      Unidirectional transmission from a RBS may be received by an unlimited number of mobile terminals. However, unidirectional traffic channel designation to those mobile terminals is not specified in the prior art. As well, cell reselection when those mobile terminals move around remains a problem since the MAHO concept relies on bidirectional communication between servers and mobile terminals. MAHO cannot support a system wherein unidirectional transmission would be effectuated from the servers towards a mobile receiver. There would be clear advantages of having a method and a mobile receiver for supporting unidirectional traffic channel assignment and cell reselection where unidirectional service can be provided from servers towards the mobile receiver.  
     SUMMARY OF THE INVENTION  
      It is therefore a broad object of this invention to provide a method and a mobile receiver for effectuating unidirectional traffic channel assignments for communication service in the downlink direction only, wherein server reselection decisions are made within the mobile receiver, based on information provided to the mobile receiver about servers. Such method and mobile receiver are used for unidirectional services between the servers and the mobile terminal and are of particular interest for large contents or for continuously transmitted contents.  
      A first aspect of the present invention is directed to a method for unidirectional transmission service from fixed servers to a mobile receiver. The mobile receiver receives on a broadcast channel a traffic channel designation for an unpaired downlink traffic channel. While the mobile receiver receives data on the unpaired downlink traffic channel, it also receives information on the neighboring servers in the form of a measurement channel for each server. The mobile receiver compares a signal quality of the first server with those of the neighbor servers and autonomously reselects a new server with better signal quality. On the new server, the mobile receiver receives a new unpaired downlink traffic channel designation. The method for unidirectional transmission from fixed servers to the mobile receiver is applicable to TDMA and to CDMA air interfaces. Neighbor servers may transmit the same or distinct data, as applicable to an application of the unidirectional service.  
      In a second aspect of the present invention, the mobile receiver additionally receives on the unpaired downlink traffic channel a downlink traffic channel identity for each of the neighbor servers. The mobile receiver upon reselection of a new server uses the downlink traffic channel identity for that new server to select the unpaired downlink traffic channel.  
      In a third aspect of the present invention, the mobile receiver uses signal strength measurements on downlink channels to evaluate signal quality of the servers.  
      In a fourth aspect of the present invention, the mobile receiver averages signal strength measurements for a brief period before making comparisons and adds a hysteresis to the present server&#39;s signal strength.  
      In a fifth aspect of the present invention, though the mobile receiver is capable of bidirectional transmission, it may send a request to receive a designation for an unpaired downlink traffic channel.  
      A sixth aspect of the present invention is directed to a CDMA or TDMA mobile receiver for receiving unidirectional transmission from fixed servers. The mobile receiver comprises a selection logic used for selecting a first server and for selecting a neighbor server when the neighbor server is received with higher signal quality than the first server. The mobile receiver comprises a decoder for decoding data. Data may take the form of voice, video or packet data services. The mobile receiver optionally comprises means for bidirectional communication and, if such means are comprised, a switch for turning on or off transmission capabilities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more detailed understanding of the invention, for further objects and advantages thereof, reference can now be made to the following description, taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  (Prior Art) shows a mobile receiver and two proximally located Radio Base Stations;  
       FIG. 2  shows an exemplary mobile receiver built according to preferred embodiment of the present invention;  
       FIG. 3   a  shows an exemplary method implementing the preferred embodiment of the present invention; and  
       FIG. 3   b  shows a variant of the method implementing the preferred embodiment of the present invention;  
       FIG. 3   c  shows another variant of the method implementing the preferred embodiment of the present invention wherein an optional request for unidirectional service is sent by another exemplary mobile receiver; and  
       FIG. 4  shows an exemplary use of the method and of the mobile receiver of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The innovative teachings of the present invention will be described with particular reference to various exemplary uses and aspects of the preferred embodiment. However, it should be understood that this embodiment provides only a few examples of the many advantageous uses of the innovative teachings of the invention. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed aspects of the present invention. Moreover, some statements may apply to some inventive features but not to others. In the following description, the terms “Radio Base Station (RBS)” and “server” may be used to represent the same concept of a node comprising at least one broadcast channel and at least one traffic channel providing service over a same coverage area or cell.  
      Mobile networks are increasingly used for new services that were not predicted at the time when conventional concepts such as Mobile Assisted Hand Off (MAHO) were defined. One service concept that could be of great interest is the capability to broadcast data on a continuous basis, intended to a plurality of concurrent users. Examples of such data may comprise video, audio information such as news, tourist information, music and the like. Such capability could use unidirectional transmission wherein a downlink traffic channel is used without being paired with a reverse, or uplink, traffic channel. The present invention provides a solution for enabling cell reselection, or handoff, from a first cell to a neighboring cell while using unidirectional transmission from a RBS to a mobile receiver. The present invention is especially applicable in cases where the content to be broadcasted is either large or continuous, as prior art methods such as broadcast Short Message Service (SMS) are not well suited to carry such content. Because mobile receivers are not transmitting, they do not generate interference. Also, unidirectional transmission from a server may be received by any number of mobile receivers. A single server built according to the preferred embodiment of the present invention can thus serve an unlimited number of mobile receivers.  
      The present invention provides a method and a mobile receiver for providing a content, for instance an audio signal, streaming video or packet data, to a user of a mobile receiver without the use of a reverse traffic channel. The invention supports both mobile terminals capable of bidirectional communication and mobile terminals that only have receiving capabilities. Since a receive-only capable mobile receiver does not send any request for expressly requesting unidirectional service from the RBS, a broadcast channel repetitively sends unpaired downlink traffic channel designations for use by an eventual mobile receiver ready to receive service. In the case of a bidirectional capable mobile receiver, either the broadcast channel unpaired downlink traffic channel designation, which is intended to a plurality of mobile users, or an unpaired downlink traffic channel designation dedicated to this specific bidirectional capable mobile terminal may be used. As the user moves around, reception quality from the unpaired downlink traffic channel may degrade and a handoff to a new server may be appropriate. Moreover, as the user moves, it may become advantageous to change the content of the unidirectional service of interest to the user. For instance, in a case where the user is located proximate to a first server, he/she receives a content from said first server. If the user moves closer to a second server, he/she would desire to receive a content from the second server. In order to maintain unidirectionality of the communication, the well-known handoff mechanisms of conventional mobile cellular networks are replaced in the present invention by an autonomous process effectuated in the mobile receiver. The mobile receiver makes autonomous decisions to select the most appropriate server among a list of available servers. The autonomous decision process relies on signal quality evaluation capabilities of the mobile receiver.  
      Referring once again to  FIG. 1 , which shows a mobile receiver and two proximally located Radio Base Stations, the mobile terminal  100  has a capability of evaluating the signal strength of downlink channels in the cell it is currently selecting as well as the signal strength of downlink channels in the neighboring cells. Conventionally, as the mobile terminal  100  moves around between cells, reselection of the best connection is effectuated, unbeknownst to the user. Two distinct cases apply and are based on principles that are mostly common to Code Division Multiple Access (CDMA) and to Time Division Multiple Access (TDMA) mobile communication.  
      Firstly, while the user is not involved in a call, the mobile terminal  100  is in standby mode. In standby mode, the mobile terminal  100  is monitoring the broadcast information received on the TDMA control channel  112  or  122  or on the CDMA paging channel  112  or  122 . At the time, communication is unidirectional most of the time, unless a brief event happens such as when the user initiates a call or when the mobile terminal  100  responds to a dedicated message, such as a page. In standby mode when none of those events occur, the mobile terminal  100  is only receiving and is not transmitting. By not transmitting, the mobile terminal  100  is also not generating any radio interference. At the same time, while in standby mode, the mobile terminal  100  evaluates the received signal strength on a downlink channel in the present cell as well as that of the neighboring cells. A TDMA mobile terminal  100  evaluates the signal strength of the present cell&#39;s control channel  112  and  122  and the signal strength of the neighboring cells&#39; measurement channels. A CDMA mobile terminal  100  evaluates the present cell as well as neighbor cells by measuring the signal strength on their respective pilot channels  116  and  126 , which are used in CDMA as measurement channels. As the user moves around between coverage areas of cells while in standby mode, if the mobile terminal  100  determines that the signal strength of a new cell has become stronger than the signal strength of the present cell, the mobile terminal  100  selects the new cell and starts monitoring the new cell&#39;s broadcast information.  
      Secondly, while the user is in an active voice or data call, it holds a dedicated traffic channel pair, said dedicated traffic channel pair being unavailable to serve any other mobile terminal  100  within a large area comprising many cells. While holding a dedicated traffic channel pair, the mobile terminal  100  is busy. In TDMA, the dedicated traffic channel pair comprises a timeslot on a downlink frequency and the same timeslot of a corresponding uplink frequency, said timeslot and said frequencies forming a combination dedicated to that mobile terminal  100 , said combination not being reused in any surrounding cells. In CDMA, the dedicated traffic channel pair comprises a frequency pair shared between many users and preferably shared between many cells, a pair of channelization codes, one in the forward link direction and one in the reverse link direction, dedicated to that mobile terminal  100  in that cell, and a pseudo random sequence specific to the cell. Because the number of available traffic channels is limited, the dedication of traffic channels to specific mobile terminals for the purposes of specific calls is one of the main factors limiting the traffic capacity of mobile networks. While busy, the mobile terminal  100  decodes the content received on the TDMA downlink traffic channel  114  or  124  or on the CDMA downlink traffic channel  114  or  124 . The mobile terminal  100  also transmits towards the RBS on the TDMA or CDMA uplink traffic channel  118  or  128 . Because it is transmitting on the uplink traffic channel  118  or  128 , the mobile terminal  100  generates radio interference. In a mobile network comprising a large number of mobile terminals, many simultaneous reverse link traffic channel  118  and  128  transmissions from mobile terminals interfere with each other. This interference phenomenon is another main factor limiting the traffic capacity of the mobile network. While receiving on the dedicated forward link traffic channel  114  or  124 , the mobile terminal  100  evaluates the received signal strength in the present cell as well as that of the neighboring cells. As a busy terminal  100  moves around between coverage of cells while busy, it transmits on the reverse link traffic channel information regarding the signal strength of the present cell and the signal strength of the neighboring cells, said information being based on measurements made in a manner that is essentially the same as the hereinabove described method used for making neighboring cell measurements while in standby mode. Despite the capability of the mobile terminal  100  to make measurements of signal quality in the present cell and in surrounding cells, the mobile terminal  100  is not allowed to make a decision on a cell reselection. The reasons are two-fold. First, the mobile terminal  100  does not know if any free traffic channel pair is available in the new cell as well as it does not know the identity (frequency and timeslot combination in the case of TDMA, frequency, channelization code and pseudo random sequence combination in the case of CDMA) of any traffic channel pair eventually available in the new cell. If the mobile terminal  100  was allowed to autonomously select a new cell, it would have to make a request to obtain a designation for a new traffic channel pair. This step would cause a pause of several seconds in the communication, which would be detrimental to most voice or data services. Second, as the mobile terminal  100  generates reverse link interference, it cannot make any estimation of an impact in terms of reverse link interference caused by a choice of a specific traffic channel pair. The mobile communication network is the only entity that is capable of making a full assessment of the impacts on interference of allocating traffic channel pairs to mobile terminals. As a result, it is conventional to use mobile terminal  100  for providing signal strength measurements solely as a means to assist cell reselection by mobile communication networks, by use of Mobile Assisted Hand Off (MAHO). A network node in control of the RBSs takes any handoff decision based in part on the MAHO measurements of the mobile terminal  100 . When the network node has taken the handoff decision, a designation for a traffic channel pair is sent to the mobile terminal  100 .  
      Referring now to  FIG. 2 , an exemplary mobile receiver  200  built according to the preferred embodiment of present invention is shown. The mobile receiver  200  comprises a receiver  210 , a selection logic  220  and a speech decoder  230 . The mobile receiver  200  may optionally comprise a transmitter  240 , a switch  250  and a speech coder  260 , as these optional elements bring to the mobile receiver  200  transmission capabilities enabling it to alternate between receive-only or transmit-receive modes. The mobile receiver  200  may further comprise many more components of a standard mobile terminal, as well known by those of ordinary skills in the art. In the case where the mobile receiver  200  comprises the optional transmitter  240 , switch  250  and speech coder  260 , the mobile receiver may additionally operate as a standard mobile terminal for bidirectional communication with a mobile network. When the switch  250  is in transmit-receive mode, the transmitter  240  and the speech coder  260  are enabled and the mobile receiver  250  operates as any standard mobile terminal. When the switch  250  is in receive-only mode, the mobile receiver  250  operates in unidirectional reception mode according to the teachings of the present invention.  
      The receiver  210  may be a conventional receiver as described by the IS-136 TDMA air interface standard, by the GSM air interface standard which also is a form of TDMA, by the IS-95 CDMA air interface standard, by the CDMA200 air interface standard, or by the Wideband Code Division Multiple Access (WCDMA) air interface standard. The receiver  210  receives the downlink traffic channel. Likewise, the transmitter  240  and the speech coder  260 , if included, are conventional and are also built according to the relevant air interface standard. The optional transmitter  240 , when present, may be used to transmit on the uplink traffic channel, otherwise known as reverse traffic channel, when the mobile receiver  200  is used for other services than the unidirectional service. The speech decoder  230  is also conventional.  
      The selection logic  220  used for cell selection differs from a conventional means found in a standard mobile terminal, such means normally being used for MAHO. If the mobile receiver  200  comprises the optional transmitter  240  and the optional switch  250 , the selection logic  220  comprises MAHO capabilities. In any case, the selection logic  220  further comprises capabilities store server information comprising a measurement channel for each server and to autonomously determine when a server shall be selected, both for receiving a broadcast channel and for receiving an unpaired downlink traffic channel. Selection of the server is accomplished by an evaluation of a quality of the servers. Quality of the servers may be evaluated by use of signal strength measurements on downlink channels, analysis of signal-to-noise ratio on received signals, measurement of bit-error-rates, and the like.  
      While  FIG. 2  shows a terminal comprising the speech decoder  230  and the optional speech coder  260 , the terminal may comprise other mechanisms to encode, decode or otherwise use and process traffic channel information. Instead of or in addition to the speech decoder  230  and speech coder  260 , the mobile receiver  200  may comprise a video decoder and a video coder, or a packet handler for treatment of received packet data information and, if the mobile terminal  200  is bidirectional capable, for preparation of packet data information to be transmitted.  
      Referring now to  FIG. 3   a  which shows an exemplary method implementing the preferred embodiment of the present invention, the mobile receiver selects a downlink broadcast channel at step  300 . Particulars of step  300  vary slightly depending on whether the mobile receiver is a standard mobile terminal or whether the mobile receiver comprises receive-only capabilities. The selection logic  220  operates, when the mobile receiver  200  is not receiving data on a traffic channel, to select the broadcast channel with the highest signal quality among possible broadcast channels. The mobile receiver  200  may find a broadcast channel according to general principles for selecting broadcast channels as described in the air interface standard from which it borrows its elements, said air interface standard comprising any of TDMA IS-136, GSM, CDMA2000, CDMA IS-95 or WCDMA. Alternatively, the selection logic  220  may comprise a pre-set list of possible broadcast channels; the list of possible pre-set broadcast channels typically comprises a small number of broadcast channels specific to cells of an application, such as for example a private network using the unidirectional reception capability of mobile receiver  200 . In this case, the receiver  210  simply monitors all broadcast channels sequentially to select a first broadcast channel with a good signal quality, as for example a first broadcast channel with sufficient signal strength for proper reception. The mobile receiver  200  may receive on the first broadcast channel a list of neighboring broadcast channels that the terminal may monitor. The mobile receiver  200  then compares the signal quality of each broadcast channel, on the present cell as well as on the neighboring cells. If the mobile receiver  200  determines that another server provides a better signal quality, it tunes to a broadcast channel of that server. Eventually, comparisons of signal quality on broadcast channels leads the mobile receiver  200  to select the broadcast channel at step  300 .  
      Considering now step  315 , the mobile receiver  200  decodes the broadcast channel content. The broadcast channel may transmit an optional parameter indicating to the mobile receiver  200  that the cell supports downlink unidirectional transmission on at least one traffic channel. In the case of a fully private network wherein all mobile receivers  200  are used for such unidirectional transmission service and wherein the cell only support said type of service, the unidirectional transmission is implied and the parameter indicating unidirectional transmission is not required.  
      To receive unidirectional service, the mobile receiver  200  needs to get access to an unpaired downlink traffic channel. This can be accomplished in various manners. In one aspect of the present invention particularly suited to a network wherein the cell only provides unidirectional service, the broadcast channel periodically transmits an unpaired downlink traffic channel designation message, said message having the particularity of not being dedicated to any specific user, rather being intended to any mobile receiver  200  able to use the unidirectional service. The unpaired downlink traffic channel designation comprises a frequency and a timeslot in the case of TDMA, or a frequency, an orthogonal code and a pseudo-random sequence in the case of CDMA. The periodicity of the unpaired downlink traffic channel designation is selected so that the user will not need to wait for an excessive length of time before getting access to the unidirectional service, also considering that an excessive rate of sending the unpaired downlink traffic channel designation would require an excessive amount of bandwidth on the broadcast channel. The periodicity may be for example in a range of every few tenths of a second to every few seconds. In the case of periodic unpaired downlink traffic channel designations on the broadcast channel, the mobile receiver  200  simply waits while receiving the broadcast channel until it detects the unpaired downlink traffic channel designation. Upon receiving the unpaired downlink traffic channel designation, the mobile receiver  200  selects the unpaired downlink traffic channel.  
      Information is required at the mobile receiver  200  to specify that the traffic channel is to be used in the forward link only. In a case where the cells and the mobile receivers  200  form a private network independent from any other network, the information that all traffic channel communication is unidirectional in the forward link, or downlink, is implemented by pre-programming of the mobile receivers  200 . Unidirectionality may also be implied by the sheer fact that all mobile receivers  200  are only capable of unidirectional communication in the private network. In an alternate aspect of the present invention particularly suited to provide unidirectional service as an optional feature in a standard mobile network also capable of bidirectional service, the broadcast channel may transmit a designation of a traffic channel, said designation comprising a specific indication that the traffic channel is unidirectional, used in the forward link only. The unpaired downlink traffic channel designation may be intended to one specific mobile receiver  200 , to a specific group of mobile receivers  200  or to any mobile receiver  200  capable of unidirectional communication. Since the indication is provided to the mobile receiver  200  by the addition of a parameter specifying unidirectional transmission, the traffic channel designation message may differ from a standard traffic channel designation message.  
      Having received the unpaired downlink traffic channel designation by any of the aforementioned methods at step  315 , the mobile receiver  200  selects the unpaired downlink traffic channel at step  320  and starts receiving the content of the unpaired downlink traffic channel at step  325 . Any optional transmitter  240  and speech coder  260  in the mobile receiver  200  are switched off or otherwise inactivated. The optional switch  250 , if present, is set in the receive-only mode. Still at step  325 , the mobile receiver  200  receives data, for instance a voice recording, which is decoded in the speech decoder  230 . At the same time, the mobile receiver receives neighbor server information on the unpaired downlink traffic channel in the form of a neighbor server measurement channel list. In an alternate aspect of the preferred embodiment, in the case of a private network, the aforementioned list of pre-set broadcast channels implemented in the selection logic  200 , as described at step  300  above, may be used as a substitute for the neighbor server measurement channel list received on the unpaired downlink traffic channel.  
      At step  330 , the mobile receiver  200  compares a received signal quality on the present cell with signal quality values of neighboring cells. In the case of TDMA, the selection logic  220  considers a received signal quality on the unpaired downlink traffic channel that the receiver  210  currently receives. In the case of CDMA, the selection logic  220  considers a pilot signal quality on the cell also comprising the unpaired downlink traffic channel that the receiver  210  currently receives. The selection logic  220  also considers signal quality estimations made at the receiver  210  on measurements channels of neighboring cells. Estimations of signal quality of the present cell and of signal quality of the neighboring cells may be done in the same manner as a standard mobile terminal built according to the relevant TDMA or CDMA air interface standard, by use of signal strength measurements. Estimations of signal quality of the present cell and of signal quality of the neighboring cells may also be done by other means such as estimation of bit-error rates, signal to noise-ratio, distortion measurements and the like. In the case of TDMA, any radio channel may be a measurement channel. In the case of CDMA, a measurement channel for a cell is a pilot channel in that cell. Preferably, signal quality estimates are averaged over a brief pre-defined period of time in order to compensate for radio propagation effects such as shadowing and Rayleigh fading and for imprecise measurements. Averaging may be made over a brief pre-defined period corresponding to a small number of TDMA frames or CDMA frames, usually on the order of a few tenths of a second up to one second. Preferably, when signal strength comparisons are used, these are made factoring in a hysteresis value, added to the signal strength value for the present cell. The hysteresis value used to avoid rapid reselections back and forth between cells received with similar signal strengths is, for example, in a range of 1 to 3 dB. If the current cell still provides the best signal strength, there is no handoff condition and the process continues at step  325 . If a neighbor server provides better signal quality, there is a handoff condition found at step  330 . The process moves to step  335  where the mobile receiver  200  selects the broadcast channel of the new cell. The process then continues at step  315  where the mobile receiver  200  waits for a periodic unpaired downlink traffic channel designation. Alternatively, if the system does not support periodic unpaired downlink traffic channel designations, a bidirectional capable mobile receiver  200  sends a request on the broadcast channel to receive a dedicated unpaired downlink traffic channel designation.  
      In conventional networks using conventional methods, a traffic channel is dedicated to each user. Any number of mobile receivers may at once receive the same data on the unpaired downlink traffic channel of the present invention. Hence, in a given cell, one single high bandwidth traffic channel may be allocated for unidirectional transmission from the server towards a large number of mobile receivers. Because only one such high bandwidth unpaired downlink traffic channel may be required, a high level of error detection and error correction encoding may be added to the data. As a result, data can be reliably transmitted to the mobile receivers without the aid of reverse link feedback from the mobile receivers.  
      Referring now to  FIG. 3   b , a variant of the method implementing the preferred embodiment of the present invention is shown. Steps  300 ,  315 ,  320  and  330  are the same as those shown in  FIG. 3   a . At step  375 , the mobile receiver  200  receives the same data and the same neighbor server measurement channel list, as described in the aforementioned description of step  325 . In addition, the mobile receiver  200  receives at step  375  additional neighbor server information in the form of downlink traffic channel identities for each of the neighboring cells. The downlink traffic channel identity comprises the same information as the unpaired downlink traffic channel designation message described at step  315 . The mobile receiver  200  does not immediately use it for selecting a downlink traffic channel. The neighboring cell data format of step  375 , with the addition of downlink traffic channel identities, comprises all required information for the mobile receiver  200  to select the unpaired downlink traffic channel of each of the neighbor servers. Comparison of signal quality evaluations on the present cell and on neighboring measurement channels is performed at step  330  in order to determine whether a handoff condition exists. If no handoff condition is present, the process continues at step  375 . If a handoff condition is found, the process continues at step  385  where the cell corresponding to the measurement channel received with the highest signal quality is selected. The downlink traffic channel identity having been received earlier in the mobile receiver  200  at step  375 , the mobile receiver  200  immediately selects the unpaired downlink traffic channel on the new cell at step  385 . The process continues on the new unpaired downlink traffic channel at step  375  where data is received from the new unpaired downlink traffic channel and where neighbor server information relative to the new cell is received. While in this case, the continuous transmission of enhanced neighboring cell data requires more bandwidth on the unpaired downlink traffic channel, the advantage of this alternate aspect of the present invention is that acquisition of the new unpaired downlink traffic channel is faster than as depicted in the description of  FIG. 3   a.    
      Reference is now made to  FIG. 3   c  which shows another variant of the method implementing the preferred embodiment of the present invention wherein an optional request for unidirectional service is sent by another exemplary mobile receiver. This exemplary mobile receiver  200  is capable of bidirectional communication. Steps  300 ,  320 ,  325 ,  300  and  335  are identical to the same steps in  FIG. 3   a . At step  317 , the mobile receiver  200  decodes the broadcast channel content. In the variant of the method of  FIG. 3   c , the broadcast parameter indicating to the mobile receiver  200  that the cell supports unidirectional transmission service on at least one traffic channel is required. The mobile receiver  200 , responsive to the broadcast parameter indicating support of unidirectional transmission, sends to the server a request to get access to unidirectional service at step  318 . In this case, the server responds by sending an unpaired downlink traffic channel designation at step  319 , said message being intended solely to the mobile receiver  200  having requested access to unidirectional service. In a system that only supports such bidirectional capable mobile receivers  200 , use of the periodic broadcast unpaired downlink traffic channel designation at step  317  is possible, but not required. The mobile receiver  200  selects the unpaired downlink traffic channel at step  320  and further operates in the same manner as shown in  FIG. 3   a . One of the advantages of using the optional request from the mobile receiver  200  prior to assigning an unpaired downlink traffic channel designation is the possibility to charge for the service since a record may be kept of such requests. The unpaired downlink traffic channel designation, or the data, when dedicated to a given mobile receiver  200 , may be encrypted. Encryption ensures that only those users that have made proper requests can have access to the service.  
       FIG. 3   c  has been shown as a variant of the exemplary method of  FIG. 3   a . It should however be understood that the exemplary method of  FIG. 3   b  could also be modified to support the bidirectional mobile receiver  200  capable of sending a request to get access to unidirectional service. Hence, steps  375  and  385  as shown on  FIG. 3   b  could replace steps  325  and  335  in the variant of the method described in  FIG. 3   c.    
      With either of the methods of  FIGS. 3   a ,  3   b  or  3   c , data in different cells may be identical or distinct. In an application where it is desired to provide data such as a news broadcast, to users in a large area, the same data is preferably provided in each cell. In another application where it is desired to provide data specific to the user&#39;s location, data may differ in each cell.  
      Referring now to  FIG. 4 , an exemplary use of the method and the mobile receiver of the present invention is illustrated.  FIG. 4  shows an art exhibition hall  400  comprising four (4) distinct sections, such as rooms  410 ,  420 ,  430  and  440 , wherein artifacts  412 ,  422 ,  432  and  442  are on display. At a conventional art exhibition, a guide would greet visitors and provide explanations on the various artifacts as visitors tour the art exhibition. Sometimes, other means are substituted for the guide. For instance, headphones may be installed near each artifact for use by the visitors as they peruse over the displays. Alternatively, tape players with headphones may be rented by visitors as they enter the exhibition hall. All of those manners of providing information have their drawbacks: Use of guides, though more personal, is costly, requires planning, may require knowledge of more than one language by the guides, and forces visitors to follow scheduled groups. Use of headphones at fixed locations may force visitors to wait when all headphones at a specific location are in use. Use of tape players may require users to move at a preset pace, consistent with the programmed content.  
      Using the method and the mobile receiver of the present invention overcomes all of those deficiencies. RBSs  414 ,  424 ,  434  and  444  are placed in rooms  410 ,  420 ,  430  and  440  in order to support unidirectional transmission. Upon entering the exhibition hall, the visitor may be offered the use of a mobile receiver from a pool of available units. A receiving-only mobile receiver used solely in the exhibition hall facilities is pre-programmed to select a first broadcast channel amongst a limited set of channels corresponding to RBSs  414 ,  424 ,  434  and  444 . If the visitor owns a bidirectional capable mobile terminal capable of unidirectional reception, as per the preferred embodiment of the present invention, he/she may use this terminal as a mobile receiver. The mobile terminal capable of bidirectional communication can be put in receive-only mode by use of a code entered by the user. The exhibition hall staff can give instructions to the visitor on which particular code is required to access the servers used in the exhibition. The mobile terminal can also be put in the receive-only mode by other means, for example, an SMS may be received by the mobile terminal, indicating that unidirectional communication is possible. The SMS may further comprise a list of one or more broadcast channels used for unidirectional communication. The user may then press a single button on the mobile terminal to indicate whether or not he/she wants to use the unidirectional communication service. In any cases, for comfort and convenience, use of a hands free earpiece is preferable.  
      As the visitor enters a first room  410  in the exhibition hall, the mobile receiver selects the broadcast channel of the first RBS  414  currently transmitting in the first room  410 . This selection is made simply because a signal from first RBS  414  is received with a better signal quality than signals from any other RBS in the exhibition hall. By use of one of the methods introduced in the foregoing description of  FIGS. 3   a  and  3   b , the mobile receiver receives an unpaired downlink traffic channel designation from the first RBS  414 . In this exemplary use, the unpaired downlink traffic channel of the first RBS  414  includes a recorded voice description of the artifacts  412  found in the first room  410 . When the visitor decides to moves to the second room  420 , the mobile receiver rapidly detects that the measurement channel of the second RBS  424  found in the second room  420  has become stronger. The mobile receiver thus starts receiving an unpaired downlink traffic channel from the second RBS  424 , said unpaired downlink traffic channel comprising a different recorded voice description describing the artifacts  422 .  
      In yet another aspect of the preferred embodiment of the present invention, the method and mobile receiver can be used in any standard mobile communication network having been adapted to the teachings of the present invention. One or several traffic channels may be set aside in a cell or in any number of cells to support unidirectional transmission. A broadcast parameter may be used to inform mobile terminals having the features of the present invention that downlink unidirectional transmission is available. Traffic channel designations for unpaired downlink traffic channels may be sent periodically on the broadcast channel of the cell or cells having unpaired downlink traffic channels to let mobile receivers select the unidirectional service. Alternatively, a bidirectional capable mobile terminal may, as directed by the user, send a request for traffic channel designation to the unidirectional service. The unidirectional service is not limited to voice or audio information, but may also comprise video or packet data service.  
      Although several aspects of the preferred embodiment of the method and of the distributor of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.