Abstract:
A method for reassigning mobile stations a new phase within a channel is disclosed. In a communication system, a plurality of mobile stations attempt to communicate on a limited number of channels. The invention first uses a general allocation method to approximately evenly distribute the mobile stations on the available channels. After a mobile station begins active communication on the system, a second allocation method is used to change the phase of some mobile stations on a channel which has become heavily loaded. In addition, a message can also be sent to mobile stations assigned to but not yet actively communicating on the heavily loaded channel indicating that the mobile stations should change to a new channel.

Description:
BACKGROUND 
     Applicants&#39; invention relates to electrical telecommunication, and more particularly to wireless communication systems, such as cellular and satellite radio systems, for various modes of operation (analog, digital, dual mode, etc.), and access techniques such as frequency division multiple access (FDMA), time divisional multiple access (TDMA), code divisional multiple access (CDMA), hybrid FDMA/TDMA/CDMA, for example. The specific aspects of the present invention are directed to techniques for enhancing bandwidth allocation, traffic and capacity management, and the throughput and quality of transactions. 
     A description follows which is directed to environments in which the system of the present invention may be applied. This general description is intended to provide a general overview of known systems and the terminology associated therewith so that a better understanding of the invention can be achieved. In North America, digital communication and multiple access techniques such as TDMA are currently provided by a digital cellular radiotelephone system called the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard TIA/EIA/IS-54-B, &#34;Dual-Mode Mobile Station-Base Station Compatibility Standard&#34;, published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA), which is expressly incorporated herein by reference. Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), TIA/EIA/IS-54-B is a dual-mode (analog and digital) standard, providing for analog compatibility together with digital communication capability. For example, the TIA/EIA/IS-54-B standard provides for both FDMA analog voice channels (AVC) and TDMA digital traffic channels (DTC). The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (KHz). 
     In a TDMA cellular radiotelephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. The number of time slots in each TDMA frame is related to the number of different users that can simultaneously share the radio channel. If each slot in a TDMA frame is assigned to a different user, the duration of a TDMA frame is the minimum amount of time between successive time slots assigned to the same user. 
     The successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute the user&#39;s digital traffic channel, which may be considered a logical channel assigned to the user. As described in more detail below, digital control channels (DCCs) can also be provided for communicating control signals, and such a DCC is a logical channel formed by a succession of usually non-consecutive time slots on the radio carrier. 
     In only one of many possible embodiments of a TDMA system as described above, the TIA/EIA/IS-54-B standard provided that each TDMA frame consists of six consecutive time slots and has a duration of 40 milliseconds (msec). Thus, each radio channel can carry from three to six DTCs (e.g., three to six telephone conversations), depending on the source rates of the speech coder/decoders (codecs) used to digitally encode the conversations. Such speech codecs can operate at either full-rate or half-rate. A full-rate DTC requires twice as many time slots in a given time period as a half-rate DTC, and in TIA/EIA/IS-54-B, each full-rate DTC uses two slots of each TDMA frame, i.e., the first and fourth, second and fifth, or third and sixth of a TDMA frame&#39;s six slots. Each half-rate DTC uses one time slot of each TDMA frame. During each DTC time slot, 324 bits are transmitted, of which the major portion, 260 bits, is due to the speech output of the codec, including bits due to error correction coding of the speech output, and the remaining bits are used for guard times and overhead signalling for purposes such as synchronization. 
     It can be seen that the TDMA cellular system operates in a buffer-and-burst, or discontinuous-transmission, mode: each mobile station transmits (and receives) only during its assigned time slots. At full rate, for example, a mobile station might transmit during slot 1, receive during slot 2, idle during slot 3, transmit during slot 4, receive during slot 5, and idle during slot 6, and then repeat the cycle during succeeding TDMA frames. Therefore, the mobile station, which may be battery-powered, can be switched off, or sleep, to save power during the time slots when it is neither transmitting nor receiving. 
     In addition to voice or traffic channels, cellular radio communication systems also provide paging/access, or control, channels for carrying call-setup messages between base stations and mobile stations. According to TIA/EIA/IS-54-B, for example, there are twenty-one dedicated analog control channels (ACCs), which have predetermined fixed frequencies for transmission and reception located near 800 MHz. Since these ACCs are always found at the same frequencies, they can be readily located and monitored by the mobile stations. 
     For example, when in an idle state (i.e., switched on but not making or receiving a call), a mobile station in a TIA/EIA/IS-54-B system tunes to and then regularly monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a call through the corresponding base station. When moving between cells while in the idle state, the mobile station will eventually &#34;lose&#34; radio connection on the control channel of the &#34;old&#34; cell and tune to the control channel of the &#34;new&#34; cell. The initial tuning and subsequent re-tuning to control channels are both accomplished automatically by scanning all the available control channels at their known frequencies to find the &#34;best&#34; control channel. When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this way, mobile stations stay &#34;in touch&#34; with the system. 
     While in the idle state, a mobile station must monitor the control channel for paging messages addressed to it. For example, when an ordinary telephone (land-line) subscriber calls a mobile subscriber, the call is directed from the public switched telephone network (PSTN) to a mobile switching center (MSC) that analyzes the dialed number. If the dialed number is validated, the MSC requests some or all of a number of radio base stations to page the called mobile station by transmitting over their respective control channels paging messages that contain the mobile identification number (MIN) of the called mobile station. Each idle mobile station receiving a paging message compares the received MIN with its own stored MIN. The mobile station with the matching stored MIN transmits a page response over the particular control channel to the base station, which forwards the page response to the MSC. 
     Upon receiving the page response, the MSC selects an AVC or a DTC available to the base station that received the page response, switches on a corresponding radio transceiver in that base station, and causes that base station to send a message via the control channel to the called mobile station that instructs the called mobile station to tune to the selected voice or traffic channel. A through-connection for the call is established once the mobile station has tuned to the selected AVC or DTC. 
     The performance of the system having ACCs that is specified by TIA/EIA/IS-54-B has been improved in a system having digital control channels (DCCHs) that is specified in TIA/EIA/IS-136, which is expressly incorporated herein by reference. Using such DCCHs, each TIA/EIA/IS-54-B radio channel can carry DTCs only, DCCHs only, or a mixture of both DTCs and DCCHs. Within the TIA/EIA/IS-136-B framework, each radio carrier frequency can have up to three full-rate DTCs/DCCHs, or six half-ram DTCs/DCCHs, or any combination in between, for example, one full-rate and four half-rate DTCs/DCCHs. 
     In general, however, the transmission rate of the DCCH need not coincide with the half-rate and full-rate specified in TIA/EIA/IS-54-B, and the length of the DCCH slots may not be uniform and may not coincide with the length of the DTC slots. The DCCH may be defined on an TIA/EIA/IS-54-B radio channel and may consist, for example, of every n-th slot in the stream of consecutive TDMA slots. In this case, the length of each DCCH slot may or may not be equal to 6.67 msec, which is the length of a DTC slot according to TIA/EIA/IS-54-B. Alternatively (and without limitation on other possible alternatives), these DCCH slots may be defined in other ways known to one skilled in the art. 
     In cellular telephone systems, an air link protocol is required in order to allow a mobile station to communicate with the base stations and MSC. The communications link protocol is used to initiate and to receive cellular telephone calls. The communications link protocol is commonly referred to within the communications industry as a Layer 2 protocol, and its functionality includes the delimiting, or framing, of Layer 3 messages. These Layer 3 messages may be sent between communicating Layer 3 peer entities residing within mobile stations and cellular switching systems. The physical layer (Layer 1) defines the parameters of the physical communications channel, e.g., radio frequency spacing, modulation characteristics, etc. Layer 2 defines the techniques necessary for the accurate transmission of information within the constraints of the physical channel, e.g., error correction and detection, etc. Layer 3 defines the procedures for reception and processing of information transmitted over the physical channel. 
     Communications between mobile stations and the cellular switching system (the base stations and the MSC) can be described in general with reference to FIGS. 1 and 2. FIG. 1 schematically illustrates pluralities of Layer 3 messages 11, Layer 2 frames 13, and Layer 1 channel bursts, or time slots, 15. In FIG. 1, each group of channel bursts corresponding to each Layer 3 message may constitute a logical channel, and as described above, the channel bursts for a given Layer 3 message would usually not be consecutive slots on an TIA/EIA/136 carrier. On the other hand, the channel bursts could be consecutive; as soon as one time slot ends, the next time slot could begin. 
     Each Layer 1 channel burst 15 contains a complete Layer 2 frame as well as other information such as, for example, error correction information and other overhead information used for Layer 1 operation. Each Layer 2 frame contains at least a portion of a Layer 3 message as well as overhead information used for Layer 2 operation. Although not indicated in FIG. 1, each Layer 3 message would include various information elements that can be considered the payload of the message, a header portion for identifying the respective message&#39;s type, and possibly padding. 
     Each Layer 1 burst and each Layer 2 frame is divided into a plurality of different fields. In particular, a limited-length DATA field in each Layer 2 frame contains the Layer 3 message 11. Since Layer 3 messages have variable lengths depending upon the amount of information contained in the Layer 3 message, a plurality of Layer 2 frames may be needed for transmission of a single Layer 3 message. As a result, a plurality of Layer 1 channel bursts may also be needed to transmit the entire Layer 3 message as there is a one-to-one correspondence between channel bursts and Layer 2 frames. 
     As noted above, when more than one channel burst is required to send a Layer 3 message, the several bursts are not usually consecutive bursts on the radio channel. Moreover, the several bursts are not even usually successive bursts devoted to the particular logical channel used for carrying the Layer 3 message. Since time is required to receive, process, and react to each received burst, the bursts required for transmission of a Layer 3 message are usually sent in a staggered format, as schematically illustrated in FIG. 2(a) and as described above in connection with the TIA/EIA/IS-136 standard. 
     FIG. 2(a) shows a general example of a forward (or downlink) DCCH configured as a succession of time slots 1, 2, . . . , N, . . . included in the consecutive time slots 1, 2, . . . sent on a carrier frequency. These DCCH slots may be defined on a radio channel such as that specified by TIA/EIA/IS-136, and may consist, as seen in FIG. 2(a) for example, of every n-th slot in a series of consecutive slots. Each DCCH slot has a duration that may or may not be 6.67 msec, which is the length of a DTC slot according to the TIA/EIA/IS-136 standard. 
     As shown in FIG. 2(a), the DCCH slots may be organized into superframes (SF), and each superframe includes a number of logical channels that carry different kinds of information. One or more DCCH slots may be allocated to each logical channel in the superframe. The exemplary downlink superframe in FIG. 2(a) includes three logical channels: a broadcast control channel (BCCH) including six successive slots for overhead messages; a paging channel (PCH) including one slot for paging messages; and an access response channel (ARCH) including one slot for channel assignment and other messages. The remaining time slots in the exemplary superframe of FIG. 2(a) may be dedicated to other logical channels, such as additional paging channels PCH or other channels. Since the number of mobile stations is usually much greater than the number of slots in the superframe, each paging slot is used for paging several mobile stations that share some unique characteristic, e.g., the last digit of the MIN. 
     FIG. 2(b) illustrates a preferred information format for the slots of a forward DCCH. The information transferred in each slot comprises a plurality of fields, and 
     FIG. 2(b) indicates the number of bits in each field above that field. The bits sent in the SYNC field are used in a conventional way to help ensure accurate reception of the coded superframe phase (CSFP) and DATA fields. The SYNC field includes a predetermined bit pattern used by the base stations to find the start of the slot. The shared channel feedback (SCF) field is used to control a random access channel (RACH), which is used by the mobile to request access to the system. The CSFP field conveys a coded superframe phase value that enables the mobile stations to find the start of each superframe. This is just one example for the information format in the slots of the forward DCCH. 
     For purposes of efficient sleep mode operation and fast cell selection, the BCCH may be divided into a number of sub-channels. A BCCH structure is known that allows the mobile station to read a minimum amount of information when it is switched on (when it locks onto a DCCH) before being able to access the system (place or receive a call). After being switched on, an idle mobile station needs to regularly monitor only its assigned PCH slots (usually one in each superframe); the mobile can sleep during other slots. The ratio of the mobile&#39;s time spent reading paging messages and its time spent asleep is controllable and represents a tradeoff between call-set-up delay and power consumption. 
     Since each TDMA time slot has a certain fixed information carrying capacity, each burst typically carries only a portion of a Layer 3 message as noted above. In the uplink direction, multiple mobile stations attempt to communicate with the system on a contention basis, while multiple mobile stations listen for Layer 3 messages sent from the system in the downlink direction. In known systems, any given Layer 3 message must be carried using as many TDMA channel bursts as required to send the entire Layer 3 message. 
     Digital control and traffic channels are desirable for reasons, such as supporting longer sleep periods for the mobile units, which results in longer battery life, for example. Digital traffic channels and digital control channels have expanded functionality for optimizing system capacity and supporting hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term &#34;macrocell&#34; generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms &#34;microcell&#34; and &#34;picocell&#34; generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments. 
     FIG. 3 is an exemplary hierarchical, or multi-layered, cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot. 
     FIG. 4 represents a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station 110 and mobile station 120. The base station includes a control and processing unit 130 which is connected to the MSC 140 which in turn is connected to the PSTN (not shown). General aspects of such cellular radiotelephone systems are known in the art, as described by U.S. Pat. No. 5,175,867 to Wejke et al., entitled &#34;Neighbor-Assisted Handoff in a Cellular Communication System,&#34; which is incorporated in this application by reference. 
     The base station 110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each base station includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the base station or cell to mobiles locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with DCCHs and DTCs that sham the same radio carrier frequency. 
     The mobile station 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the mobile station to lock on to, and determines on which cell the mobile should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described in U.S. Pat. No. 5,353,332 to Raith et al., entitled &#34;Method and Apparatus for Communication Control in a Radiotelephone System,&#34; which is incorporated in this application by reference. 
     To increase the user&#39;s &#34;talk time&#34;, i.e., the battery life of the mobile station, a digital forward control channel (base station to mobile station) may be provided that can carry the types of messages specified for current analog forward control channels (FOCCs), but in a format which allows an idle mobile station to read overhead messages when locking onto the FOCC and thereafter only when the information has changed; the mobile sleeps at all other times. In such a system, some types of messages are broadcast by the base stations more frequently than other types, and mobile stations need not read every message broadcast. 
     The systems specified by the TIA/EIA/IS-54-B and TIA/EIA/IS-136 standards are circuit-switched technology, which is a type of &#34;connection-oriented&#34; communication that establishes a physical call connection and maintains that connection for as long as the communicating end-systems have data to exchange. The direct connection of a circuit switch serves as an open pipeline, permitting the end-systems to use the circuit for whatever they deem appropriate. While circuit-switched data communication may be well suited to constant-bandwidth applications, it is relatively inefficient for low-bandwidth and &#34;bursty&#34; applications. 
     Packet-switched technology, which may be connection-oriented (e.g., X.25) or &#34;connectionless&#34; (e.g., the Internet Protocol, &#34;IP&#34;), does not require the set-up and tear-down of a physical connection, which is in marked contrast to circuit-switched technology. This reduces the data latency and increases the efficiency of a channel in handling relatively short, bursty, or interactive transactions. A connectionless packet-switched network distributes the routing functions to multiple routing sites, thereby avoiding possible traffic bottlenecks that could occur when using a central switching hub. Data is &#34;packetized&#34; with the appropriate end-system addressing and then transmitted in independent units along the data path. Intermediate systems, sometimes called &#34;routers&#34;, stationed between the communicating end-systems make decisions about the most appropriate route to take on a per packet basis. Routing decisions are based on a number of characteristics, including: least-cost route or cost metric; capacity of the link; number of packets waiting for transmission; security requirements for the link; and intermediate system (node) operational status. 
     Packet transmission along a route that takes into consideration path metrics, as opposed to a single circuit set up, offers application and communications flexibility. It is also how most standard local area networks (LANs) and wide area networks (WANs) have evolved in the corporate environment. Packet switching is appropriate for data communications because many of the applications and devices used, such as keyboard terminals, are interactive and transmit data in bursts. Instead of a channel being idle while a user inputs more data into the terminal or pauses to think about a problem, packet switching interleaves multiple transmissions from several terminals onto the channel. 
     Packet data provides more network robustness due to path independence and the routers&#39; ability to select alternative paths in the event of network node failure. Packet switching, therefore, allows for more efficient use of the network lines. Packet technology offers the option of billing the end user based on amount of data transmitted instead of connection time. If the end user&#39;s application has been designed to make efficient use of the air link, then the number of packets transmitted will be minimal. If each individual user&#39;s traffic is held to a minimum, then the service provider has effectively increased network capacity. 
     Packet networks are usually designed and based on industry-wide data standards such as the open system interface (OSI) model or the TCP/IP protocol stack. These standards have been developed, whether formally or de facto, for many years, and the applications that use these protocols are readily available. The main objective of standards-based networks is to achieve interconnectivity with other networks. The Internet is today&#39;s most obvious example of such a standards-based network pursuit of this goal. 
     Packet networks, like the Internet or a corporate LAN, are integral parts of today&#39;s business and communications environments. As mobile computing becomes pervasive in these environments, wireless service providers such as those using TIA/EIA/IS-136 are best positioned to provide access to these networks. Nevertheless, the data services provided by or proposed for cellular systems are generally based on the circuit-switched mode of operation, using a dedicated radio channel for each active mobile user. 
     A few exceptions to data services for cellular systems based on the circuit-switched mode of operation are described in the following documents, which include the packet data concepts. 
     U.S. Pat. No. 4,887,265 and &#34;Packet Switching in Digital Cellular Systems&#34;, Proc. 38th IEEE Vehicular Technology Conf., pp. 414-418 (June 1988) describe a cellular system providing shared packet data radio channels, each one capable of accommodating multiple data calls. A mobile station requesting packet data service is assigned to a particular packet data channel using essentially regular cellular signalling. The system may include packet access points (PAPS) for interfacing with packet data networks. Each packet data radio channel is connected to one particular PAP and is thus capable of multiplexing data calls associated with that PAP. Handovers are initiated by the system in a manner that is largely similar to the handover used in the same system for voice calls. A new type of handover is added for those situations when the capacity of a packet channel is insufficient. 
     These documents are data-call oriented and based on using system-initiated handover in a similar way as for regular voice calls. Applying these principles for providing general purpose packet data services in a TDMA cellular system would result in spectrum-efficiency and performance disadvantages. 
     U.S. Pat. No. 4,916,691 describes a new packet mode cellular radio system architecture and a new procedure for routing (voice and/or data) packets to a mobile station. Base stations, public switches via trunk interface units, and a cellular control unit are linked together via a WAN. The routing procedure is based on mobile-station-initiated handovers and on adding to the header of any packet transmitted from a mobile station (during a call) an identifier of the base station through which the packet passes. In case of an extended period of time between subsequent user information packets from a mobile station, the mobile station may transmit extra control packets for the purpose of conveying cell location information. 
     The cellular control unit is primarily involved at call establishment, when it assigns to the call a call control number. It then notifies the mobile station of the call control number and the trunk interface unit of the call control number and the identifier of the initial base station. During a call, packets are then routed directly between the trunk interface unit and the currently serving base station. 
     The system described in U.S. Pat. No. 4,916,691 is not directly related to the specific problems of providing packet data services in TDMA cellular systems. 
     &#34;Packet Radio in GSM&#34;, European Telecommunications Standards Institute (ETSI) T Doc SMG 4 58/93 (Feb. 12, 1993) and &#34;A General Packet Radio Service Proposed for GSM&#34; presented during a seminar entitled &#34;GSM in a Future Competitive Environment&#34;, Helsinki, Finland (Oct. 13, 1993) outline a possible packet access protocol for voice and data in GSM. These documents directly relate to TDMA cellular systems, i.e., GSM, and although they outline a possible organization of an optimized shared packet data channel, they do not deal with the aspects of integrating packet data channels in a total system solution. 
     &#34;Packet Data over GSM Network&#34;, T Doc SMG 1 238/93, ETSI (Sep. 28, 1993) describes a concept of providing packet data services in GSM based on first using regular GSM signalling and authentication to establish a virtual channel between a packet mobile station and an &#34;agent&#34; handling access to packet data services. With regular signalling modified for fast channel setup and release, regular traffic channels are then used for packet transfer. This document directly relates to TDMA cellular systems, but since the concept is based on using a &#34;fast switching&#34; version of existing GSM traffic channels, it has disadvantages in terms of spectrum efficiency and packet transfer delays (especially for short messages) compared to a concept based on optimized shared packet data channels. 
     Cellular Digital Packet Data (CDPD) System Specification, Release 1.0 (July 1993), which is expressly incorporated herein by reference, describes a concept for providing packet data services that utilizes available radio channels on current Advanced Mobile Phone Service (AMPS) systems, i.e., the North American analog cellular system. CDPD is a comprehensive, open specification endorsed by a group of U.S. cellular operators. Items covered include external interfaces, air link interfaces, services, network architecture, network management, and administration. 
     The specified CDPD system is to a large extent based on an infrastructure that is independent of the existing AMPS infrastructure. Commonalities with AMPS systems are limited to utilization of the same type of radio frequency channels and the same base station sites (the base station used by CDPD may be new and CDPD specific) and employment of a signalling interface for coordinating channel assignments between the two systems. 
     Routing a packet to a mobile station is based on, first, routing the packet to a home network node (home Mobile Data Intermediate System, MD-IS) equipped with a home location register (HLR) based on the mobile station address; then, when necessary, routing the packet to a visited, serving MD-IS based on HLR information; and finally transferring the packet from the serving MD-IS via the current base station, based on the mobile station reporting its cell location to its serving MD-IS. 
     Although the CDPD System Specification is not directly related to the specific problems of providing packet data services in TDMA cellular systems that are addressed by this application, the network aspects and concepts described in the CDPD System Specification can be used as a basis for the network aspects needed for an air link protocol in accordance with this invention. 
     The CDPD network is designed to be an extension of existing data communications networks and the AMPS cellular network. Existing connectionless network protocols may be used to access the CDPD network. Since the network is always considered to be evolving, it uses an open network design that allows the addition of new network layer protocols when appropriate. The CDPD network services and protocols are limited to the Network Layer of the OSI model and below. Doing so allows upper-layer protocols and applications development without changing the underlying CDPD network. 
     From the mobile subscriber&#39;s perspective, the CDPD network is a wireless mobile extension of traditional networks, both data and voice. By using a CDPD service provider network&#39;s service, the subscriber is able seamlessly to access data applications, many of which may reside on traditional data networks. The CDPD system may be viewed as two interrelated service sets: CDPD network support services and CDPD network services. 
     CDPD network support services perform duties necessary to maintain and administer the CDPD network. These services are: accounting server; network management system; message transfer server; and authentication server. These services are defined to permit interoperability among service providers. As the CDPD network evolves technically beyond its original AMPS infrastructure, it is anticipated that the support services shall remain unchanged. The functions of network support services are necessary for any mobile network and are independent of radio frequency (RF) technology. 
     CDPD network services are data transfer services that allow subscribers to communicate with data applications. Additionally, one or both ends of the data communications may be mobile. 
     To summarize, there is a need for a system providing general purpose packet data services in D-AMPS cellular systems, based on providing shared packet-data channels optimized for packet data. This application is directed to systems and methods that provide the combined advantages of a connection-oriented network like that specified by the TIA/EIA/IS-136 standard and a connectionless, packet data network. 
     One important aspect in such systems is the allocation of channels or bandwidth. One example of such channel allocation for IS-136 is mobile assisted channel allocation (MACA). In IS-136, a MACA message is received before assigning the traffic channel and is typically sent on the broadcast control channel (BCCH). For example, procedures used in making contention- or reservation-based access attempts may be sent in an access parameter message on the fast BCCH. Examples of such IS-136 random access parameters include maximum busy/reserved information, maximum retries information, maximum repetitions information, and a maximum stop counter. Since MACA reports are used before assigning traffic channels, MACA does not provide any information after the mobile station accesses the system. 
     Another important aspect of cellular telephone communication systems is equalization which is used to compensate for irregularities or deficiencies in the radio medium. An equalizer is primarily used in receiving circuits for the purpose of reducing the effects of multipath propagation and, in a cellular system, the effects of relative motion between the transmitter and receiver. This is described, for instance, in WO 88/05981, which relates to a TDMA system which includes so-called adaptive equalization. The setting of the equalizer incorporated in the radio receiver is contingent on synchronizing words that are time multiplexed with data words transmitted from the radio transmitter. With the aid of these synchronizing words, the equalizer can be set so as to compensate for the dispersion properties of the medium. Radio receivers which include equalizers are often used for high symbol rate communication (&gt;100 kbit/s), the bit sensitivity of which due to multipath propagation is greater than the bit sensitivity of lower symbol rate communication. One disadvantage of using equalizers is that they increase a receiver&#39;s complexity and power consumption. 
     The absence of an equalizer affords the advantage of enabling noncoherent demodulation to be applied, which results in a lower degree of complexity in the receiver and a lower current consumption. In addition, a robust receiver is obtained with rapidly varying radio channels, due to high vehicle speeds. The disadvantage lies in the fact that the demodulation cannot be carried out with time dispersion, which constitute a considerable part of the symbol time. 
     SUMMARY 
     According to one embodiment of this invention, a method for reassigning mobile stations a new phase within a channel is disclosed. In a communication system, a plurality of mobile stations attempt to communicate on a limited number of channels. The invention first uses a general allocation method to approximately evenly distribute the mobile stations on the available channels. After a mobile station begins active communication on the system, a second allocation method is used to change the phase of some mobile stations on a channel which has become heavily loaded. In addition, a message can also be sent to mobile stations assigned to but not yet actively communicating on the heavily loaded channel indicating that the mobile stations should change to a new channel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of Applicants&#39; invention will be understood by reading this description in conjunction with the drawings in which: 
     FIG. 1 schematically illustrates pluralities of Layer 3 messages, Layer 2 frames, and Layer 1 channel bursts, or time slots; 
     FIG. 2(a) shows a forward DCC configured as a succession of time slots included in the consecutive time slots sent on a carrier frequency; 
     FIG. 2(b) shows an example of an IS-136 DCCH field slot format; 
     FIG. 3 illustrates an exemplary hierarchical, or multi-layered, cellular system; 
     FIG. 4 is a block diagram of an exemplary cellular mobile radiotelephone system, including an exemplary base station and mobile station; 
     FIG. 5 illustrates one example of a possible mapping sequence; 
     FIG. 6 illustrates an example of PDCH reassignment; 
     FIG. 7 illustrates a flow chart according to one embodiment of this invention; 
     FIG. 8 illustrates a full rate mobile station receiving a new phase assignment; and 
     FIG. 9 illustrates a double rate mobile station receiving a new phase assignment. 
    
    
     DETAILED DESCRIPTION 
     To aid in the understanding of the present invention, a description for one possible mapping sequence is illustrated in FIG. 5. It will be apparent to one skilled in the art that the present invention is not limited to this mapping sequence but also applies to other mapping sequences as well. FIG. 5 shows a dedicated packet digital control channel (PDCH) example of how one L3 message is mapped into several Layer 2 frames, an example of a Layer 2 frame mapping onto a time slot, and an example of time slot mapping onto a PDCH channel. The length of the forward packet digital control channel (FPDCH) time slots and reverse packet digital control channel (RPDCH) bursts are fixed. There are three forms of RPDCH bursts which have different fixed lengths and the FPDCH slot and the full-rate PDCH are assumed to be on the physical layer in FIG. 5. In the present invention, the TDMA frame structure is the same as for IS-136 DCCH and DTC. In the interest of maximal throughput when a multi-rate transmission is used (double rate PDCH and triple rate PDCH), an additional FPDCH slot format is specified. 
     The digital control channel (DCCH) of IS-136 is used to indicate PDCH operation. FIG. 6 illustrates the relationship between PDCH&#39;s belonging to one cell (or more specifically, having a common mother DCCH) and DCCH&#39;s in different cells (more specifically, indicated in the DCCH neighboring list as candidates for DCCH reselection). A mobile station always first goes to a DCCH at initial cell selection. On the DCCH, the support for PDCH is indicated. If the DCCH indicates support for one or more dedicated PDCH&#39;s, the carrier frequency of one PDCH (beacon PDCH) is provided. The mobile station then registers itself on the beacon PDCH and may be reassigned by the system to another dedicated PDCH. A more detailed description of the beacon PDCH is provided in U.S. application Ser. No. 08/544,488, entitled &#34;A Method for System Registration and Cell Reselection&#34; filed herewith and expressly incorporated herein by reference. 
     In a communication system, a plurality of mobile stations are attempting to communicate on a limited number of channels wherein each channel may contain a plurality of timeslots. From an efficiency point of view, it is advantageous not to have all of the mobile stations assigned to one channel, but rather to have them spread out over all the channels. One phase allocation method is described below and is illustrated in FIG. 7. Thus, the system, during packet data channel assignment, can assign mobile stations to channels in such a manner that the mobile stations are approximately evenly distributed among the available channels. For example, the mobile stations can be assigned to channels based upon their mobile station identification MSID or the last digit of their phone number. The distribution can also be influenced by attributes of the mobile stations as set forth during the registration process. For example, the mobile station may indicate that it is a full-rate, double-rate, or triple-rate mobile station. Thus, the system could consider one triple-rate mobile station to have the same potential load as three full-rate mobile stations when assigning channels at mobile station registration. Thus, the system might assign two triple-rate mobile stations to one channel while assigning six full-rate mobile stations to another channel since the two triple-rate mobile stations have the same potential load as the six full-rate mobile stations. 
     After the mobile stations have been assigned to a packet data channel, the mobile stations are in a &#34;waiting-for-packet&#34; state wherein the mobile station is not communicating with the system. Mobile stations in the &#34;waiting-for-packet&#34; state can read all of the timeslots except for the broadcast slots. In addition, a mobile station may be in a sleep mode wherein the mobile station regularly wakes up for short periods of time to read a paging slot or channel. 
     Since a mobile station may have less transmission capability and receiving capability than the channel, for instance, when a full-rate mobile station is assigned to a triple-rate channel, the system may have problems communicating with the mobile station. A full-rate mobile station can only listen to one of the three slots, i.e., there are three possible phases (slot 1, slot 2, slot 3). For example, a full-rate mobile station will only be listening to one phase of its assigned channel, e.g., a PPCH subchannel (PPCH --  SUBCH). A double-rate mobile station operates on two of the three possible slots (slots 1 and 2, slots 2 and 3, and slots 1 and 3). Finally, a triple-rate mobile station operates on all of the slots, hence there is no rendezvous problem which needs to be solved. However, for mobile stations operating in full-rate and double-rate, the phase of the mobile station and the system must be equal. There are two activities for which this rendezvous problem must be addressed: when the mobile station is in a &#34;waiting-for-packet&#34; state; and when the mobile station is in a sleep mode. 
     For mobile stations in the sleep mode, the present invention solves the rendezvous problem by only paging mobile stations on a predetermined phase using a full-rate channel of the channel since all mobile stations must be able to operate at the full-rate. For example, the system only pages the mobile stations in the first phase of the channel. In addition, the particular paging slot which is contained in one TDMA out of many TDMA frames can be determined using the permanent mobile station&#39;s identity which can be a MIN or an IMSI. 
     The paging slots are only assigned to a full-rate and phase for several reasons. First, there will typically be fewer mobile stations camping on a dedicated PDCH than a DCCH. This holds true even if all of the mobile stations are packet data capable and all users have activated their packet data service account since all PDCH mobiles camp on the DCCH after extensive non-activity. Secondly, the required paging capacity for mobiles camped on the PDCH is reduced compared with IS-136 operation since the paging area is limited to one cell. Finally, the layer 2 frames are longer compared to the frames in IS-136, if less channel coding is used. To faster the ability of grouping multiple pages into a single layer 2 frame, avoiding scattering the pages over all of the PDCH slots, e.g., on a triple rate PDCH, there should not be too many paging slots in relation to the number of mobile stations camping on a PDCH. 
     When mobile stations have been assigned a PDCH channel but are not in the sleep mode, the mobile stations are reading all of the slots according to its capabilities. As a result, the mobile stations can be assigned evenly to the different phases based upon some parameter, for example the mobile station identification number. Furthermore, the mobile stations may determine which phase (a subset of slots) to read according to Tables 1 and 2 illustrated below. 
     
                       TABLE 1______________________________________Mobile Station PARCH Phase Allocation   Mobile Station PARCH Channel Usage (Active Mode)Mode of Calculate .O slashed.Mobile  Full-Rate  Double Rate Triple RateStation PDCH       PDCH        PDCH______________________________________Full-Rate   .O slashed. = Undefined              .O slashed. =                          .O slashed. = PPCH.sub.-- SUBCH              PPCH.sub.-- SUBCH                          mod 3              mod 2Double Rate   Not applicable              .O slashed. = Undefined                          .O slashed. = PPCH.sub.-- SUBCH                          mod 3Triple Rate   Not applicable              Not applicable                          .O slashed. = 0______________________________________ 
    
     
                       TABLE 2______________________________________    Mobile Station PARCH Phase (Active Mode)    Slot Number Assignments (mod 4)Mode of  Full-Rate   Double Rate Triple RateMobile Station    PDCH        PDCH        PDCH______________________________________Full-Rate    All PDCH Slots                Master + .O slashed.                            Master + .O slashed.Double Rate    Not applicable                All PDCH slots                            Master + .O slashed. and                            Master + .O slashed. + 1Triple Rate    Not applicable                Not applicable                            All PDCH slots______________________________________ 
    
     An alternative solution for the rendezvous problem for mobile stations in a sleep mode or a &#34;waiting-for-packet&#34; state, is to send the mobile station the necessary synchronization in a registration response message. For example, the registration response message may contain the rate and phase information. In this alternative, the PPCH subchannel does not need to be tied to the permanent mobile station identity and the paging channel must not be restricted to full-rate. In addition, the assigned phase of a mobile station in the &#34;waiting-for-packet&#34; state can also be given in the registration response message. 
     Once a mobile station begins actively communicating with the system, the phase of the mobile station can be further reassigned so as to correct any allocation problems arising out of the general allocation method described above. In this embodiment, a Layer 2 command can be used to inform the mobile station that it should change its phase, i.e., the mobile station should shift the phase on which it is transmitting and receiving. In this embodiment, the frequency of the mobile stations that are already communicating with the system are not changed. Thus, if there are too many mobile stations transmitting on a certain channel, only the phase within the channel can be shifted. However, a Layer 3 message can be used to tell mobile stations that have not yet began communicating that they should switch to another channel so that when they begin to communicate, they will not be operating on an overloaded channel. 
     A change in the phase allocation affects the forward and reverse channel. In order to avoid severe time constraints on the mobile station, the mobile station is allowed to maintain its current phase allocation for the next scheduled transmission event (one slot) for a full rate transmission and for the next two scheduled transmission events (two slots) for a double rate transmission. FIGS. 8 and 9 illustrate examples of full rate and double rate mobile stations receiving a new phase assignment. As illustrated in FIGS. 8 and 9, the phase reassignment occurs after a full frame. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.