High power short message service using dedicated carrier frequency

A communication system and method for transmitting relatively short data messages in the communication system. A dedicated frequency is sequentially switched into each of a plurality of satellite beams or traffic channels to transmit data messages at an increased power level to provide an increased signal margin. The increased power level of the dedicated frequency can be combined with coding and bit and message repetition to further increase the signal margin.

FIELD OF THE INVENTION
 The present invention generally relates to radiocommunication systems and
 more particularly relates to a system and method for reliably transmitting
 alphanumeric messages via radiocommunication signals under non-ideal
 conditions.
 BACKGROUND OF THE INVENTION
 Referring to FIG. 1, a typical cellular mobile radiocommunication system is
 shown. The typical system includes a number of base stations similar to
 base station 110 and a number of mobile units or stations similar to
 mobile 120. Voice and/or data communication can be performed using these
 devices or their equivalents. The base station includes a control and
 processing unit 130 which is connected to the MSC (mobile switching
 center) 140 which in turn is connected to the public switched telephone
 network (not shown).
 The base station 110 serves a cell and includes a plurality of voice
 channels handled by 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. The voice
 channel transceiver broadcasts the traffic or voice channels which can
 include digital control channel location information.
 When the mobile 120 first enters an idle mode, it periodically scans the
 control channels of base stations like base station 110 for the presence
 of a paging burst addressed to the mobile 120. The paging burst informs
 mobile 120 which cell to lock on or camp to. The mobile 120 receives the
 absolute and relative 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 the candidate cells and determines which cell the
 mobile should lock to. 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. These
 adjacent cells are periodically scanned while monitoring the primary
 control channel to determine if there is a more suitable candidate.
 Additional information relating to specifics of mobile and base station
 implementations can be found in U.S. patent application Ser. No.
 07/967,027 entitled "Multi-Mode Signal Processing" filed on Oct. 27, 1992
 to P. Dent and B. Ekelund, now U.S. Pat. No. 5,745,523 the entirety of
 which is incorporated herein by reference. It will be appreciated that the
 base station may be replaced by one or more satellites in a
 satellite-based mobile radiocommunication system.
 To increase radiocommunication system capacity, digital communication and
 multiple access techniques such as Frequency Division Multiple Access
 (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple
 Access (CDMA) may be used. The objective of each of these multiple access
 techniques is to combine signals from different sources onto a common
 transmission medium in such a way that, at their destinations, the
 different channels can be separated without mutual interference. In a FDMA
 system, users share the radio spectrum in the frequency domain. Each user
 is allocated a part of the frequency band which is used throughout a
 conversation. In a TDMA system, users share the radio spectrum in the time
 domain. Each radio channel or carrier frequency is divided into a series
 of time slots, and individual users are allocated a time slot during which
 the user has access to the entire frequency band allocated for the system
 (wideband TDMA) or only a part of the band (narrowband TDMA). Each time
 slot 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. CDMA combines FDMA and TDMA. In a CDMA system,
 each user is assigned a unique pseudorandom user code to uniquely access
 the frequency time domain. Examples of CDMA techniques include spread
 spectrum and frequency hopping.
 In a TDMA system, 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 traffic channel, which is considered to be a
 logical channel assigned to the user. The organization of TDMA channels,
 using the GSM standard as an example, is shown in FIG. 2. The TDMA
 channels include traffic channels TCH and signalling channels SC. The TCH
 channels include full-rate and half-rate channels for transmitting voice
 and/or data signals. The signalling channels SC transfer signalling
 information between the mobile unit and the satellite (or base station).
 The signalling channels SC include three types of control channels:
 broadcast control channels (BCCHs), common control channels (CCCHs) shared
 between multiple subscribers, and dedicated control channels (DCCHs)
 assigned to a single subscriber. A BCCH typically includes a frequency
 correction channel (FCH) and a synchronization channel (SCH), both of
 which are downlink channels. The common control channels (CCCHs) include
 downlink paging (PCH) and access grant (AGCH) channels, as well as the
 uplink random access channel (RACH). The dedicated control channels DCCH
 include a fast associated control channel (FACCH), a slow associated
 control channel (SACCH), and a standalone dedicated control channel
 (SDCCH). The slow associated control channel is assigned to a traffic
 (voice or data) channel or to a standalone dedicated control channel
 (SDCCH). The SACCH channel provides power and frame adjustment and control
 information to the mobile unit.
 The frequency correction channel FCH of the broadcast control channel
 carries information which allows the mobile unit to accurately tune to the
 base station. The synchronization channel SCH of the broadcast control
 channel provides frame synchronization data to the mobile unit.
 Using a GSM-type system as an example, the slow associated control channel
 SACCH can be formed by dedicating every 26th TDMA frame to carrying SACCH
 information. Each SACCH frame includes 8 time slots (1 SACCH slot for each
 traffic slot in the frame), allowing one unique SACCH channel for each
 mobile communication link. The base station or satellite sends commands
 over the SACCH channel to advance or retard the transmission timing of the
 mobile unit to achieve time alignment between different mobile bursts
 received at the base station or satellite.
 The random access channel RACH is used by the mobiles to request access to
 the system. The RACH logical channel is a unidirectional uplink channel
 (from the mobile to the base station or satellite), and is shared by
 separate mobile units (one RACH per cell is sufficient in typical systems,
 even during periods of heavy use). Mobile units continuously monitor the
 status of the RACH channel to determine if the channel is busy or idle. If
 the RACH channel is idle, a mobile unit desiring access sends its mobile
 identification number, along with the desired telephone number, on the
 RACH to the base station or satellite. The MSC receives this information
 from the base station or satellite and assigns an idle voice channel to
 the mobile station, and transmits the channel identification to the mobile
 through the base station or satellite so that the mobile station can tune
 itself to the new channel. All time slots on the RACH uplink channel are
 used for mobile access requests, either on a contention basis or on a
 reserved basis. Reserved-basis access is described in U.S. patent
 application No. 08/140,467, entitled "Method of Effecting Random Access in
 a Mobile Radio System", which was filed on Oct. 25, 1993, now U.S. Pat.
 No. 5,420,864 and which is incorporated in this application by reference.
 One important feature of RACH operation is that reception of some downlink
 information is required, whereby mobile stations receive real-time
 feedback for every burst they send on the uplink. This is known as Layer 2
 ARQ, or automatic repeat request, on the RACH. The downlink information
 preferably comprises twenty-two bits that may be thought of as another
 downlink sub-channel dedicated to carrying, in the downlink, Layer 2
 information specific to the uplink. This flow of information, which can be
 called shared channel feedback, enhances the throughput capacity of the
 RACH so that a mobile station can quickly determine whether any burst of
 any access attempt has been successfully received. As shown in FIG. 2,
 this downlink information is transmitted on channel AGCH.
 Transmission of signals in a TDMA system occurs in a buffer-and-burst, or
 discontinuous-transmission, mode: each mobile unit transmits or receives
 only during its assigned time slots in the TDMA frames on the mobile
 unit's assigned frequency. 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. The mobile unit,
 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.
 To increase mobility and portability, radiocommunication subscribers tend
 to prefer mobile units having a relatively small, omnidirectional (and
 accordingly, less powerful) antenna over mobile units having a large or
 directional antenna. Because of this preference, it is sometimes difficult
 to provide sufficient signal strength for the exchange of communication
 signals between typical mobile units having a small, omnidirectional
 antenna and a mobile switching center (MSC) or satellite. This problem is
 particularly serious in satellite-based mobile radiocommunications.
 A satellite-based mobile radiocommunication system provides
 radiocommunication services to particular geographical areas of the earth
 using one or more partially overlapping satellite beams. Each satellite
 beam has a radius of up to about 1000 KM. Due to the power limitations of
 a satellite, it is not practical to provide a high link margin in every
 beam simultaneously.
 Because mobile satellite links are severely power limited, communication is
 typically limited to line-of-sight channels with Ricean fading. Ricean
 fading occurs from a combination of a strong line-of-sight path and a
 ground-reflected wave, along with weak building-reflected waves. These
 channels require a communications link margin of approximately 10 dB or
 more to achieve voice communication in ideal or near-ideal conditions,
 such as when the mobile radiotelephone unit antenna is properly deployed
 and the unit is in an unobstructed location. In these near-ideal channels,
 the mobile unit can successfully monitor the paging channel to detect
 incoming calls. In non-ideal conditions, such as when the mobile unit
 antenna is not deployed or the mobile unit is in an obstructed location
 (e.g., inside a building) reflected waves, including ground-reflected and
 building-reflected waves, become dominant. The channels in these non-ideal
 conditions are characterized by flat Rayleigh fading (the most severe type
 of fading) with severe attenuation. In such channels, a link margin of as
 much as 30 dB or more is required to achieve reliable voice or data
 communication, and the mobile unit in this case cannot monitor the paging
 channel to detect incoming calls. In these non-ideal conditions, a short
 message service (SMS) is desirable. Due to the power limitations of the
 satellite, SMS is particularly effective when used in non-ideal conditions
 to alert a mobile station user of an incoming call. The mobile station
 user may then change locations to receive or return the call. The term
 "link margin" or "signal margin" refers to the additional power required
 to offer adequate service over and above the power required under ideal
 conditions--that is, a channel having no impairments other than additive
 white Gaussian noise (AWGN). "Impairments" include fading of signal
 amplitude, doppler shifts, phase variations, signal shadowing or blockage,
 implementation losses, and anomalies in the antenna radiation pattern.
 Whether transmitting voice or data, it is frequently desirable to increase
 the signal margin to ensure reliable radiocommunication performance,
 particularly in power-limited satellite applications. Known methods of
 increasing the link margin of a signal include expanding the channel
 bandwidth to achieve frequency selectivity or to use forward error
 correction coding (such as convolutional coding), increasing signal power,
 and bit repetition (which may be viewed as a form of forward error
 correction coding). Each of these methods has significant limitations.
 Bandwidth expansion is typically achieved by known methods such as signal
 spreading and low bit rate error correction coding, and results in a
 signal which is less sensitive to fading. Bandwidth expansion reduces
 spectrum allocation efficiency. Further, in a SMS application, if the
 expanded bandwidth of the voice channel is different from the bandwidth of
 the message channel, two separate and complete radios (one for each
 service) will be required in the mobile unit, thereby complicating its
 design. Also, a coherent Rake receiver or equalizer is also typically
 required to reduce delay spread, further complicating the design of the
 mobile unit. Bandwidth expansion may also be implemented by repeated
 transmissions of the entire voice or data message. However, under the
 non-ideal conditions of interest, this method is not effective because
 each repetition is typically below the noise floor (that is, does not have
 a sufficient margin), resulting in a high error rate and preventing the
 coherent integration of the repetitions.
 Increasing signal power may also be used to provide a higher margin. Due to
 the power limitations of the satellite, this is typically not a practical
 approach. In addition to increasing the cost of the system, increased
 transmission power also makes it more difficult to control co-channel
 interference, particularly in TDMA systems with narrow re-use margins.
 Accordingly, large power increases from the satellite to the mobile unit
 may be provided only during periods of relatively light use. Further,
 because the mobile unit is even more power limited than the satellite,
 this technique is typically practical only in one direction, from the
 satellite to the mobile unit.
 Bit repetition may also be used to increase the margin. Bit repetition
 results in a lower error rate than message repetition, particularly in
 non-ideal conditions. Bit repetition causes transmission delay, which is
 not desirable for voice signals, for obvious reasons. However,
 transmission delay may be acceptable for data communications, such as a
 SMS feature, provided that the delay is kept to a reasonable minimum. Bit
 repetition is achieved by transmitting individual bits or modulation
 symbols, or packets of bits or modulation symbols, a plurality of times
 such that all repetitions are contiguous or contained within the same time
 slot or slots of successive TDMA frames. The receiver integrates the
 energy from each repetition to create a signal having a higher margin. As
 noted above, bit repetition can cause significant delay, depending upon
 the length of the message. To achieve a 30 dB signal margin, each bit will
 have to be repeated 1000 times. A typical short message has between 32 and
 64 characters in the GSM system, the European digital standard, up to 245
 characters in the DAMPS (Digital Advanced Mobile Phone Service IS-136)
 system currently used in the United States, and up to 160 characters in
 the DECT (Digital European Cordless Telephone) system. Assuming a GSM
 system having TDMA frames of 18.64 ms, with 16 slots per frame and 114
 data bits/slot, the minimum delay for receiving a 64 character message,
 not including propagation time, is as follows:
EQU 64 bits.times.8 bits/character.times.1000 repetitions/bit.times.18.64
 ms/slot.times.1/114 slot/data bit=84 seconds.
 Such a delay is highly undesirable, even for data transmission.
 It has been suggested to implement a short message service in a
 satellite-based telecommunications system using 2 different satellite
 transponders, one for voice and one for data. Different channel bandwidths
 are also used in this implementation. The use of multiple transponders and
 bandwidths results in an overly complex system.
 Accordingly, it would be desirable for a radiocommunication system to allow
 for transmission of signals at an increased signal margin without
 significant delay and without a significant increase in power.
 It would be further desirable for a communication system to allow for
 transmission of signals with an increased signal margin without requiring
 expansion of the channel bandwidth, multiple bandwidths, or multiple
 transponders.
 It would also be desirable for a TDMA communication system to allow for
 transmission of signals with an increased signal margin without requiring
 a change in the structure or organization of TDMA frames.
 It would be further desirable for a mobile radiocommunication system to
 allow for transmission of data messages originating from either a mobile
 unit or from a satellite or base station with an increased signal margin.
 It would be further desirable for a communication system to selectively
 increase the signal margin of a communication link for the transmission of
 data messages.
 SUMMARY OF THE INVENTION
 The above-noted and other limitations of conventional communication systems
 and methods are overcome by the present invention, which provides for a
 high-penetration transmission method for transmitting short alphanumeric
 messages in which signal margin is increased by a combination of bit
 repetition and a relatively small increase in power. According to
 exemplary embodiments, the combination of bit repetition and a relatively
 small increase in power avoids the unacceptable delays characteristic of
 systems which rely solely on repetition to increase the signal margin.
 Likewise, the combination of repetition and a relatively small increase in
 power avoids the co-channel interference problems of systems which rely
 solely on increased power to increase the signal margin.
 According to an exemplary embodiment of the present invention, a mobile
 radiocommunication system is provided with a short message service feature
 for transmitting alphanumeric messages to and from a mobile unit. In order
 to ensure reliable transmission over channels having severe attenuation,
 the data message is encoded; the encoded message is divided into packets
 or groups of one or more bits each; each packet is transmitted multiple
 times over a dedicated carrier frequency at a power level greater than the
 power level for voice transmission; and the transmissions are integrated
 and checked for errors at the receiver to form a signal having an
 increased signal margin. The carrier frequency for transmitting message
 data for a satellite is multiplexed with, or switched into, the satellite
 beams. Thus, a given satellite beam exchanging voice and control data with
 a particular subscriber over an assigned frequency will stop exchanging
 voice and control data when the dedicated carrier frequency is switched
 into the beam, and will instead be used to transmit message data. When the
 dedicated frequency is switched out of one beam and into another beam, the
 first beam will again be used to exchange voice and control information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 While the following description is directed toward a short message service
 implemented in a satellite-based radiocommunication system, it will be
 appreciated that the present invention may also be applied to other types
 of communication systems.
 In a satellite-based mobile radiocommunication system, a communication link
 for transmitting voice or data may be established between a mobile station
 and either a standard telephone or a second mobile station through one
 satellite, multiple satellites, or a combination of one or more satellites
 and the PSTN (public switched telephone network). Such a system, as shown
 in FIG. 3, may be desirable to achieve a broad geographical coverage in
 which few or no base stations are present, and additional base stations
 are not practical, such as in rural areas. Due to the inherent power
 limitations of satellites, voice communications links between the
 satellite and the mobile station require ideal or near-ideal conditions;
 that is, conditions such as line-of-sight communication with the mobile
 station's antenna properly deployed. In non-ideal conditions, such as when
 the mobile station is shadowed (e.g., inside a building, etc.) or when the
 mobile antenna is not properly deployed, the power or signal margin
 requirements for communication increases significantly due to the
 increased attenuation in the channel. In such situations (shown as MUz in
 FIG. 3), Rayleigh fading often prevents satisfactory communication, and it
 is therefore desirable to send a short alphanumeric message to the mobile
 station. The message may be used, for example, to inform the subscriber of
 an incoming call. The present invention ensures reliable transmission of
 the message by providing for an efficient technique for increasing signal
 margin without significant delay, power increase, or co-channel
 interference.
 For purposes of illustration only, and without limiting the scope of the
 invention, a satellite-based GSM radiocommunication system using TDMA
 channels may be assumed to exhibit the following conditions. The
 communication channel has no line of sight component and is subject to
 flat Rayleigh fading with severe attenuation. As will be appreciated by
 those of skill in the art, Rayleigh (or multipath) fading is a phenomenon
 which occurs when multipath waves form standing-wave pairs due to
 reflection from the physical structures in a service area. The
 standing-wave pairs summed together form an irregular wave fading
 structure. When the mobile unit is stationary, it receives a constant
 signal. However, when the mobile unit is moving, the fading structure
 causes fading to occur which increases as the mobile unit moves faster.
 The mean signal level of the non-ideal Rayleigh channel is approximately
 20-30 dB below the signal level of a near-ideal line-of-sight channel.
 In order to ensure reliable transmission of a short message to the mobile
 unit in non-ideal conditions, the signal margin must be increased.
 According to the present invention, bit repetition and power increase can
 be combined to provide an increased signal margin without significant
 delay.
 It will be appreciated that decibels (dB) are units used to express ratios
 of power, current, or voltage. Specifically, a power ratio (P2/P1) may be
 expressed in decibels by the formula dB=10 log (P2/P1). A signal margin of
 30 dB requires a power ratio of 1000, since 10 log 1000=30. Thus, to
 achieve this signal margin solely by bit repetition, each bit must be
 repeated 1000 times and the signal margin from each repetition must be
 integrated at the receiver, resulting in the 82 second delay calculated
 above. However, to achieve a 15 dB margin, the required power ratio is
 only 31.623, since 10 log 31.623=15. Thus, a 30 dB signal margin may be
 provided by increasing the power by 15 dB and repeating each bit
 approximately 31 times. Using this technique, the bit-repetition delay for
 a 64 character message is (64 characters.times.8 bits/character.times.31
 repeats/bit.times.18.64 ms/slot.times.1/114 slot/bits) approximately 2.5
 seconds. As a result, the bit repetition delay is maintained at a
 reasonable level, and the power increase is also maintained at a
 reasonable level, thereby avoiding co-channel interference. It will be
 appreciated that many different combinations of repetitions and power
 increases are possible to achieve successful communication in Rayleigh
 fading environments without significant delay. Further, rather than
 repeating individual bits of a digital signal, groups of bits may be
 repeated.
 Referring now to FIG. 4, a flow chart describing the transmission of a
 short message using the transmission method of the present invention is
 shown. Referring now to FIG. 4, a flow chart describing the transmission
 of a short message using the transmission method of the present invention
 is shown. In step 100, a sending party inputs a message to be transmitted
 to a receiving subscriber. The message may be input into the communication
 system directly by the sending party through a mobile unit, a standard
 telephone, a computer terminal, or equivalent device, or the message may
 be input indirectly by calling an operator at a service center who inputs
 the message into the system. The message address is used to determine
 which satellite beam or assigned frequency is being used to serve the
 recipient subscriber. In step 102, the information bits comprising the
 short message are encoded by an encoder located at the transmitter with an
 error detection code, such as CRC. The encoded message constitutes one or
 more codewords, each containing codeword bits or symbols. It should be
 recognized that the transmitter may be the satellite, a base station, or a
 mobile unit.
 In step 104, bit repetition can be employed, such that each of the codeword
 bits or symbols output by the encoding means are repeated N times to form
 a packet containing N bits. It will be apparent that, instead of repeating
 individual bits or symbols, groups of two or more bits or symbols, or the
 entire codeword or codewords could also be repeated.
 Packets are then transmitted in step 106 such that each slot within a TDMA
 frame includes one or more packets of repeated bits, error detection
 coding bits, and a sync burst to enable the receiver to estimate the
 channel quality. All bits comprising the encoded short message are
 transmitted in this fashion. If bit repetition and message repetition are
 employed, once the entire encoded message has been transmitted, the
 transmission of the message (in the form of packets of N codeword bits)
 can be repeated M times to achieve the desired signal margin. It will be
 appreciated that, since the short message may be transmitted from a
 satellite, base station, or mobile station, encoding and transmitting
 functions are provided in each of these devices. It will also be
 appreciated that, in order to implement the technique of the present
 invention, means are included in the transmitter to determine the number
 of bit repetitions N, message repetitions M, and power increase necessary
 to achieve the signal margin required for successful transmission of the
 message.
 Referring still to FIG. 4, in step 108, the receiving device (i.e., the
 mobile unit, satellite, base station, or equivalent device) samples the
 received signal, including the repeated encoded message bits, error
 detection bits, and channel quality estimation bits, and generates a
 metric sum of the form
 ##EQU1##
 where r.sub.ij is the sampled received signal corresponding to the ith
 repetition of the information bit or symbol S in packet repeat j, and
 C.sub.j is the corresponding estimate of channel quality. In step 110 a
 decoder contained in the receiving device decodes each encoded bit or
 symbol in the TDMA slot from the metric sums, using soft combining or
 majority logic voting, or other suitable decoding methods. To implement
 soft combining, the decoder adds metric sums as
 ##EQU2##
 and makes a bit or symbol decision based on the sum. To implement majority
 logic voting, the decoder makes a preliminary bit or symbol decision for
 each metric y.sub.j and then a makes a final bit or symbol decision by
 comparing all of the preliminary decisions. Thus, if the decoder has made
 M preliminary decisions, the decoder will determine that the corresponding
 information bit is a 1 if more than half of the preliminary decisions were
 1; otherwise, the decoder will determine that the corresponding
 information bit is a 0. The same logic is used to decode a bit that is a
 0. To prevent an error which might result if exactly half of the
 preliminary decisions are 0 and exactly half of the preliminary decisions
 are 1, M is chosen to be an odd number. The decoded bits are combined
 coherently and the multiple transmissions of the message are combined
 coherently to generate a message signal having an increased margin. Those
 of ordinary skill in the art will appreciate that the processing unit 180
 (FIG. 1) can perform the integration.
 In step 112, an error detector contained in the receiving apparatus detects
 errors based on the CRC error detection coding provided at the
 transmitting apparatus. If no errors are detected, the message is
 displayed on the receiving subscriber's mobile unit in step 114. If an
 error is detected, then the message is not displayed at the receiving
 apparatus, the user is notified of an erroneous message by a displayed
 error message or by an audio signal, and the receiver requests the
 transmitter to retransmit the message or the erroneous parts of the
 message in accordance with a bidirectional radio protocol.
 According to the present invention, repeated transmission of messages may
 be used in combination with repetition of individual portions of a
 message. That is, individual portions of the message may be transmitted
 multiple times, and when the entire message has been transmitted by means
 of the repeated transmissions of the message portions, the entire message
 may be transmitted again.
 According to an exemplary embodiment of the present invention, the
 transmission of message data in step 106 is performed during a time
 interval when a carrier frequency, dedicated to transmitting message data,
 is switched into the satellite beam and/or replaces the carrier frequency
 assigned to the recipient subscriber. The dedicated carrier frequency is
 switched into each satellite beam (e.g., by switching a local oscillator
 in the transmitter) at predetermined message intervals. The switching may
 be performed by a multiplexer located in the satellite or base station, or
 by other suitable means, and the message intervals may be selected in a
 variety of ways. According to one example, the predetermined message
 interval may be during the slow associated control channel (SACCH) frames
 or other frames selected for use as message frames in a short message
 service. Such a short message service is disclosed in applicants'
 copending, commonly assigned application, U.S. patent application Ser. No.
 08/578,945, entitled "High Power Short Message Service Using TDMA Frames",
 filed on Dec. 27, 1995, now pending the entirety of which is incorporated
 herein by reference. According to a second example, the predetermined
 message interval may be during slots selected from the broadcast control
 channel (BCCH) or other group of n slots occurring in each frame or group
 of frames. Such a short message service is disclosed in applicants'
 copending, commonly assigned application, U.S.patent application Ser. No.
 08/579,015, entitled "High Power Short Message Service Using Broadcast
 Control Channel", filed Dec, 27, 1995, now U.S. Pat. No. 5,822,310, the
 entirety of which is incorporated herein by reference. It will be
 appreciated that other intervals may be selected as message intervals for
 a short message service using the dedicated carrier frequency according to
 the present invention.
 When the dedicated carrier frequency is switched into a beam and/or
 replaces a frequency, the power used to transmit signals to the receiver
 is increased to a level greater than the power level used to transmit
 signals over the traffic or control channels. The receiver synchronizes
 itself (e.g., using the processing unit 180 of FIG. 1) to receive the
 message over the dedicated frequency in a manner which will be described
 later.
 Referring now to FIG. 5, a timing diagram of a SMS service according to an
 exemplary embodiment of the present invention is shown. The high power
 short message service (HP-SMS) is provided via a dedicated carrier of,
 e.g., 200 KHz, which is staggered in time with, e.t., BCCH bursts. f.sub.0
 is the frequency of the dedicated carrier for HP-SMS, and f.sub.i is a
 preselected carrier frequency with BCCH time multiplexed with traffic
 channels (only BCCH bursts are shown in the FIG. 5). Power is supplied for
 the f.sub.0 -carrier only during the predetermined message intervals,
 which in this case are the FCH and SCH channels in the f.sub.i -bursts;
 power is not supplied to the f.sub.i carrier during the predetermined
 message intervals.
 In this embodiment, the FCH and SCH provide 4 bursts per multi-frame of
 message capacity. A multi-frame is chosen to consist of 51 frames, i.e.,
 51.times.16 time-slots, and lasts for 470 ms. As shown in FIG. 5, these 4
 bursts occur in frames 1,13,26 and 40 (that is with frame spacings of 12,
 13, 12, 14); a fifth burst can also be used to include a unique word.
 Alternatively, all the 4 message bursts can be contiguous in time. In both
 transmission methods, the message frames are orthoganally staggered in
 different beams, to ensure that the 8 beams transmitting BCCH at the same
 time do not transmit message data at the same time. The message slot is
 therefore transmitted only in one beam at a time, maintaining a
 substantially constant transmitter load.
 Referring now to FIG. 6, each message burst contains 156.25 bits, which
 include 8.25 bits for guard and ramp up/down, and 6 tail bits. The
 remaining 142 bits are divided into a 14 bit beam ID and 128 data-code
 bits. The 128 data-code bits can be K orthogonal code patterns selected by
 log.sub.2 K information bits. The 128 data-code bits corresponds to 7
 message bits. Assuming 2 repetitions of a 126-bit message, 252 bits
 (2.times.126) must be delivered. Hence, a message can be delivered in 36
 HP-SMS bursts. This corresponds to a message delivery in 9.times.51 TDMA
 frames or 8.47 seconds/message. With 121 beams the number of messages
 delivered per second from a satellite will be (121/8.47).apprxeq.14
 messages.
 In one implementation, the receiver synchronizes itself to receive the data
 message in two steps, coarse synchronization and fine synchronization.
 In the coarse synchronization step, synchronization is achieved to within
 approximately 7 seconds using, for example, the energy profile method.
 According to the energy profile method, synchronization is performed based
 on the strength of the signal received by the receiver. As will be
 discussed in more detail below, the power profile method accurately
 acquires initial synchronization over the HP-SMS channel, even at very low
 signal to noise ratios (SNRs).
 In the fine synchronization step, synchronization to the bit level is
 obtained by correlating the orthogonal data-codes in each burst. The
 correlations are added over the repeats to provide an adequate step to
 reliably decode message data. In this step, synch is achieved to within a
 fraction of a channel symbol interval.
 In this embodiment, the receiver synchronizes specifically to the HP-SMS
 FCH and SCH channels, and the sleep cycle can be determined in accordance
 with these channels. In the HP-SMS mode, the mobile wakes up for 4 slots
 every 51 TDMA frames, implying a duty cycle of 1/204. However, in the
 HP-SMS mode the mobile may also wake up to check if it can receive a
 normal calling channel or a cellular channel. This will typically require
 the mobile to wake up for another 4-8 slots every 51 frames, resulting in
 a duty cycle of around 1.5%.
 The beam ID in each message burst can be used as a pointer to indicate the
 BCCH carrier in a beam. The BCCH is transmitted on a limited number of
 frequencies and never more than one per beam. The mobile terminal can
 achieve coarse synchronization to the BCCH format using signal strength
 profiles of the message bursts only, and then can obtain fine
 synchronization to the bit level by correlating with the known data codes
 in the message bursts.
 To maximize capacity, the dedicated carrier frequency can hop on all beams
 in one of every three multi-frames, and can be flexible in the other two
 multi-frames. To accommodate peak HP-SMS capacity in a beam, a message
 burst can request a user to look for a message in a time slot at a later
 time.
 The power profile method will now be described for the case when the four
 message slots occur together. The method is easily extended to the case
 where the message slots are staggered.
 The receiver samples the signal at one sample per bit and accumulates the
 signal samples over a duration equal to four time slots (625 bits;
 assuming 156.25 bits per slot). It stores the accumulated value in a bin.
 Over a duration equal to 3 multi-frames, there are 612 bins. After a time
 duration of 3 multi-frames, the accumulated power over four time-slots at
 that time is added to the corresponding bin. After sufficient
 accumulations, the maximum bin valve is chosen to represent the interval
 where the data message occurs in the beam.
 The power profile method for performing coarse synchronization will now be
 described in more detail. A received signal includes useful real signal a
 and complex noise x+jy, where x and y are Gaussian random variables with
 unit variance. Thus, the carrier-to-noise ratio C/N is given by a.sup.2
 /2.
 The instantaneous power is defined as
EQU P=(a+x).sup.2 +y.sup.2. (1)
 The mean power can be shown to be
 ##EQU3##
 The standard deviation of P can be shown to be
 ##EQU4##
 In equations (1)-(3), the subscript s indicates the fact that the useful
 real signal component is present in the received signal.
 In the absence of a signal component, the mean and the standard deviation
 of the resulting noise power vector is:
EQU m.sub.n =2
EQU .sigma..sub.n =2. (4)
 When power is accumulated over 3k multi-frames, i.e., over a time equal to
 1.41k seconds, the number of samples over which the power is averaged is
 k.times.625. Let S denote the random variable corresponding to the average
 power in the bin where the useful real signal is present, and the power is
 accumulated over 3k multi-frames. By invoking the central limit theorem, S
 can be approximated by a Gaussian random variable with mean m.sub.s and
 standard deviation .sigma..sub.as =.sigma..sub.s /625+L k. Similarly, the
 random variable N, which is the average power in a bin where the useful
 real signal component is not present, can be approximated as a Gaussian
 random variable with mean 2 and standard deviation .sigma..sub.an =2/625+L
 k.
 Since the power profile method chooses the maximum among the different
 bins. the method succeeds when S is greater than the maximum of the random
 variables N corresponding to the other 611 bins. If M is used to denote
 the random variable which is the maximum of the 611N bins, the cdf of M
 can be expressed in terms of the cdf of the N's as
EQU P.sub.M (x)=[P.sub.N (x)].sup.611 (5)
EQU P.sub.c =Prob {S&gt;M} (6)
 In terms of the probability density functions ps(x) of S and P.sub.M (x) of
 M, the correct detection probability is
 ##EQU5##
 This can be rewritten as
 ##EQU6##
 The synchronization performance of the power profile method is shown in
 FIG. 7. The probability of correct detection P.sub.c is plotted against
 the carrier-to-noise ratio C/N for different acquisition times (values of
 k).
 The carrier-to-noise ratio at a nominal operating point can be found using
 the link budget for the message channel, as shown in Table 1.
 TABLE 1
 C/N calculation for the HPSMS channel
 Item Allowance
 EIRP 46.6 dBW
 Path Loss -182 dB
 Margin -27 dB
 kT 228.6 dBW/Hz
 G/T -24 dB/K
 Noise bandwidth 51.4 dBHz
 C/N -9.2 dB
 A typical synchronization acquisition performance of the power profile
 method on the message channel at a nominal operating point is summarized
 in Table 1.
 TABLE 1
 Acquisition Performance at Nominal Operating Point
 Acquisition Time Acquisition Probability
 2.82 s 0.8377
 5.64 s 0.9931
 8.46 s 0.9998
 11.28 s 1.0
 In an implementation where message slots are borrowed from the broadcast
 control channel BCCH, the BCCH bursts occur every 16th slot. The slot on
 which the BCCH bursts occur is staggered from beam to beam in a 16-cell
 pattern. With 121 beams, approximately 8 beams at a time are transmitting
 BCCH, while the other 113 are transmitting traffic. As mentioned above, in
 4 out of 51 frames, the BCCH slot is staggered with the dedicated message
 carrier, which also serves the FCH and SCH functions.
 In an exemplary implementation, the message bursts are transmitted at 7 dB
 above the power provided to edge of coverage traffic channels (8 watts).
 Hence, the message bursts are transmitted at 40 watts. The BCCH bursts are
 transmitted at 16 watts which is 3 dB above the traffic channels. Since
 the BCCH signals in 8 beams are transmitted simultaneously the total power
 is 64 watts. Hence, the BCCH and message channels together take up 128
 watts of the spacecraft power. This is 18.6% of the spacecraft S-band RF
 power (689 watts).
 Preferably, increased signal margin for the message channel is provided by
 a combination of additional power, spreading by a 128 chip code, message
 repeats and/or error correction coding. In the following example, coding
 gain is not explicitly included. If the traffic channel provides a 7 dB
 minimum margin over an AWGN channel, 9 dB additional power over the
 traffic channel provides a 17 dB margin over an AWGN channel with rate 2/3
 coding. The spreading gain achieved by using a 128 chip code for 7
 information bits provides an additional gain of 11 dB (10log.sub.10
 (128/7.times.2/3)) relative to the rate 2/3 coding. Further, message
 repetition with soft decision decoding provides another 3 dB gain. Thus,
 the overall margin over an AWGN channel is 9 dB (power increase)+7 dB
 (voice margin)+12.6 dB (spreading)+3 dB (message repeat)=31.6 dB.
 The following table summarizes the merging and performance thresholds of
 the traffic, paging and message channels.
 TABLE 3
 Performance Thresholds
 Parameter Traffic BCCH HP-SMS
 EIRP (dBw) 37.6 37.6 46.6
 Margin (dB) -- 17 30
 C/No (dB) 53.4 43.4 39.4
 FER 1% 1% 1%
 Ec/No (dB) -0.9 -10.9 -14.9
 While the foregoing description includes numerous details and
 specificities, it is to be understood that these are merely illustrative
 of the features and principles of the present invention, and are not to be
 construed as limitations. Many modifications will be readily apparent to
 those of ordinary skill in the art which do not depart from the spirit and
 scope of the invention, as defined by the following claims and their legal
 equivalents.