MXPA00009876A - Increasing traffic capacity in a cellular communications system by change of traffic channel rate - Google Patents

Abstract

The present invention dynamically increases the capacity of a cellular radio communications system to meet temporary high traffic demands effectively and economically. The current traffic load at a particular base station cell area is determined (14). If the determined traffic load exceeds a threshold (16), a higher rate traffic channel over which a dual rate mobile station is communicating is handed over to a lower rate traffic channel available in that cell area (18). A list is maintained for those dual rate mobile stations currently assigned to higher rate traffic channels. Before making the handover from the higher rate traffic channel to the lower rate traffic channel, it is determined whether a handover is permitted. If so, the intra-cell handover is preferably made to a traffic channel which is currently already supporting another lower rate call. Otherwise, any available lower rate traffic channel is assigned. At call setup, if the current traffic load in the cell area exceeds the threshold, a call request involving a dual rate mobile station is assigned a lower rate traffic channel (20). If the traffic load in the cell area decreases, a call intitially set up or subsequently handed over to a lower rate channel because of high traffic load may optionally be handed over to a higher rate channel (17).

Description

INCREASE OF TRAFFIC CAPACITY IN A CELLULAR COMMUNICATION SYSTEM BY CHANGE OF CHANNEL SPEED FIELD OF THE INVENTION The present invention relates to an increased traffic capacity in a cellular radio system, and more particularly, to an increased traffic capacity by changing a traffic channel speed requested or initially selected at a speed bit rate when additional traffic capacity is required. BACKGROUND AND COMPENDIUM OF THE INVENTION In the Global System for Mobile Communications (GSM), two bit rates are defined for voice coders: full speed and half speed. The full speed corresponds to a bit rate of 13Kbit / s, and the average speed corresponds to a bit rate of 6.5Kbit / s. A traffic channel in the GSM system can support a full speed voice call or two half speed voice calls. Voice coding significantly decreases the bit rate at the radio interface (higher bit rates absorb too much of the frequency spectrum) while still providing acceptable voice quality. In general, the voice coding sends information as to the voice (instead of the voice itself) from which the speech signal in the receiver can be reconstructed. GSM also employs time division multiple access (TDMA) with each radio frequency carrier divided into 8 time segments (TS). A time segment in a TDMA type box is known as a physical channel and each duplex pair of frequency carriers includes 8 physical channels. Information of vos and information of another type are sent in logical traffic channels (TCH) represented in the channels of physical time segments. In full speed traffic channels, a user is assigned a physical channel / single time segment. In the case of medium speed traffic channels, two mobile stations share the same physical channel / time segment with each mobile station making alternate use of the allocated time segment. Mobile phone system operators live in a highly competitive world. To be successful, a mobile phone system operator must provide the following: * Coverage - a call can be established anywhere. * Capacity - a call can be established at any time. * Clarity - The call must be clear, without interference or interruptions.

The present invention focuses on this capability. The traditional approach to increasing capacity is to equip base stations with the maximum number of possible transceivers and invest in high-capacity base station controllers. While it is a correct strategy in highly populated areas with traffic demands distributed regularly over time, in areas where traffic demand is high only for short periods of time such as, for example, some streets during the morning or At night, this strategy is less suitable. The difficult problem is how to satisfy temporarily high demands of traffic in an effective and economic way. Accordingly, it is a primary object of the present invention to dynamically increase the capacity of the system to satisfy temporarily high traffic demands effectively and economically. It is a further object of the present invention to provide an enhanced capability to provide service for connections that require a full speed traffic channel.

A certain percentage of mobile stations can currently (and with increasing numbers in the future) transmit and receive at different bit rates. For purposes of explaining the present invention (and in no way to limit the present invention), some Mobile stations are considered capable of transmitting and receiving both full-speed channels and half-speed channels. Such mobile stations are known as having a "two speed" capability. Two-speed mobile stations indicate their two-speed capability to the cellular network through an initial channel request message or possibly in periodic log messages. Higher speed channels can be used under favorable traffic conditions but medium speed channels are used during temporarily high traffic conditions. The present invention increases the capacity in a cellular radio system in accordance with the following method. Initially, a higher speed traffic channel is established for communication with a mobile station located in a particular cell area. The current traffic load in this, cell area is determined. If the determined traffic load exceeds a threshold, the highest speed traffic channel in which a dual speed mobile station communicates is transferred to another lower speed traffic channel available in this cell area. For currently active traffic channels, a list is maintained for the two-speed mobile stations currently assigned to higher-speed traffic channels.

Before making a transfer from the highest speed traffic channel to the lowest speed traffic channel, it is determined whether said transfer is allowed. If this is the case, the transfer between cells is preferably carried out towards a traffic channel which is already currently supporting another lower speed call. Otherwise, any available lower speed traffic channel is allocated. At the time of establishing a call, if the current traffic load in the cellular area exceeds the threshold, a call request that includes a two-speed mobile is assigned to a lower rate traffic channel. If the traffic load in the cell area decreases, a call initially established or subsequently transferred to a lower speed channel due to the high traffic load may be transferred to a higher speed channel. BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects, features and advantages as well as other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments in accordance with that illustrated in the accompanying drawings in which reference numbers they refer to the same parties in the various views. The drawings are not necessarily to scale, the illustration of the principles of the invention being emphasized. Fig. 1 is a flow diagram illustrating a general "channel velocity change" procedure in accordance with a first example embodiment of the present illustration; Fig. 2 is a functional block diagram of a GSM-type mobile radiocommunications system in which the present invention can be incorporated in accordance with a second exemplary embodiment of the present invention; Figure 3 is a diagram illustrating in a conceptual manner a traffic channel that supports a single total speed channel or two half speed channels; Figure 4 depicts more detailed function block diagrams of specific blocks and used in Figure 2; and Figures 5A and 5B are flow charts illustrating flow charts of exemplary procedures in accordance with the second exemplary embodiment of the present invention. • DETAILED DESCRIPTION OF THE DRAWINGS In the following description, for purposes of explanation and not limitation, specific details are presented such as, for example, particular modalities, equipment, techniques, etc., in order to offer a thorough understanding of the invention. However, it will be apparent to a person skilled in the art that the present invention can be practiced in other modalities that depart from these specific details. For example, while a specific embodiment of the present invention is described in the context of a GSM-type cellular telephone network, those skilled in the art will note that the present invention can be implemented in any cellular telephony system. In other cases, detailed descriptions of well-known methods, interfaces, devices and signaling techniques are omitted so as not to obscure the description of the present invention with unnecessary details. Since the invention can be applied to any cellular telephone communication system, a first exemplary embodiment of the present invention is described below in general terms. Flow diagram No. 1 presents the steps of the Channel Speed Change routine (block 10) Considering a light or moderate traffic load initially, traffic channels are established for mobile calls in a cell, sector or other particular indication area in a first traffic speed (block 12) in accordance with procedures for requesting and assigning traffic. normal channels. The traffic load is determined in the cell or in the location area (block 14). A decision is made in block 16 in the sense that if the traffic load for this cell or location area exceeds a threshold, that is, if there has been a temporary increase in traffic demand. If this is not the case, mobile communications continue as they were established (block 17). However, if the traffic load exceeds a threshold, a two-speed mobile communication is taking place in the traffic channel at a first speed that is transferred to another traffic channel where the communication is carried out at a second speed. lowest (block 18). In addition, new incoming two-speed mobile calls are set as higher speed calls until the traffic load decreases below the threshold. If the traffic load decreases significantly, calls can be transferred from the lowest speed traffic channels to the highest available traffic speed channels (block 17). Accordingly, the present invention offers a temporary increase in the traffic capacity of a cell through the use of lower speed capabilities of certain two-speed mobile stations operating in the cell. Because the communication connections are shifted within the service cell, i.e., intracellular transfers, there is no increase in the level of interference that could be experienced in the case of using intercell transfers to other cells. Another advantage of the present invention is that no additional equipment is required. Reserve base station transceivers, but often underutilized, are not required to ensure sufficient capacity for temporarily high traffic demands. Another advantage of this invention is that a mobile communication operator can control how the traffic demands of each cell are handled by changing the traffic load threshold to appropriate values. A threshold with appropriate value ensures that only a minimum number of mobile stations that have two-speed capability are affected by intracell transfers to lower speed traffic channels during heavy traffic conditions. The present invention also selects the channel rate for a particular traffic channel when a call is established based on a current cell traffic load level such that during periods of heavy traffic, communications involving two-speed mobile stations are establish using the lower speeds to further increase the traffic capacity. A second example embodiment of the present invention will be described below applied to a GSM-based cellular communication system. However, those skilled in the art will note that the present invention is not limited in any way to cellular communication systems based on GSM. Reference is now made to Figure 2 which illustrates a block format that functions typical in the case of a cellular communication system 30 based on the GSM cell. A Compuerta Mobile Switching Center (GMSC) 36 connects the cellular communications network with other telecommunications networks including the Public Switched Telephone Network (PSTN) 32, Internet 34, other cellular networks and other types of communication networks such as the Digital Network of Integrated Services (ISDN). The Gateway Mobile Switching Center 36 is connected to one or several mobile switching centers (MSCs) 38. Mobile switching 38 has been interconnected to one or more base station controllers (BSCs) 42. The BSC handles all related functions with radio including administration and remote control of the base stations 44 as well as the management of connections with mobile stations 46, including transfers. For BSC 42 it is connected to several base stations (BSs) 44 communicating with mobile radio stations (MS). The MSC gate 36 is the interface point in the mobile radio network for calls to mobile subscribers. Even though GMSC 36 is illustrated as a separate node for a clear illustration, can be located with an MSC 38. Each mobile switching cell 38 performs telephony switching functions associated with calls involving a mobile station (MS) 46 which includes connections to other telecommunications networks 32 and 34 and routing of calls originating in a mobile station. Each MSC 38 is associated with a Visitor Location Register (VLR) 40 which includes a database containing information of the required temporary subscriber MSC 38 in order to offer services to mobile stations in the area of higher service MSCs. Typically, when a mobile station enters a visiting network or service area, it is registered with the VLR 40 that requests later and receives data regarding the moving mobile station from the Home Location Register (HLR) 41 of the mobile station and stores said information. As a result, when the visiting mobile station participates in a call, the VLR 40 already has the necessary information to establish a call. While the VLR 40 can be an independent node, it is preferably integrated with its associated MSC to eliminate signaling between the two nodes. The address location register (HLR) 41 is a database node that stores and manages subscriptions. For each mobile "domicile" subscriber, HLR 41 contains permanent subscriber data such as the Mobile Station ISDN number (MSISDN) that uniquely identifies mobile phone subscribers in a PST numbering plan, and an identity of International Mobile Subscriber (IMSI), which is a unique identity assigned to each mobile subscriber and used for signaling in the entire network. All the subscriber information related to the network is connected to the IMSI. The HLR 41 contains a service announcement that a mobile subscriber can employ together with the current subscriber numbers corresponding to the address of the VLR 40 that is currently serving the mobile subscriber. Once a serving MSC / VLR is identified (in the home network or in a visiting network), a call directed to the mobile station 46 is routed by this service MSC to a base station 44 associated with the cell in which is currently located the mobile station called. Using established well-known protocols as well as possibly documented in several GSM standards, it establishes a call connection on the radio interface between the base station 44 and the mobile station 46. During the establishment of a call, a call channel is assigned. logical traffic to a radio connection between a base station and a mobile station based on the information as to the available channel characteristic, and in the present invention in particular, based on the capabilities of the mobile station. When the connection is established in an assigned traffic channel, such as a time segment of the DTMA type, signal strength and voice quality are monitored and sent to the BSC, which can initiate the transfer of the base connection. in these reports. When a call is initiated, it is the BSC that allocates an available traffic channel (TCH) to the mobile station. In current GSM systems, a traffic channel (TCH) can either support a single full speed communication as shown at 50 in Figure 3 or two half speed communications as indicated at 22 and 24 in Figure 3. An example full speed communication corresponds to a bit rate of 13 Kbit / s, and an example half-speed communication corresponds to a bit rate of 6.5 Kbit / s. It is understood that reference to a full speed traffic channel refers to a traffic channel that is currently supporting a single full speed call, and reference to a medium speed traffic channel means refers to a channel that is supporting currently one or two half-speed calls. More detailed block diagrams of our BSC 42, base station 44, and a two-speed mobile station 46 are described below in relation to Figure 4. For purposes of illustration and description only function blocks are shown. However, these functions can be performed using any suitable electronic circuit, including DSP. AGIC, main processor programmed appropriately, etc. The traffic load in each cell, sector or base station location area is monitored by the BSC 42. However, this operation or function can be performed by other entities such as MSC 38, each base station, or another radio network control entity. The BSC 42 includes a traffic load detector 50 which, in this simplified example, monitors the traffic load in the area X that is served by a base station 44. The BSC 42 carries out similar tasks for each area of its type for each base station under control. A traffic load detector 50 includes a comparator 52 and a busy full-traffic traffic channel counter 54 (TCH) 54. The counter 54 is implemented and decremented to track a current total number of traffic channels occupied for this area of traffic. calls "X". A traffic channel is considered busy if it is not available to support a full speed call, that is, the channel is partially or fully assigned for traffic commuted by its equipment or switched by packet. The output of the counter 54 is input to a comparator 52 together with a threshold value (T) which may correspond to a certain number of traffic channels established by the mobile network operator. When the counter output exceeds the threshold, the generator generates a traffic load signal that can be used to establish a high load marking at the controller 56 for the cell area X. Preferably, the threshold value (T) incorporates hysteresis in order to minimize unwanted switching of channel speeds. Thus, the controller 56 receives channel requests from several base stations 44 and sends channel assignments for several mobile communications to the base stations. The controller 56 has access to a memory 58 for storing a list of mobile stations capable of two speeds currently assigned to a full speed traffic channel. By executing various channel assignments and channel outputs, the controller 56 increments and decreases, respectively, the full speed TCH counter 54 occupied for each base station area. Each base station 44, such as the base station used for area X, includes numerous transceivers used to establish and maintain several channels on the radio interface with mobile stations 46 receiving service in an X area. A simplified example of a transceiver of base station illustrated in block 44 includes a baseband processing circuit 60 switchably connected through a switch 62 to one of a full speed encoder / decoder 64 and half speed encoder / decoder 66 also connected in a manner switchable to an opposite terminal via a switch 70 for a transceiver circuit 72 and antenna 76. A base station controller 74 performs several base station control operations including adjusting the position of the switches 62 and 70 according to whether a particular communication should be carried out on a full or average speed traffic channel speed. The two-speed mobile station 46 illustrated in FIG. 4 includes, among other elements, a baseband processing circuit 80 switchably connected through a switch 82 to one of a full speed encoder / decoder 84 and a a half-speed coder / decoder 86 also switchably connected at its respective output terminals through the counter 88 to the transceiver circuit 90 and antenna 94. A two-speed mobile station controller 92 performs numerous control functions including the control of the position of the switches 82 and 88 according to whether a full speed channel assignment or half speed channel (initially or as a result of a transfer), it is received by the mobile station. The operation of the embodiment of the second example of the present invention is now described in combination with the flow chart illustrated in Fe 5A. It is understood that a change in the channel speed is supported and allowed by the base station as well as by at least some of the mobile stations that are receiving service from said base station at present. A change in channel speed can be blocked if it is not supported or allowed. For example, certain types of calls such as data calls initially assigned to a full speed call may, in some cases, not be allowed or be changed to a half-speed call after the call has been initiated as a speed connection. complete The first step is for the traffic load detector to determine the traffic load corresponding in the second mode to the number of full-speed traffic channels currently allocated (block 100) which employ the output of the full-speed counter 54 occupied, the threshold input, and the comparator 52 in the traffic detector 50. A decision is made in block 102 as to whether the current traffic load exceeds the threshold (which preferably incorporates a hysteresis value). If this is not the case, the control returns to block 100 to monitor the traffic load without a change in channel speed being necessary at this time. If the traffic load in this cell area exceeds the threshold, the TSC controller 56 generates a list 58 of the two-speed mobile stations in this cell area X currently assigned to a full-speed transit channel that can employ a channel of medium speed traffic, if necessary (block 106). A decision is made in block 108 by the controller 56 in the sense that there are two-speed mobile stations included in the list. If not, the control returns to block 100 to repeat the previous procedures. However, if there are three-speed mobile stations currently assigned to a full-speed traffic channel entered in the list, another decision is made in block 110 in the sense of whether a traffic channel associated with the base station for the Area X has an idle half-speed connection. In accordance with what is described above in combination with Fe 3, a traffic channel can support two half-speed traffic channel connections. It is more efficient to fully occupy this traffic channel with two half-speed calls. A transfer of the current call in the full-speed traffic channel for the two-speed mobile station at the top of the list is transferred to the remaining unoccupied half-speed connection (block 112). The transfer frees up a full speed traffic channel for other call requests some of which may require a full speed traffic channel during this period of high traffic demand. Alternatively, the freed velocity channel can be used to support two new half-speed call connections. The BSC controller 56 generates the necessary transfer commands for the base station controller 74 and the mobile station controller 92. When the mobile station 46 initially sends a "channel request" message to its service base station, the The base station includes the channel request message with a "requested channel" message sent to the BSC. A portion of the channel request indicates the types of channels in which the mobile station can communicate such as full speed, half speed, etc. A "channel assignment" command is then sent to the mobile station from the BSC through the service base station containing a channel description of a new or initial average traffic channel for which it is due. establish or transfer the call. The base station activates the new half-speed traffic channel. In the case of transfers, when the new traffic channel is recognized, the BSC controller 56 sends a message to the mobile station through the old full speed traffic channel with information regarding the new medium speed traffic channel frequency, time, and power output. The mobile station is tuned to the new frequency and sends transfer access pulses in the appropriate time slot. Once these pulses are detected and recognized, a complete transfer message is transmitted by the mobile, and the old full speed traffic channel is deactivated making it available for assignment to other communications. On the other hand, if no traffic channel supports a current half-speed connection with the other idle half-speed connection, a decision is made in block 114 as to whether there is a traffic channel with both medium-speed channel connections. unoccupied, that is, a traffic channel available to support a full speed connection or two half speed connections. If this is not the case, then the list of two-speed mobile stations 58 (block 122) is deleted, and the process is repeated in block 100. On the other hand, if there is a medium-speed channel connection available, a transfer for the mobile station at the top of the list 58 from a current full speed connection to a new half-speed connection in the idle traffic channel identified in block 114 (block 116). When the transfer is performed either in blocks 112 or 116, the mobile station station entry at the top of list 58 is removed (block 118). A decision is then made in block 120 to determine whether the traffic load is less than the threshold by an amount of hysteresis. If this is not the case, if the cell continues to experience tense traffic conditions, you can add additional capacity by changing the channel speed from full speed connections to half speed connections. On the other hand, if the traffic load is sufficiently decreased below the threshold, the list is deleted in block 22, and the process is repeated starting in block 100. If the traffic load exceeds the threshold in decision block 102 , the speed controller BSC 56 also tries to increase the capacity by assigning medium speed traffic channels where it is allowed / where it is possible for new call requests, see markers A in figure 5A, and the description in the flow chart illustrated in Figure 5B. For a heavy traffic load condition, each new channel request is received (block 130), and the speed capability of the mobile station associated with the channel request (block 132) is determined based on the indication of the types of channels that the mobile station can handle contained in the channel request message. If a full speed traffic channel is required (whatever the reason), the channel establishment proceeds to assign a full speed traffic channel considering that one is available. However, if a full speed traffic channel is not required for the current call request, a half speed traffic channel is allocated (block 136) and the control returns to block 100 in figure 5. When it is detected that the traffic load has been sufficiently decreased, an optional procedure allows the current half-speed calls to be traffic sufficiently decreased, an optional procedure allows the half-speed calls in progress to be selectively transferred to available full-speed channels. Now let's consider the following simple example. We will consider that the base station 44 has a total of 4 transceivers, where each transceiver includes 8 time segment channels. Therefore, the base station includes a total of 32 time segments in which three are used for control channel signaling leaving 29 time segments for traffic channels. Again, all base station transceivers are considered capable of carrying full speed channels and half speed channels. Half of the mobile stations are considered capable of two-speed communications and the other half is capable of full-speed communication only. The traffic load threshold is set at 75% with a hysteresis of 5%. Translated to a number of traffic channels for this example, the threshold is 22 busy traffic channels with a hysteresis of a traffic channel. Thus, a high load marker is set when 23 or more traffic channels are currently occupied, and it is reinitialized when at least 21 traffic channels are currently occupied. We will now consider that traffic channels are currently employed in the following way: two traffic channels are assigned as packetized data channels, ten traffic channels are occupied by mobile stations capable only of full speed, one traffic channel is occupied by two half-speed calls, two traffic channels are occupied by a half-speed call for each traffic channel, seven traffic channels are occupied by two-speed mobile stations that are currently using a full-speed connection. In total 22 traffic channels are busy, and therefore the heavy traffic load marker is not set. Let's now assume that another full speed call is established for a two-speed mobile station. Then the intense traffic load marker is set. The base station controller 56 generates a list of two-speed mobile stations that are candidates for transfers. The list includes 8 entries corresponding to the eight two-speed mobile stations that are currently using full-speed connections. These full speed connections are transferred one at a time to a half speed channel until the intense load situation is mitigated. For example, a first full speed connection is shifted to a half speed connection that is occupied in one of the partially allocated traffic channels already carrying a half speed call connection. After this first transfer, the occupied traffic channel number is equal to 22, considering that no new calls have been established, which is not sufficiently low to reinitialize the heavy load marker. A second connection is shifted to the inactive part of the second of the partially allocated traffic channels. The number of busy traffic channels is reduced to 21 which again is not sufficiently low to reinitialize the heavy load marker. A third full speed connection is shifted to a half speed channel on one of the inactive full speed channels. A fourth connection is moved to the other half-speed connection available in this same traffic channel. By transforming the four full-speed connections into four new half-speed connections, the number of busy traffic channels is reduced from 23 to 20 as a result, four additional traffic channels are available to service other calls at a cost relatively minimal to make intracellular transfers. Sufficient traffic demand increases are sufficiently satisfied without additional base station transceivers. While the present invention has been described in relation to particular embodiments, those skilled in the art will recognize that the present invention is not limited to the specific embodiments described and illustrated herein. Different formats, embodiments, and adaptations, in addition to those illustrated and described, as well as numerous variations or modifications and equivalent arrangements may also be employed to implement the invention. Accordingly, while the present invention has been described in relation to its preferred embodiments, it will be understood that this disclosure is illustrative and exemplary only of the present invention and is for the sole purpose of providing a thorough disclosure of the present invention. Accordingly, the invention is limited only by the spirit and scope of the appended claims.

Claims (1)

1. CLAIMS A method to increase the capacity in a cellular communication system in which mobile stations communicate in traffic channels, which comprises the grounds of: establishing a traffic channel of a first speed for communication with a mobile station located in a cell area particular; determine a traffic load in the area of the cell; and if the traffic load in the cell area exceeds a threshold value, change the first-rate traffic channel to a second-rate traffic channel. The method according to claim 1, further comprising: if the traffic load in the cell area exceeds a threshold value, establishing a new second rate traffic channel in response to a channel request involving a mobile station in the area of the cell. The method according to claim 2, further comprising: determining whether the mobile stations identified in the channel requests have the ability to communicate in a second rate channel before the step of establishing a new traffic channel. The method according to claim 1, wherein the first velocity is a higher velocity than the second velocity. The method according to claim 1, further comprising: determining whether the mobile station includes a two-speed capability to communicate using any of the first rate and second rate, where the step of changing includes the determination of whether the mobile station has the capacity of two speeds. The method according to claim 5, further comprising: for a mobile station capable of two speeds, determining whether to change to a second-rate channel communication. The method according to claim 1, wherein the step of changing includes: carrying out an intracellular transfer of communication from the first-rate traffic channel to the second-rate traffic channel. The method according to claim 1, wherein the second rate corresponds to a bit rate of 6.5 Kbit / s and the first rate corresponds to a bit rate of 13 Kbit / s. 9. The method according to claim 1, wherein the second speed communication occupies half the capacity occupied by the first speed communication. The method according to claim 1, further comprising: assigning the second speed communication to a traffic channel that currently supports another second speed communication. The method according to claim 1, further comprising: compiling a list of two-speed mobile stations that have the ability to communicate using any of the first rate and the second highest rate. The method according to claim 11, further comprising: the use of hysteresis in the determination step. 13. A method for increasing the capacity in the cellular communication system wherein mobile stations in cell areas communicate with corresponding base stations in traffic channels, comprising the steps of: in response to a request to establish a traffic channel for a communication with a mobile station in a cell area, determining a traffic channel velocity capability of the mobile station; determine a cell-area traffic load; and if the cell area traffic load exceeds a threshold and if the mobile station has the ability to communicate using any of a higher first rate and a lower second rate, establish a traffic channel for mobile station communication in the second lowest speed. The method according to claim 13, wherein the second lowest speed corresponds to a bit rate of 6.5Kbit / s and the first highest speed corresponds to a bit rate of 13Kbit / s. 15. The method according to claim 13, where the second lowest speed communication occupies one half of a traffic channel and the first highest speed communication occupies an entire traffic channel. The method according to claim 13, further comprising: switching the second lower speed communication to a traffic channel that currently supports another lower second speed communication. 17. The method according to claim 13, further comprising: compiling a list of mobile stations that have the ability to communicate at both the lowest second rate and the first highest rate. The method according to claim 13, further comprising: if the traffic load in the cell area is below a threshold, establish a traffic channel for the mobile station communication at the first highest speed. 19. The method according to claim 18, further comprising: employing hysteresis in the determination step. The method according to claim 13, further comprising: if the traffic load in the cell area exceeds a threshold, examine the first-rate traffic channel connections and change one of the traffic channel connections of first speed to a second speed traffic channel. 21. In a cellular communication system in which mobile stations communicate in traffic channels with base stations, each base station has a corresponding cell, a controller that coordinates the establishment of a first-rate traffic channel for communication with a mobile station served by a base station, comprising: a memory storing traffic channel velocity capabilities of several mobile stations served by the base station, data processing circuit, connected to the memory programmed to perform the following tasks : determining a traffic load in the cell corresponding to the base station, if the traffic load in the cell exceeds a threshold, reassigning the communication to a lower second traffic speed channel. The controller according to claim 21, wherein the mobile station has the ability to communicate in traffic channels with a first speed and a second speed. The controller according to claim 21, wherein the data processing circuit executes an intracell transfer from the first-rate traffic channel to the second-rate traffic channel. The controller according to claim 21, wherein the second rate corresponds to a bit rate of 6.5Kbit / s and the first rate corresponds to a bit rate of 13Kbit / s. 25. The controller according to claim 21, wherein the second speed communication occupies half the capacity occupied by the first speed communication. 26. The controller according to claim 21, wherein the data processing circuit assigns the second speed communication to a traffic channel that is currently supporting another second speed communication. 27. The controller according to claim 21, wherein the memory stores a list of two-speed mobile stations that have the ability to communicate using both the first rate and the second highest rate. 28. The controller according to claim 21, wherein if the traffic load in the cellular area exceeds a threshold, the data processing circuit establishes new second-rate traffic channels in response to requests for channels involving mobile stations. in the cell area. 29. The controller according to claim 21, wherein the controller is located in a base station controller connected to several base stations. 30. The controller according to claim 21, wherein the controller is located in a mobile communication center connected to several base stations. The controller according to claim 21, the data processing circuit comprises: a counter for counting the number of traffic channels occupied, and a comparator for comparing a counter output with the threshold. SUMMARY OF THE INVENTION The present invention dynamically increases the capacity of a cellular radio communication system to meet temporarily high traffic demands efficiently and economically. The current traffic load is determined in a particular base station cell area (14). If the determined traffic load exceeds a threshold (16), a higher speed traffic channel in which a mobile station communicates at a double speed is transferred to a lower speed traffic channel available in this cell area ( 18). A list is maintained for dual speed mobile stations currently assigned to higher speed traffic channels. Before transferring from the highest speed traffic channel to the lowest speed traffic channel, it is determined whether a transfer is allowed. If this is the case, the intra-cell transfer is preferably carried out to a traffic channel that is already supporting another lower speed call. Otherwise, any available lower speed traffic channel is allocated. When a call is established, if the current traffic load in the cell area exceeds the threshold, a call request involving a dual speed mobile station is assigned to a lower speed traffic channel (20). If the traffic load in the cell area decreases, a call initially established or subsequently transferred to a lower speed channel due to a high traffic load may optionally be transferred to a higher speed channel (17).