United States Patent (19)
`Jasper et al.
`
`IIII IIH|IIII
`USOO5533004A
`Patent Number:
`11)
`5,533,004
`45) Date of Patent:
`Jul. 2, 1996
`
`(54) METHOD FOR PROVIDING AND
`SELECTING AMONGST MULTIPLE DATA
`RATES IN A TIME DIVISION MULTIPLEXED
`SYSTEM
`
`(75) Inventors: Steven C. Jasper, Hoffman Estates;
`Kenneth J. Crisler, Wheaton, both of
`Il.
`73 Assignee: Motorola, Inc., Schaumburg, Ill.
`
`21 Appl. No.: 334,982
`(22 Filed:
`Nov. 7, 1994
`6
`I51) Int. Cl. ........................................... "On H04B 122
`I52 U.S. Cl. .............................. 370/11; 370/84; 3095 .3;
`8
`375/265; 375/298
`(58) Field of State, 375,261.263,26i",s'',
`s
`awways
`s
`y
`371143
`
`56
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`l/1993 Hurlbut et al. ...................... 370/100.1
`5, 182,746
`3/1993 Halbert-Lassalle et al. ............. 370/1
`5,197,061
`5,233,629 8/1993 Paik et al. ............................... 375/262
`5,305,352 4/1994 Calderbank et al. ................... 375/261
`5,369,637 11/1994 Richardson et al. ...................... 370/84
`5,377,194 12/1994 Calderbank ......................... 375/261 X
`Primary Examiner-Douglas W. Olms
`Assistant Examiner-Min Jung
`Attorney, Agent, or Firm-Christopher P. Moreno
`(57)
`ABSTRACT
`o
`In a Radio Frequency (RF) communication system using
`Time Division Multiple Access (TDMA) having time slots
`of a common duration, a quantity of information bits to be
`transmitted is provided. Based at least in part on the number
`of information bits to be transmitted, a modulation technique
`is selected from a plurality of modulation techniques. Based
`at least in part on the modulation technique selected and the
`common duration of the time slots, the information bits are
`formatted into blocks, each block containing an equal num
`ber of information bits. The blocks are transmitted in the
`time slots such that a predetermined symbol rate is main
`tained.
`
`1/1985 Acampora .............................. 370/95.3
`4,495,619
`5,117,453 5/1992 Piasecki et al. ........................ 379/100
`
`23 Claims, 3 Drawing Sheets
`
`PACKET BLOCK(168 BITS)1-0
`- - - - - - - - - - - - -
`CONVOLUTIONAL
`
`
`
`-304
`
`-02
`ERGBERSF")
`ENCODED BLOCK
`(168/R BITS)
`BIT TOSYMBOL MAPPER
`305
`(K BITS/SYMBOL)
`303
`- - - - - - - - - - - - -
`
`
`
`|
`
`FORMAT CODE RATE NODULATION RATE
`NUMBER
`(R)
`(K)
`
`INTERLEAVE SIZE
`
`
`
`EFFECTIVE DATA
`RATE(BITS/SYMBOL)
`(
`
`- 309
`
`s
`
`4: 1
`6:1
`
`6
`
`Qualcomm Incorporated
`Exhibit 1012
`Page 1 of 9
`
`

`

`U.S. Patent
`
`Sheet 1 of 3
`
`5,533,004
`F I G. f.
`
`Jul. 2, 1996
`102
`
`\ 103
`
`MOBILE
`RADIO
`
`104
`
`101
`
`105
`
`BASE
`
`100
`
`204
`
`FI G. 2
`
`200
`
`201
`
`
`
`DIGITAL SIGNAL
`PROCESSOR
`
`203
`
`
`
`
`
`
`
`CONVOLUTIONAL
`ENCODER(RATE R)
`304
`ENCODED BLOCK
`(168/R BITS) h
`BIT TO SYWBOL WAPPER
`(K BITS/SYMBOL)
`
`- 309
`NORMAL TRANSMISSION UNIT(168 SYMBOLS)
`
`Page 2 of 9
`
`

`

`U.S. Patent
`
`Jul. 2, 1996
`
`Sheet 2 of 3
`
`5,533,004
`FI G. 4
`EFFECTIVE DATA
`RATE(BITS/SYMBOL)
`
`twig at
`(K)
`2 (QPSK)
`
`INTERLEAVE SIZE
`(BLOCKS/SLOT)
`
`3/4
`
`8 (2560A)
`
`|
`
`6
`
`||
`
`6:1
`
`
`
`
`
`O
`
`5
`
`256CA
`
`
`
`QPSK
`
`16QAM
`
`64QAM
`
`256QAM
`
`FI G. 6
`
`
`
`QPSK 2
`501
`16QAM 2.
`64-QAM
`
`1/6 1/4
`
`1/2
`t(SLOTS)
`
`1
`
`FI G 6
`
`610
`
`BLOCK
`630
`BLOCK
`640
`BLOCK
`
`BLOCK 1
`620
`
`601
`
`602
`
`630
`BLOCK 4
`640
`640
`BLOCK 5
`BLOCK 6 h60
`
`603
`
`620
`BLOCK 2
`630
`630
`BLOCK
`BLOCK 2
`640
`640
`640
`BLOCK 4
`BLOCK 2
`BLOCK 3
`1 SLOT
`
`Page 3 of 9
`
`

`

`U.S. Patent
`
`Jul. 2, 1996
`
`Sheet 3 of 3
`
`5,533,004
`
`FI G. 7
`
`TRANSMISSION
`
`700
`
`
`
`
`
`INFORMATION BITS
`
`701
`
`
`
`SELECT
`NODULATION TECHNIQUE
`
`FORMAT BLOCKS
`(IDENTICAL NUMBER OF BITS)
`
`TRANSMIT BLOCKS AT
`CONSTANT SYMBOL RATE
`
`
`
`702
`
`
`
`
`
`703
`
`704
`
`Page 4 of 9
`
`

`

`5,533,004
`
`1
`METHOD FOR PROVIDENG AND
`SELECTING AMONGST MULTIPLE DATA
`RATES IN A TIME DIVISION MULTIPLEXED
`SYSTEM
`
`FIELD OF THE INVENTION
`This invention relates generally to data communication
`Systems
`
`10
`
`15
`
`20
`
`25
`
`BACKGROUND OF THE INVENTION
`Radio frequency (RF) communication systems for the
`transmission of data information (i.e. binary coded informa
`tion) are well-known in the art. RF data communication
`systems generally provide a single channel data rate to their
`users. In these systems, the modulation and error coding are
`designed to provide acceptable performance for users at the
`edge of the desired coverage area, where generally worst
`case signal quality conditions are experienced.
`It is well-known that, at signal quality levels typical of
`those found in closer proximity to a transmitting antenna
`(rather than at the edge of a radio coverage area), higher data
`rates with corresponding higher data throughputs are pos
`sible. It is also well-known that a relatively wide dynamic
`range of signal quality levels (e.g. 20-80 dB or decibels)
`typically exists within the coverage area of a mobile radio
`communication system. Therefore, users of prior art data
`communication systems who experience signal quality lev
`els significantly above those found near the fringe of the
`coverage area generally suffer a lower grade of performance,
`in terms of data throughput, than would otherwise be pos
`sible.
`In the field of wireline telecommunications, data modems
`35
`that provide multiple data rates in response to signal quality
`levels are well-known. The methods used in this art, how
`ever, are not well-suited for application to radio data systems
`in general, and particularly to radio systems employing Time
`Division Multiple Access (TDMA). In TDMA systems, the
`radio channel is divided into a series of time slots of
`predetermined constant duration, which are typically further
`grouped into frames, each frame containing a predetermined
`number of time slots. Multiple users are allowed to access
`the radio communication channel by transmitting in one or
`more time slots in each frame. Thus a complete communi
`cation is composed of a series of multiple transmissions,
`such that the duration of each transmission is equal to the
`time slot duration.
`Radio data communication methods typically transmit
`data in variable length messages referred to as packets.
`Packets are formed by dividing the data into a series of
`fixed-size protocol units referred to as blocks. The combi
`nation of the data block size, the data transmission rate, and
`the TDMA slot size determines how effectively the TDMA
`55
`channel can be used. For example, if an integer number of
`blocks would not fit evenly into each time slot, the capacity
`representing the fractional block may go unused, reducing
`the available throughput of the channel. Alternately, a syn
`chronization method could be implemented to permit all of
`the data capacity to be utilized, but such techniques are often
`complex. This additional complexity manifests itself in
`increased cost and in additional communication overhead
`that also reduces available throughput.
`Assuming a predetermined time slot duration, it is pos
`sible to choose a block size that avoids these problems for
`a single transmission data rate. A problem arises, however,
`
`40
`
`45
`
`50
`
`60
`
`65
`
`2
`when seeking to provide a channel that will support multiple
`data rates.
`Accordingly, a need arises for providing a plurality of data
`rates for use with an RF data system so that users may select
`that data rate that provides the best performance for their
`signal quality level. It is further desired that the multiple data
`rates be provided in a manner such that a TDMA commu
`nication channel can be utilized efficiently by a packet data
`protocol.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram of a Radio Frequency commu
`nication system in accordance with the present invention.
`FIG. 2 is a block diagram of a radio device that may be
`used to implement the present invention.
`FIG. 3 is a block diagram of a general Forward Error
`Correction (FEC) and formatting procedure in accordance
`with the present invention.
`FIG. 4 is a table illustrating combinations of modulation
`rates and code rates in accordance with the present inven
`tion.
`FIG. 5 is a diagram illustrating the duration of data blocks
`using multiple data rates in accordance with the preferred
`embodiment.
`FIG. 6 is a diagram illustrating the format of TDMA time
`slots using multiple data rates in accordance with the pre
`ferred embodiment.
`FIG. 7 is a flow chart illustrating the method of the
`preferred embodiment.
`
`DETAILED DESCRIPTION
`The following paragraphs describe in detail a method for
`maximizing data communication system throughput in a
`fashion that avoids the shortcomings revealed in the fore
`going discussion of the background art. The method
`described combines multiple Forward Error Correction
`(FEC) procedures with multiple modulation constellations,
`resulting in multiple data rates optimized for a given signal
`quality measure to provide maximum data throughput for
`signal conditions.
`In a preferred embodiment, the method may be applied to
`a Radio Frequency (RF) communication system using
`TDMA (Time Division Multiple Access) to integrate mul
`tiple services, such as voice and packet data, within the same
`RF communication channel. Of course, the principles
`described herein are equally applicable to many other types
`of communication systems as well.
`Referring to FIG. 1, the RF communication system (100)
`of the preferred embodiment makes use of one or more RF
`communication channels (101) to provide a variety of com
`munication services, among them voice and data (i.e. binary
`coded information) communications. Each RF communica
`tion channel (101) in fact is comprised of two RF frequen
`cies (102,103), about which the radio signals are modulated.
`One of the RF frequencies is referred to as the inbound
`frequency (102) and is used for the transmission of infor
`mation from mobile radio devices (mobile units) (104) to
`base radio devices (base units) (105). The second of the RF
`frequencies is referred to as the outbound frequency (103)
`and is used for the transmission of information from base
`units (105) to mobile units (104). Frequency assignments are
`typically made in a manner such that there is a constant
`spacing or offset between the inbound and outbound fre
`quency of a communication channel. Further, in the pre
`
`Page 5 of 9
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`5,533,004
`
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`3
`ferred embodiment, the offset between adjacent communi
`cation channels (i.e. between adjacent inbound frequencies
`or adjacent outbound frequencies) is 25 kHz.
`The configuration of radio units (104, 105) in accordance
`with the preferred embodiment is illustrated in the block
`diagram of FIG. 2. Information to be transmitted is provided
`by a user input device (201). The user input device may be
`a keyboard in the case of a mobile unit (104) or a network
`or computer interface in the case of a base unit (105).
`Information supplied by the user input device (201) is
`applied to a microprocessor (202) and a digital signal
`processor (203). The microprocessor (202) and the digital
`signal processor (203) workin concert to encode and format
`the information for transmission. The encoding and format
`ting operations are generally executed via algorithms which
`are typically implemented with sequences of software com
`mands stored in the memory devices provided (204, 205).
`The memory devices (204, 205) will also typically contain
`read/write memory used to store the information during
`processing., After the information is encoded and formatted,
`20
`it is passed by the digital signal processor (203) to the RF
`unit (206) for transmission on the RF channel (101). The
`microprocessor (202) acts to control features of the RF unit
`(e.g. timing) to ensure that the information transmission is
`compatible with the requirement of the RF channel (101).
`RF signals received by the RF unit (206) are applied to the
`digital signal processor (203) for demodulation and decod
`ing. As in the transmit case, the microprocessor (202) acts to
`control the RF unit (206) and the digital signal processor
`(203) in accordance with predetermined reception algo
`rithms. After the received information is decoded, the infor
`mation is presented to the user output device (207). The user
`output device (207) may be a data terminal display in the
`case of a mobile unit (104) or a network or computer
`interface in the case of a base unit (105). Thus the radio unit
`(200) depicted in FIG. 2 acts to transmit and receive
`information between user devices and the RF channel (101).
`Each of the inbound and outbound frequencies (102, 103)
`comprising the RF channel (101) are divided in time into a
`continuous series of time slots of equal or common duration.
`In the preferred embodiment, the common duration of each
`time slot is 15 ms. Using a multiple access method well
`known in the art as Time Division Mutliple Access (TDMA),
`information is transmitted in the communication channel in
`bursts equal to the size of the time slots. In the preferred
`embodiment, the bursts are modulated onto the RF fre
`quency using a Quadrature Amplitude Modulation (QAM)
`technique. QAM techniques are well-known by those skilled
`in the art as a means of modulating information organized
`into two-dimensional or complex symbols. Complex sym
`50
`bols are comprised of two scalar values, an in-phase value
`and a quadrature-phase value. These values are typically
`taken from a discrete set of values, with each value repre
`senting a binary coded number. For example, a 2 bit (binary
`digit) number would be represented by one of four possible
`55
`values. The set of values represented by the complex sym
`bols is referred to as a QAM constellation. The number of
`distinct values represented by a single symbol (the number
`of in-phase values times the number of quadrature-phase
`values) is typically used to describe the order or size of a
`QAM constellation. Thus, a QAM technique using 4 discrete
`values for each component is referred to as 16 QAM.
`The QAM technique of the preferred embodiment is used
`to transmit symbols at a constant symbol rate. Hence, given
`the common duration of the time slots, a common number of
`symbols may be transmitted in each time slot. Some number
`of the symbols in each time slot are used for synchronization
`
`30
`
`4
`and other purposes not directly involved in the transfer of
`user information. The number of symbols remaining, 168 in
`the preferred embodiment, are used to communicate user
`information. The group of 168 symbols is referred to as a
`Normal Transmission Unit (NTU).
`In the discussion that follows, the application of the
`present invention to the communication of packet data is
`described. Those of reasonable skill in the art should rec
`ognize how the concepts disclosed herein could be applied
`to other forms of communication. Radio data communica
`tion methods typically transmit data in variable length
`messages referred to as packets. Packets are formed by
`dividing the data into a series of fixed-size protocol units
`referred to as blocks. The first block of a packet is typically
`referred to as the header block and contains addressing and
`other data communication control information. Subsequent
`blocks typically contain the user data to be communicated
`by the communication system. In addition to the user data or
`header information, each block is also configured with error
`detection coding, e.g. a Cyclic Redundancy Check (CRC)
`code, to permit the receiving unit to determine if errors
`occurred due to fading, noise, or interference during the
`transmission of the block. In the preferred embodiment, each
`block, including user data and CRC coding, is comprised of
`165 bits of information.
`The Forward Error Correction (FEC) coding and format
`ting procedure used to transmit the data blocks will be
`described in conjunction with the block diagram shown in
`FIG.3. The transmit procedure is described here; the receive
`process is a straightforward reversal of the transmit process.
`In the preferred embodiment, these processes would be
`implemented in the digital signal processor (203) depicted in
`FIG. 2.
`User data is first parsed into blocks as described earlier
`(not shown in FIG.3). Blocks (301), each 165 bits long, are
`first encoded using a trellis encoder (304). Conceptually, the
`trellis encoder (304) may be viewed as the combination of
`a convolutional encoder (302) and a bit-to-symbol mapper
`(303), where the designs of the coder and mapper have been
`jointly optimized to achieve desirable performance under a
`predetermined range of signal quality. The convolutional
`encoder (302) operates to encode the input data using a shift
`register memory element at an encoding rate R, where R is
`the ratio of input bits to output bits and is generally less than
`one. For example, a coder of rate /2 will produce 2 output
`bits for every input bit. The convolutional coder may also
`append additional input bits, referred to as flush bits, to the
`end of the input block which are used to return the encoder
`shift register memory to a known state following encoding.
`In the preferred embodiment, 3 flush bits are appended by
`the convolutional encoder (302). Thus, the convolutional
`encoder (302) produces an output encoded block (305) of
`length 168/R bits. The encoded block (305) is fed into a
`bit-to-symbol mapper (303) that takes groups of encoded
`bits and produces complex symbols. This operation also has
`a characteristic rate, the number of bits grouped into each
`symbol K. Together, the coding and symbol mapping steps
`form a trellis encoder (304) with coding rate R and constel
`lation size 2.
`The output of the trellis encoder (304) is a symbol block
`(306) containing 168/KR complex symbols. This symbol
`block is inserted into a symbol block buffer (307) of length
`KR blocks. After KR symbol blocks are collected in the
`buffer, a symbol block interleaver (308) combines the blocks
`to form a single NTU (309) of 168 symbols. This NTU (309)
`is output to a RF modulation process for transmission
`according to principles well-known in the art.
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`It should be clear from the preceding discussion that the
`parameters K and R can be adjusted to control the number
`(K*R) of blocks (301) assembled per NTU (309), allowing
`variable user data rates to be supported while constraining
`K*R to be an integer. Four combinations which satisfy these
`criteria are summarized in FIG. 4. For example, consider the
`first combination in FIG. 4. Here a rate /2 code is used so that
`the convolutional encoder (302) would produce 336 output
`bits from the 165 input bits of each block (301). In this case,
`a 4 QAM (also known as Quadrature Phase Shift Keyed
`(QPSK)) constellation is used which groups 2 bits into each
`symbol. Thus, 168 symbols are produced from each block
`(301). Hence, one such encoded block can be placed in a
`single NTU. Thus the effective information bit to transmitted
`symbol ratio is 1:1.
`Now consider the second combination in FIG. 4. Here a
`rate % code is also used. However, in this case, a 16 QAM
`constellation is used which groups 4 bits into each symbol.
`Thus, 84 symbols are produced from each block (301).
`Hence, 2 such encoded blocks Can be placed in a single
`NTU comprised of 168 symbols producing an effective
`information bit to transmitted symbol ratio of 2:1. Note that
`since the NTU in either case occupies a single channel time
`slot, the effective data rate of the second combination is
`twice that of the first. Similarly, the third and fourth com
`25
`binations in FIG. 4 illustrate other code rate and QAM
`constellations that are utilized to produce effective informa
`tion bit to transmitted symbol ratios of 4:1 and 6:1, respec
`tively, so that data rates four and six times that of the first
`combination are realized.
`The operation of the encoder is further illustrated by the
`timing diagrams of FIG. 5 and FIG. 6. In FIG. 5, the transmit
`time duration of a single encoded block is shown for each of
`the encoding combinations shown in FIG. 4. The QPSK
`encoded block (501) has a duration equal to the full NTU.
`35
`Alternately, the 16 QAM encoded block (502), the 64 QAM
`encoded block (503), and the 256 QAM encoded block (504)
`have durations of /2, 4, and/6 of the full NTU respectively.
`FIG. 6 illustrates NTUs formatted using each of the
`40
`encoding combinations shown in FIG. 4. The first NTU
`(601) is comprised of a single QPSK eneoded block (610).
`The second NTU (602) is comprised of two 16 QAM
`encoded blocks (620). The third NTU (603) is comprised of
`four 64 QAM encoded blocks (630). Finally, the fourth NTU
`45
`(604) is comprised of six 256 QAM encoded blocks (640).
`Each of the NTUs (601-604) have a common duration such
`that each occupies a single channel time slot.
`In an alternate embodiment, the bit-to-symbol mapper
`(303) of FIG.3 may take groups of K/2 encoded bits and
`produce scalar values. The output of the trellis encoder (304)
`is a scalar block (306) containing 336/KR scalar values. The
`scalar block is inserted into the scalar block buffer (307) of
`length KR blocks. After KR scalar blocks are collected in the
`buffer, the symbol block interleaver (308) combines the
`55
`blocks, while simultaneously combining pairs of scalar
`values to form complex symbols, to form a single NTU
`(309) of 168 symbols. This NTU (309) is output to a RF
`modulation process for transmission according to principles
`well-known in the art.
`Note that packets are always encoded into an integer
`number of NTUs. Thus, channel capacity is never wasted
`within a string of blocks transmitted at any of the available
`data rates.
`It is intended that the different data rate options described
`here be used to optimize the net rate of user throughput
`which will vary according to the RF channel characteristics.
`
`50
`
`6
`It is well understood in the art that as the signal quality is
`improved, data communications at higher data rates, with
`correspondingly higher throughput, are feasible. The deter
`mination of the RF channel signal quality is not the subject
`of this invention. There exist many well known means of
`signal quality estimation, such as Received Signal Strength
`Indicators (RSSI), Bit Error Rate (BER) measures and the
`like, any of which could be used effectively with the present
`invention.
`A typical packet communication would begin with the
`data unit, either base or mobile, selecting an initial or default
`modulation technique for its first transmission. The default
`selection may be based on a predetermined computation of
`which technique would have the highest probability of use.
`Alternately, the default selection may be based on the initial
`value of one or more of channel quality estimation measures.
`Subsequent transmissions would then take advantage of one
`or more of the available signal quality estimation measures
`to update the data rate selection.
`Alternately, the modulation technique may be chosen as a
`function of the quantity of user data to be transmitted.
`Specifically, a lower rate modulation technique would be
`chosen if the time required to send the data at the lower rate
`was the same as the time required to send the data at the
`higher rate. To illustrate this, consider an example where the
`signal quality is determined to be sufficient to use the 256
`QAM modulation technique. If the data packet to be trans
`mitted is 9-12 blocks in length, it would require 2 slots to
`transmit at 256 QAM whereas it would require 3 slots using
`the 64 QAM modulation technique. In general, 256 QAM is
`chosen for packets longer than 8 blocks since the transmit
`time will always be less than that of a slower modulation
`technique. However, for a packet that is 7 or 8 blocks long,
`the transmit time will be 2 slots for either modulation. In this
`case, 64 QAM is used since the effective throughput is the
`same while the reliability of the data transmission is
`improved. Similar relationships exist for each of the modu
`lation techniques such that the higher rate technique is
`chosen for packets longer than a threshold value and a lower
`rate may be chosen for packets of shorter duration.
`In yet another embodiment, the modulation selection may
`be a function of the destination, or intended recipient, of the
`data packet. For example, if the packet is intended for a
`single recipient, the modulation technique may be chosen as
`described above. However, if the packet is intended for more
`than one recipient, the preferred modulation technique may
`be one well suited to provide acceptable throughput to the
`recipient experiencing the worst signal quality as compared
`to the other recipients. Thus, a modulation technique with a
`lower effective data rate would typically be chosen for data
`transmitted to a plurality of recipients. Alternately, a lower
`rate technique (with correspondingly higher transmission
`reliability) may be chosen for particularly important recipi
`ents or particularly critical data.
`The data transmission operation of the present invention
`will now be described with reference to the flow chart (700)
`depicted in FIG. 7. The process begins with a quantity of
`information bits provided to be transmitted to at least one
`intended recipient (701). In step 702, one of a plurality of
`modulation techniques, wherein each of the modulation
`techniques has a corresponding effective data transmission
`rate, is selected. The information bits are then formatted into
`blocks (703) such that each block contains 165 bits of
`information. The blocks are then transmitted in at least one
`of the channel time slots using the selected modulation
`technique (704). Note that, regardless of the choice of
`modulation technique, the formatted blocks are transmitted
`at a constant symbol rate on the RF channel.
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`In a system of the present invention, it is important for the
`receiver of a data packet to know the modulation technique
`that was used by the transmitter so that proper decoding of
`the transmitted data can be effected. In the inbound direc
`tion, prior to sending each data packet, a mobile unit may
`transmit a short preamble that includes an indication of the
`data rate that has been selected for the transmission of its
`packet. This short preamble is always transmitted using a
`known predetermined modulation technique so that the base
`unit can decode its contents and use the information con
`tained therein for properly decoding the following data
`packet. Thus, each mobile unit may independently select a
`data rate for each packet.
`The series of packets transmitted on the outbound channel
`is typically completely independent of the inbound packets.
`The modulation technique in use on the outbound channel is
`signaled by an additional data field, referred to as the Slot
`Descriptor Block (SDB), included in each time slot. As in
`the case of the inbound packet preamble, the SDB is
`transmitted using a known predetermined modulation tech
`nique. Thus, a receiving unit first decodes the SDB to
`determine how to decode the remainder of the slot, inde
`pendent of any traffic on the inbound side of the same
`channel. Similarly to the inbound, the data rate outbound
`may be varied on a packet-by-packet basis. However, since
`each outbound slot has a SDB associated with it, the data
`rate may also be changed on a slot-by-slot basis if desired.
`Thus, the invention as described provides a combination
`of multiple Forward Error Correction (FEC) coding rates
`and multiple modulation constellations which are used to
`effect multiple channel data rates. Multiple data rates pro
`vide multiple levels of throughput while maintaining an
`efficient use of the TDM slot resource. The constant symbol
`rate of the channel is unchanged, only the information
`content of each symbol is altered. Further, means are pro
`vided for each data unit, mobile and fixed, to independently
`choose the preferred data rate in response to several factors,
`among them packet length and data recipient.
`What is claimed is:
`1. A method comprising the steps of:
`providing a communication channel having a plurality of
`time slots, wherein each of the plurality of time slots
`has a common duration;
`providing a quantity of information bits to be transmitted
`to at least one intended recipient;
`providing a plurality of modulation techniques having
`corresponding effective data transmission rates;
`selecting one of the plurality of modulation techniques to
`produce a selected modulation technique;
`regardless of which of the plurality of modulation tech
`niques was selected:
`formatting the information bits into at least one block,
`such that an identical number of information bits are
`always contained within a single block;
`transmitting the at least one block in at least one of the
`time slots at a predetermined constant symbol rate
`using the selected modulation technique.
`2. The method of claim 1, wherein the step of transmitting
`the at least one block in at least one of the time slots includes
`the step of transmitting the at least one block such that all of
`the time slots will only contain an integer number of the
`blocks.
`3. The method of claim 1, wherein the step of providing
`a communication channel includes the step of providing a
`65
`communication channel that is one of a plurality of com
`munication channels, wherein the plurality of communica
`
`40
`
`45
`
`50
`
`55
`
`60
`
`5,533,004
`
`8
`tion channels are offset from adjacent communication chan
`nels by 25 kHz.
`4. The method of claim 3, wherein the step of providing
`a communication channel having a plurality of time slots
`includes the step of providing a communication channel
`having a plurality of time slots, wherein each time slot has
`a duration of 15 milliseconds.
`5. The method of claim 1, wherein the step of providing
`a plurality of modulation techniques includes the step of
`providing a plurality of modulation techniques, wherein one
`of the plurality of modulation techniques comprises a 64
`quadrature amplitude modulation technique.
`6. The method of claim 5, wherein the step of providing
`a plurality of modulation techniques includes the step of
`providing a plurality of modulation techniques, wherein one
`of the plurality of modulation techniques comprises a 16
`quadrature amplitude modulation technique.
`7. The method of claim 6, wherein the step of providing
`a plurality of modulation techniques includes the step of
`providing a plurality of modulation techniques, wherein:
`one of the plurality of modulation techniques comprises a
`256 quadrature amplitude modulation technique; and
`one of the plurality of modulation techniques comprises a
`quadrature phase shift keying modulation technique.
`8. The method of claim 1, wherein the step of selecting
`one of the plurality of modulation techniques includes the
`step of selecting a predetermined one of the plurality of
`modulation techniques as an initial default selection.
`9. The method of claim 1, wherein the step of selecting
`one of the plurality of modulation techniques includes the
`step of selecting one of the plurality of modulation tech
`niques at least as a function of the quantity of information
`bits.
`10. The method of claim 9, wherein the step of selecting
`one of the plurality of modulation techniques at least as a
`function of the quantity of information bits includes the step
`of using a first modulation technique when the quantity of
`information bits are greater in number than a predetermined
`quantity.
`11. The method of claim 10, wherein the step of selecting
`one of the plurality of modulation techniques at least as a
`function of the quantity of information bits includes the step
`of using a second modulation technique when the quantity of
`information bits are lesser in number than a predetermined
`quantity, wherein the second modulation technique has a
`lower effective data rate than the first modulation technique.
`12. The method of claim 1, wherein the step of selecting
`one of the plurality of modulation techniques includes the
`step of selecting one of the plurality of modulation tech
`niques as a function of the at least one intended recipient.
`13. The method of claim 12, wherein the step of selecting
`one of the plurality of modulation.techniques as a function
`of the at least one intended recipient includes the step of
`selecting a first modulation technique when there is only one
`intended recipient.
`14. The method of claim 13, wherein the step of selecting
`one of the plurality of modulation techniques as a f