United States Patent
`
`[191
`
`[11] Patent Number:
`
`5,757,813
`
`Date of Patent:
`Raith
`[45] May 26, 1998
`
`
`
`USOOS757813A
`
`[54] METHOD FOR ACHIEVING OPTIMAL
`CHANNEL CODING IN A COMMUNICATION
`SYSTEM
`
`[75]
`
`Inventor: Alex Krister Raith. DLn'ham. NC.
`
`[73] Assignee: Telefonaktiebolaget LM Ericsson.
`Stockholm. Sweden
`
`[21] Appl. No.: 544,491
`
`[22] Filed:
`
`Oct. 18, 1995
`
`
`Int. Cl.6
`[51]
`. .............................. G06F 11/00
`
`[52] US. Cl.
`............
`.. 37115.5; 371/51
`[58] Field of Search ............................... 371/55. 5.4. 5.1;
`375/225; 370/951. 95.3. 84. 79; 340/825.44.
`311.1
`
`[56]
`
`References Cited
`US. PATENT DOCUMENTS
`
`6/1989 Wilson et a1.
`............................ 371/32
`4,841,526
`12/1989 Felix ............
`370/941
`4,887,265
`
`...... 370/60
`4/1990 Goodman .
`4,916,691
`
`.
`.. 455/33.l
`12/1992 Wejke et a1.
`5.175.867
`............................... 379/59
`5,353,332 10/1994 Raith et a1.
`
`FOREIGN PATENT DOCUMENTS
`
`7/1986 European Pat. Off. .
`188 271
`197 545 10/1986 European Pat. 01f. .
`423 485
`4/1991
`European Pat. Off. .
`627 827
`12/1994 European Pat. Oil. .
`2 160 392
`12/1985 United Kingdom .
`W092/22162 10/1992 WIPO .
`
`OTHER PUBLICATIONS
`
`“Mobile Station—Base Station Compatibility Standard for
`Dual-Mode Wideband Spread Spectrum Cellular System.”
`TIA/EIA/IS—QS—A. pp. 6—7 through 6—11. 6—19 through
`6—22. 6—29 through 6—40 (May 1995).
`
`“Cellular System Dual—Mode Mobile Station—Base Station
`Compatibility Standard”. ElA/TIA Inter/m Standard.
`IS—54—B. pertinent pages only. Apr. 1992.
`
`K. Felix. “Packet Switching in Digital Cellular Systems”.
`Proc. 38th IEEE Vehicular Technology Conf. pp. 414—418.
`Jun. 1988.
`
`P. Decker et al.. “A General Packet Radio Service Proposed
`for GSM". GSM in a Future Competitive Environment.
`Helsinki. Finland. pp. 1—20. Oct. 13. 1993.
`
`P. Decker. “Packet Radio in GSM". European Telecommu-
`nications Standards Institute (ETSI). T Doc SMG 4 58/93.
`pp. 1—13 (odd pages only). Feb. 12. 1993.
`
`1. Ham'a’l'ainen et al.. “Packet Data Over GSM Network”. T
`
`Doc SMG 1 238/93. ETSI. pp. 1-8. Sep. 28, 1993.
`
`Primary Examiner—Phung M. Chung
`Attorney, Agent, or Firm—Burns. Doane. Swecker &
`Mathis. L.L.P.
`
`[57]
`
`ABSTRACT
`
`A method for indicating a change in coding rate so as to
`maintain synchronization between a communication system
`and a mobile station. A mobile station can request either to
`increase or decrease the degree of channel coding. The
`system can grant the request and send an indication to the
`mobile station indicating the new degree of coding. The
`indication is provided outside the field in which the coding
`rate is going to be changed. A modulation symbol alphabet
`can also changed.
`
`10 Claims, 8 Drawing Sheets
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`Broadcom v. Ericsson
`Broadcom V. Ericsson
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`Broadcom 1024
`
`

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`May 26, 1998
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`

`5.757.813
`
`1
`METHOD FOR ACHIEVING OPTIMAL
`CHANNEL CODING IN A COMMUNICATION
`SYSTEM
`
`BACKGROUND
`
`invention relates to electrical
`Applicants‘
`telecommunication. and more particularly to wireless com—
`munication 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-S4—B. “Dual-Mode Mobile Station-
`Base Station Compatibility Standard”. published by the
`Telecommunications Industry Association and Electronic
`Industries Association (TIA/BIA) which is expressly incor-
`porated 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/
`ElA/IS-54-B standard provides for both FDMA analog voice
`channels (AVC) and TDMA digital n'aflic channels (UTC).
`The AVCs and DTCs are implemented by frequency modu-
`lating 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’s digital traflic 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.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`In only one of many possible embodiments of a TDMA
`system as described above. the TWEIA/IS-54—B standard
`provided that each TDMA frame consists of six consecutive
`time slots and has a duration of 40 milliseconds (msec).
`
`65
`
`2
`Thus. each radio channel can carry from three to six UFCs
`(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’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 bufier-and—burst. or discontinuous—nansmission. 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/BIA!
`IS-S4-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
`T1AlEIA/IS-54-B system tunes to and then regularly moni-
`tors 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
`“lose" radio connection on the control channel of the “old“
`cell and tune to the control channel of the “new” 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 “best" 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 “in touc ” 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 num-
`ber (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
`
`

`

`5.757.813
`
`3
`matching stored MIN transmits a page response over the
`particular control channel to the base station. which for-
`wards 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 trans-
`ceiver 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.
`
`is
`The performance of the system having ACCs that
`specified by TIA/EIA/ISS4—B has been improved in a system
`having digital control channels (DCCHs) that is specified in
`TIA/ELAIIS— 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 D'l‘Cs/DCCHs. or six halfrate 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
`'I'IA/EIA/IS-54B. Alternatively (and without limitation on
`other possible alternatives).
`these DCCH slots may be
`defined in other ways known to one skilled in the art.
`is
`In cellular telephone systems. an air link protocol
`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 resid-
`ing 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 infor-
`mation 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 corre-
`sponding 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 T1AlEIA/l36 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.
`
`4
`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 indi-
`cated 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’s type. and possibly padding.
`Each Layer 1 burst and each Layer 2 frame is divided into
`a plurality of dilferent 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/EIAlIS— 136 standard.
`
`As shown in FIG. 2(a). the DCCH slots may be organized
`into superfiames (SF). and each superframe includes a
`number of logical channels that carry diflerent kinds of
`information. One or more DCCH slots may be allocated to
`each logical channel in the superfiame. 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
`superfiame. each paging slot is used for paging several
`mobile stations that share some unique characteristic. tag.
`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
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`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 super-
`frame phase (CSFP) and DATA fields. The SYNC field
`includes a predetermined bit pattern used by the base
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`5
`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 eflicient 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’s time spent reading paging mes—
`sages and its time spent asleep is controllable and represents
`a tradeotf between call-set—up delay and power consump-
`tion.
`Since each TDMA time slot has a certain fixed informa-
`tion 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 traflic 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 “macrocell”
`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
`“microcell” and “picocell” 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 oflice 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 diflerent
`trafiic 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 rep
`resented 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 exem-
`plary 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 radio-
`telephone systems are known in the art. as described by US.
`
`6
`Pat. No. 5.175.867 to Wejke et al.. entitled “Neighbor-
`Assisted Handotf in a Cellular Communication System.”
`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 trans—
`ceiver 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 share the same radio carrier frequency.
`The mobile station 120 receives the information broadcast
`on a connol channel at its voice and control channel trans-
`ceiver 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 proxi-
`mate to the cell with which the control channel is associated.
`as described in US. Pat. No. 5.353.332 to Raith et al..
`entitled “Method and Apparatus for Communication Control
`in a Radiotelephone System.” which is incorporated in this
`application by reference.
`To increase the user’s “talk time”. 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 sta-
`tions need not read every message broadcast.
`The systems specified by the TIA/EIA/IS-S4-B and TIA/
`EMS—136 standards are circuit-switched technology.
`which is a type of “connection-oriented” 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 appropri-
`ate. While circuit- switched data communication may be well
`suited to constant-bandwidth applications. it is relatively
`inefficient for low-bandwidth and “bursty” applications.
`Packet-switched technology. which may be connection-
`oriented (e.g.. X25) or “connectionless” (e.g.. the Internet
`Protocol. “IP”). 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 efliciency of a channel in handling rela-
`tively short. bursty. or interactive transactions. A connec-
`tionless 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 “packetized” with the appropriate
`end-system addressing and then transmitted in independent
`units along the data path. Intermediate systems. sometimes
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`7
`called “routers”. 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 consid-
`eration path metrics. as opposed to a single circuit set up.
`ofiers application and communications flexibility. It is also
`how most standard local area networks (LANs) and wide
`area networks (WANs) have evolved in the corporate envi-
`ronment. Packet switching is appropriate for data commu-
`nications because many of the applications and devices
`used. such as keyboard terminals. are interactive and trans-
`mit 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’ 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's application has been designed to make
`efiicient use of the air link. then the number of packets
`transmitted will be minimal. If each individual user’s traflic
`is held to a minimum then the service provider has effec—
`tively increased network capacity.
`Packet networks are usually designed and based on
`industry-wide data standards such as the open system inter-
`face (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’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’s business and communications envi-
`ronments. As mobile computing becomes pervasive in these
`environments. wireless service providers such as those using
`'I'IAIEIA/IS-l36 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 “Packet Switching in Digital
`Cellular Systems”. Free. 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 particu—
`lar 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
`
`8
`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
`regllar voice calls. Applying these principles for providing
`general purpose packet data services in a TDMA cellular
`system would result in spectrum-efliciency 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 mobilestation—initiated han-
`dovers and on adding to the header of any packet transmitted
`from a mob