HK1038843A1 - Signal decoding with or without second synchronization word in a mobile communication system - Google Patents

Abstract

A time division multiple access (TDMA) radio communication system (100) accommodates signal decoding by a mobile station (130) with and without a second synchronization word (214). The radio communication system includes at least one base station (102, 104) to transmit radio signals during a succession of time slots (202, 204, 206) including a current time slot (202) and a subsequent time slot (204). The radio signals include an indication of whether transmission of the subsequent time slot is guaranteed. The system further includes at least one mobile station to receive the radio signals in accordance with the indication.

Description

Signal decoding with or without second sync word in mobile communication system

Applications relating thereto

This non-provisional application claims priority to provisional application 60/085,710, filed on 1998 at 5/15 and patentees are Paul w.

Background

The present invention relates generally to transmitting and receiving signals in a mobile radio communication system. In particular, the present invention relates to decoding signals in a mobile radio communications system, suspending transmissions from a base station of the system for selected time periods.

In a Time Division Multiple Access (TDMA) wireless communication system, each transmitter in the system is assigned or allocated a time slot for transmitting wireless signals. All other transmitters are stationary during the time slot so that the designated receiver can clearly receive the radio signal. If the radio communication system is a two-way mobile system, such as a cellular radiotelephone system, the radio carrier frequency in the first frequency band is divided into time slots for communication from the base station to the plurality of mobile stations, and the radio carrier frequency in the second frequency band is divided into time slots for communication from different ones of the plurality of mobile stations to the base station. The base stations communicate with mobile stations within a fixed geographic area near the base stations, with other neighboring base stations serving mobile stations in the vicinity.

During certain time slots, the base station transmits control and timing information. Such information includes, for example, the identity of the mobile station specified by the call received on the communication contact channel, and synchronization and timing information on the synchronization or Sync channel. The mobile stations in the system receive the synchronization channel and synchronize their timing with the base station timing using information in the synchronization channel. Typically, each slot also includes a known data pattern to facilitate the mobile station in estimating the phase and amplitude of the channel, which is useful for decoding the symbols transmitted in the slot.

It is known to use a synchronization word at the beginning of the current slot and a synchronization (Sync) word at the beginning of the next slot and at the end of the current slot to improve the reception and decoding of symbols. The current or first sync word may not be reliably received because of fading or other interference on the channel. By storing received signal samples including the second sync word, the stored signal samples can be decoded using either the first or second sync word or both. This technique is described in U.S. patent 5,335,250 issued to Dent et al at 8/2 of 1994, and in U.S. patent 5,841,816 issued to Dent et al at 11/24 of 1998. Both of these patents are commonly assigned with the present application to the applicant of the present invention and are incorporated herein by reference. Some mobile stations are currently in use and they rely on receiving the second sync word for optimum performance.

In some mobile radio systems it is proposed to suspend or change the transmission of information including a synchronisation word when a time slot is not allocated. If a time slot is not allocated, no mobile station is currently using the time slot. This may result in a change in the phase, amplitude or direction transmitted by the base station in a subsequent time slot relative to the current time slot, which may reduce the operating conditions of existing mobile stations without the use of the present invention. However, it is desirable to avoid transmitting information by the base station in unallocated timeslots to reduce interference to the receiver in the surrounding area. Eliminating the transmission of sync words and other information during unallocated timeslots reduces or eliminates this co-channel interference, improving overall performance in the radio system. This improvement is threatened by reduced quality due to the removal of the mobile station's ability to utilize the second sync word.

For a mobile station that relies on receiving the second sync word to improve communication, if the unassigned slot allows a slot to be assigned to the mobile station, then eliminating the sync word or a change in characteristics, such as timing, phase or direction of transmission in the unassigned slot, can create problems. The mobile station may not be able to successfully decode the channel or may take too much time to search for the second sync word. Therefore, there is a need for a method and apparatus for providing performance that eliminates certain time slots transmitted by a base station in a mobile radio communication system without degrading the remaining time slots.

Brief description of the drawings

FIG. 1 is a block diagram of a wireless communication system;

fig. 2 is a diagram illustrating a communication format in the wireless communication system of fig. 1;

fig. 3 is a flow chart illustrating a method for operating a base station in the radio communication system of fig. 1;

fig. 4 is a flow chart illustrating a method for operating a first type of mobile station in the wireless communication system of fig. 1; and

fig. 5 is a flow chart illustrating a method for operating a second type of mobile station in the wireless communication system of fig. 1.

Detailed description of the presently preferred embodiments

Referring now to the drawings, FIG. 1 illustrates a wireless communication system 100 including at least one base station 102,104 and at least one mobile station 130. The wireless communication system 100 may be any two-way wireless communication system such as a cellular radiotelephone system, a Personal Communication System (PCS), a trunked radio system, or others. In the illustrated embodiment, the wireless communication system 100 IS a cellular radiotelephone system operating in accordance with the interim standard IS-136 promulgated by the telecommunications industry association/electronic industry association (TIA/EIA).

In fig. 1, system 100 is illustrated with two base stations, including base station 102 and base station 104. Typically, all base stations are similarly configured, and the structure and operation of the base stations will be described in connection with base station 102. The base stations of the system, including base station 102 and base station 104, are linked together to form a network. The network is controlled by a Mobile Switching Center (MSC) 106. The MSC106 is coupled to each base station 102,104 by landline, telephone link, or wireless link. The MSC106 controls the operation of the overall network, provides a connection to the Public Switched Telephone Network (PSTN), and controls the interoperation of base stations to provide, for example, functionality for handover of radio communications between two base stations and a single mobile station in the system 100.

Base station 102 is a template of the structure and operation of the base station. Base station 102 includes a controller 110, a transmitter 112, a receiver 114, a memory 116, a clock 118, and an antenna 120. The controller 110 controls the overall operation of the base station 102. Each base station provides two-way radio communication with one or more mobile stations in a fixed geographical area proximate to the base station. Each geographic area may be divided into sectors with each sector being served by an antenna or portion of antennas 120.

Transmitter 112 transmits radio signals to mobile stations, such as mobile station 130 within a fixed geographic area surrounding base station 102, using antenna 120. The transmitter 112 may include functions such as coding, insertion, and modulation. Receiver 114 converts radio signals received on antenna 120 into digital data for use by base station 102. The receiver may include functions such as filtering, demodulation, and decoding. Memory 116 stores data and instructions for operating controller 110 and for use by other circuitry within mobile station 102.

The clock 118 provides a time base for operating the base station 102. In general, devices operating in system 100 must be close to synchronous. Including the base station, MSC106 and mobile stations. The clock 118 receives the synchronization signal from the MSC106 and, in turn, is used to provide the synchronization signal to a mobile station, such as the mobile station 130.

Those skilled in the art will appreciate that the base stations 102,104 may likewise include other functionality and other circuitry. These functions include managing call initiation with the mobile station and handoff of communications with the mobile station from one base station to another.

The mobile station 130 is a template of a mobile station that may operate in the radio communication system 100. The mobile station may be any radio capable of two-way wireless communication with a distant base station, such as a cellular telephone, PCS telephone or other two-way radio. In the illustrated embodiment, mobile station 130 IS a mobile telephone capable of operating in accordance with IS-136. Mobile station 130 in the illustrated embodiment includes an antenna 132, a receive path 134, a transmit path 136, a controller 138, a memory 140, a user interface 142, a battery 144, a clock 146, and a synthesizer 148.

Receive path 134 receives the wireless signals detected at antenna 132 and generates digital data for use within mobile station 130. The receive path 134 includes an analog front end 150, a demodulator 152, and a decoder 154. The analog front end 150 includes a low noise amplifier and appropriate filters for detecting and filtering the radio signal received at the antenna 132. Analog front end 150 may further include circuitry, such as a mixer coupled to synthesizer 148, for shifting the frequency of the broadcast radio signal down to a lower frequency for processing in mobile station 130 with relative ease. Demodulator 152 determines an estimate of the multipath propagation channel by processing the received signal samples with a synchronization word or known symbol pattern and then processes the signal samples with the channel estimate to produce soft decisions that compensate for multipath and inter-symbol interference. The soft decisions are then passed from the demodulator 152 to the decoder 154, which performs error correction coding to produce error corrected information to the controller 138.

The transmit path 136 converts digital data generated by the controller 138 into radio signals for transmission using the antenna 132. Transmit path 136 includes an encoder 160, a modulator 162, and a transmitter 164. The encoder 160 encodes the digital data provided by the controller 138 into a format required for communication in the system 100, including insertion. The encoded data is provided to a modulator 162 that modulates a carrier signal provided by the synthesizer 148 with the data. The modulated carrier is provided to a transmitter 164 which provides functions such as power amplification and filtering. The amplified carrier is then applied to an antenna 132 for transmission to a remote base station, such as base stations 102, 104.

Controller 138 controls the overall functionality of mobile station 130. The controller 138 preferably operates as a microcontroller, digital signal processor, or a combination thereof, and is responsive to data and instructions stored in the memory 140. The user interface 142 allows control of the mobile station by a user, typically including a keypad, microphone, speaker and display. The battery 144 provides operating power for the mobile station 130. Clock 146 provides timing for the circuitry of mobile station 130. In particular, the clock 146 provides timing signals for use by the controller 138 so that the mobile station 130, including the controller 138, may still be synchronized with the rest of the communication system 100.

In the illustrated embodiment, the wireless communication system 100 is a Time Division Multiple Access (TDMA) wireless communication system. That is, in system 100, each mobile transmitter in the system, when activated, is assigned or allocated a time slot for transmitting wireless signals. All other mobile transmitters in the same cell using the same frequency are stationary during the time slot. So that the designated receiver can clearly receive the radio signal on the channel. Of course, other mobile stations may independently use the same time slot in another cell or another frequency.

An example of a TDMA radio communication system IS a mobile telephone system according to the interim standard IS-136. IS-136 specifies a communication frame having six time slots, however, IS typically assigned in pairs to constitute a full-rate channel. The full rate channel is effectively a three time slot TDMA system. By combining time slots on the uplink frequency with time slots on the downlink frequency, a duplex channel is established for establishing bidirectional communication between the base station and a particular mobile station.

Fig. 2 illustrates a communication format in the wireless communication system of fig. 1. Fig. 2 shows a portion of a communication frame 200 that includes three time slots, a first time slot 202, a second time slot 204, and a third time slot 206. The time slots 202,204,206 are all transmitted by the base stations in the system 100 of FIG. 1.

Each slot includes a synchronization word. The first time slot 202 therefore includes a sync word 212, the second time slot 204 includes a sync word 214 and the third time slot 206 includes a sync word 216. A Sync word or Sync word comprises a predefined data pattern in a predefined location near the beginning of each time slot. The sync word is used by the receiving mobile station for synchronizing timing and decoding the transmitted time slots.

Each time slot and the synchronization word associated with it are specified by the transmitting base station for reception by a single mobile station of a neighboring base station. However, in current IS-136 systems, all three slots are transmitted all the time, so the specified mobile station can receive other slots, such as subsequent slots and better than its specified slot. In IS-136, the transmission format exhibits time inverse symmetry so that the mobile station can demodulate its information forward from the first sync word or backward from the second sync word. However, in future systems, when no data is transmitted, the time slot may be omitted. To reduce interference to non-designated receivers but deprive designated second sync word receivers of rights.

It is known to improve the reception and decoding of symbols by mobile stations using the synchronization words of the current time slot and the subsequent time slot. This technique is described in U.S. patent No.5,335,250 issued to Dent et al at 8/2 of 1994, and U.S. patent No.5,841,816 issued to Dent et al at 24/11 of 1998. Both of these patents are commonly assigned with the present application to the applicant of the present invention and are incorporated herein by reference. These incorporated by reference decoding techniques or other suitable decoding techniques are referred to herein as a first decoding algorithm or technique. According to the presently disclosed embodiments, the mobile station will use a second decoding algorithm or technique when the base station cannot guarantee that the sync word is transmitted in a subsequent time slot.

According to the decoding and demodulation techniques described in the incorporated references, the mobile station receives a time slot of one TDMA frame period that is designated or allocated. With the first decoding algorithm, the mobile station decodes the received signal by means of known symbols, called first sync words, which are transmitted near the beginning of the allocated time slot, and by means of known symbols, called second sync words, which are transmitted near the beginning of the subsequent time slot. Stated another way, the base station transmits a first predefined data symbol and an unknown data symbol. A sync word or other known data pattern constitutes a first predefined data symbol. The particular data designated for a particular mobile station is an unknown data symbol. Unknown data symbols may be encoded as voice data, control data, or other data. In some systems, the base station transmits a second predefined data symbol, such as a second synchronization word in a subsequent time slot.

In some systems, a known symbol, e.g., the second sync word, at the beginning of a subsequent slot is not guaranteed to be transmitted by the base station. This may be achieved, for example, to reduce co-channel interference of the receiver in the surrounding area. If subsequent time slots are not allocated to the base station's mobile station, the base station may suspend transmission during the unallocated time slots, including the synchronization word, to reduce co-channel interference. Alternatively, the base station may use directional antennas to allow the broadcasting of time slots into specific sectors of an area serviced by the base station. Such a sector may be different from the sector in which the mobile station assigned the current time slot is located. Therefore, the second slot may not be received coherently with the previous slot data due to the change in beam direction.

Due to these possibilities, it is not possible for the base station to guarantee the transmission of a second predefined data symbol, e.g. a synchronization word in a subsequent time slot. Furthermore, it may not be useful to transmit a second predefined data symbol, if any, whose timing or phase or amplitude of the subsequent time slot is substantially changed by the base station for any particular reason. Because the synchronization word used by the mobile station is used to synchronize its timing and reception to the base station, any change in these parameters can cause loss of synchronization and require re-synchronization of the mobile station to the base station.

When it is known that symbols, such as sync words, are not guaranteed to be transmitted by the base station at the beginning of a subsequent time slot, or are not guaranteed to be continuous in timing, phase, amplitude or direction with the transmission in the current time slot, the base station in accordance with the present invention provides a signal to the mobile station indicating that a second sync word cannot be relied upon. Stated another way, the base station sends an indication when transmission of the second predefined data symbol is not guaranteed.

The mobile station decodes a transmission from the base station using at least a first predefined data symbol. If the signal transmitted by the base station indicates that the transmission of the second predefined data symbol or the second synchronization word is reliable, the mobile station will decode the slot received from the base station using the first predefined data symbol, the second predefined data symbol, or both, in accordance with the disclosure that the "first" algorithm is incorporated. Any suitable algorithm or others described in the incorporated references may be used.

When the mobile station receives an indication that the second predefined data symbol or the second sync word cannot be relied upon, the mobile station decodes the signal received in the selected time slot using a second decoding algorithm. The second decoding algorithm only requires known symbols to be transmitted in the selected slot. This second decoding algorithm may be any of the variations described in the above-incorporated references, implemented by constantly setting the quality of the second sync word to the lowest quality value, as long as an indication that the second sync word is unlikely to be relied upon is maintained. Any other suitable decoding algorithm may be used, using only one guaranteed sync word.

Thus, when an indication is transmitted, the mobile station decodes the unknown data symbols using the first predefined data symbols, and when no indication is transmitted, decodes the unknown data symbols using the first predefined data symbols and the second predefined data symbols. The indication may be any suitable data or signal contained within the control signal information transmitted by the base station to the mobile station. In one embodiment, the indication is sent during call setup or handoff. Call setup occurs when two-way communication is initiated between the mobile station and the base station. Call initiation occurs when the mobile station indicates that the user wishes to make an outgoing call or when the base station has signaled an incoming call to the mobile station over a communication contact channel. Handoff occurs when the mobile station moves from a first base station coverage area to a second base station coverage area. When these two situations occur, basic control information is exchanged between the mobile station and the base station.

In one embodiment, the base station broadcasts an indication to all mobile stations in its coverage area or sectors in its coverage area. This can be achieved using a Broadcast Control Channel (BCCH) known as the digital control channel or DCC. The indication informs the mobile station that the base station is using an operating mode that does not guarantee availability of the second synchronization word. In response, all mobile stations in the sector or coverage area use the second decoding algorithm.

In another embodiment, the mobile station provides an indication to the base station that the mobile station needs a second sync word for optimal decoding of the slot information from the base station. This is appropriate for compatibility with mobile stations having such requirements. According to this embodiment, the mobile station transmits a pointer indicating that the mobile station needs to transmit the second predefined data symbol. In response to this pointer, the base station according to the invention reliably transmits the second predefined data symbol even when no data is transmitted in the following time slot. The base station does not suspend the second sync word and does not change the transmission phase and amplitude or any other necessary characteristics, but may omit the transmission of data following the second word if no data needs to be transmitted.

The pointer may comprise a predefined data pattern or an operating mode pointer for the mobile station. The pointer may be sent to the base station with control information provided by the mobile station.

Alternatively, the pointer may be included in the standard information sent by the mobile station to the base station. For example, each mobile station has a protocol version number that is transmitted to the base station upon initiation of communication therebetween. The protocol version number relates to a modification of the standard, e.g., IS-136, being used by the mobile station. In response to the protocol version number or any other similar pointer, the base station may decide whether the mobile station expects the second sync word or can be reliably run without sending the second sync word. Moreover, one of the two modes of operation may be set to a constant value with a very constant condition, selected only based on an appropriate determination by the base station. Further, in another embodiment, the base station may decide which mode to operate by utilizing an identifier, such as the Electronic Serial Number (ESN) of the mobile station, sent by the mobile station during call setup. By comparing the ESN for the mobile station with values stored in a memory at the base station or elsewhere in the communication network, the base station can release the characteristics of the mobile station from the subscriber database, including which operating mode is provided for the mobile station. Thus, the base station determines a transmission requirement for the mobile station and transmits a predefined second data symbol in response to the transmission requirement of the mobile station.

Fig. 3 is a flowchart for explaining a method of operating a base station in the wireless communication system 100 of fig. 1. The method begins at step 302 and at step 304 the base station determines whether a second synchronization word is required by the Mobile Station (MS). This determination may be made in any suitable way, such as by receiving signal information from the mobile station, including an implicit pointer indicating that the mobile station requires a second sync word, or receiving an identifier or other information from the mobile station and comparing the identifier to stored data to determine the transmission requirements of the mobile station. In the embodiment shown in fig. 1, the controller 110 of the base station 102 constitutes a means for determining the type of a particular mobile station in radio communication with the base station. The controller 110 may operate in conjunction with the memory for this purpose, or with other data processing equipment of the system, such as the MSC 106.

If the base station determines that the mobile station requires the second sync word, the base station transmits a first slot including the first sync word in step 306. In step 308, the base station transmits at least a second synchronization word. In this manner, a first predefined data symbol (first sync word) and a second predefined data symbol (second sync word) are provided to the mobile station for accurate demodulation and decoding of unknown data symbols transmitted by the base station in the first time slot. In the embodiment of fig. 1, the transmitter 112, alone or in conjunction with the controller 110, forms a means, responsive to the type of the particular mobile station, for transmitting radio signals to the particular mobile station during at least a portion of the current time slot and the subsequent time slot when the particular mobile station is of the first type. The method then ends at step 310.

If the base station determines that the mobile station does not require the second sync word in step 304, the base station transmits the first slot and the first sync word in step 312. With respect to the embodiment of fig. 1, the transmitter 112, alone or in conjunction with the controller 110, forms a means, responsive to the particular type of mobile station, for transmitting a radio signal to the particular mobile station during the current time slot, the radio signal being adapted to another type of mobile station when the particular mobile station is of the second type. Then, in step 314, the base station determines whether a second time slot (i.e., the time slot immediately following the first time slot) has been allocated. If the base station is using a second time slot of the same radio channel, two-way radio communication is performed with a second mobile station. If the base station does not communicate with any mobile station during that time slot, or the mobile station is assigned a time slot in a different direction, the transmission during the second or subsequent time slot may be suspended or changed. If the second time slot is allocated, the base station will transmit the second time slot and a second sync word, which is required by the mobile station to which the second time slot is allocated, in step 316.

If, at step 314, the base station determines that the second time slot is not allocated, at step 318, the base station determines whether there is a need to change its radio transmission. Such a need would exist, for example, if the second time slot had been allocated to a different sector located in the geographic area served by the base station. If no change is needed, control passes to step 316 to send a second slot and a second sync word. However, if a change is required, the base station makes the necessary change in transmission in step 320 and transmits the next slot in step 322. The method ends at step 310.

Fig. 4 serves as a flow chart illustrating a method for operating a first type of mobile station in the wireless communication system of fig. 1. The first type of mobile station needs or expects a second synchronization word to be transmitted by the base station. The method begins at step 402.

At step 404, the mobile station is still in the loop and attempts to locate a control transmission from the Base Station (BS). If no appropriate transmission is located, the mobile station continues the search. The mobile station may attempt to initiate a call by initiating two-way radio communication with the base station or may attempt to hand-off communication from a first base station to a second base station that is located.

In the context of fig. 1, the receive path 134 constitutes a means for receiving and decoding signals from a distant base station. The analog front end 150, demodulator 152 and decoder 154 are adapted to perform these functions. The design and implementation of these circuits, including hardware and software, and alternative embodiments, are more fully described in the incorporated reference material. The new pointer signal provided by the present invention indicates whether the base station transmission is of the first type or the second type. The first type of transmission includes symbols, such as synchronization words in subsequent time slots, that mobile station 130 may use to decode the base station's transmission. The second type of base station transmission does not reliably include the second sync word. The decoder 154, in combination with the controller 138, forms a means, responsive to the indication, for decoding the communication signals received from the base stations 102,104 in the time slots allocated by the TDMA frame period. The decoder 154 and controller 138 may be suitably implemented as any combination of hardware or software that performs these functions.

In one embodiment, once a base station has been located, the mobile station determines to inform the base station that it needs the second synchronization word for best performance at step 406. In other embodiments, it may occur that step 406 is not performed specifically. Software or other programs, however, control the operation of the mobile station and may simply perform the next step, depending on the mode of operation of the mobile station.

If the mobile station does not require the second sync word, it may send a pointer at step 408. As described above, the pointer may be data or other information contained in a signal, such as a control signal. The control signal may be, for example, an Electronic Serial Number (ESN) or a Mobile Identification Number (MIN) uniquely attributed to the mobile station. The pointer indicates the type of mobile station to base station. It is in turn determined whether the base station needs to transmit a second predetermined data symbol in a subsequent time slot into the time slot allocated to it. Alternatively, the pointer may be a specific class flag or protocol version number that is communicated to the base station and used by the base station to determine that the mobile station needs to communicate the second sync word. Further, the mobile station may only transmit identification information, such as an identifier similar to an electronic serial number, which is then used by the base station to determine the transmission requirements of the mobile station.

In step 408, the mobile station receives the first time slot transmitted by the base station and receives at least the second Sync word. In step 410, the mobile station demodulates the first slot by means of the second sync word. More generally, the mobile station demodulates and decodes the first time slot using any of the decoding techniques described in the references incorporated above, or any other suitable technique. The mobile station determines whether more time slots are forthcoming or the call has terminated at step 412. If the call continues, control returns to step 408. If the call has terminated, the method ends at step 416.

Fig. 5 serves as a flow chart illustrating a method for operating a second type of mobile station in the wireless communication system of fig. 1. The second type of mobile station does not require or expect the second sync word to be transmitted by the base station. However, in one embodiment, the second type of mobile station utilizes the second sync word to optimize its performance if the second sync word is available. The method starts at step 502. Step 504 is similar to step 404, as described above in connection with the first type of mobile station. In step 506, the mobile station sends a pointer indicating that the second sync word may be omitted.

If the mobile station does not require the second sync word, the mobile station of the second type may receive an indication from the base station as to whether the second sync word is available at step 508. In step 510, the mobile station receives a first time slot including up to a second sync word. In step 512, the mobile station checks whether a second sync word has been indicated as available or useful. If so, the mobile station proceeds to step 514 and applies a first demodulation algorithm using the first and/or second sync words. If not, the mobile station proceeds to step 516 and demodulates the first slot using only the first syncword using the second algorithm. Then at step 518, a soft decision is made from the subsequent first slot whether to demodulate, deinterleave and decode at step 514 using the first algorithm or at step 516 using the second algorithm.

If the call is to continue at step 520, return to step 510 to receive the next first time slot. Alternatively, control returns to step 508 if each time slot indicates that second sync word availability is provided, otherwise, if the call has terminated, the method ends at step 522.

From the foregoing, it can be seen that the illustrated embodiments provide a method and apparatus for accommodating the elimination or modification of certain synchronization words transmitted by a base station in a mobile radio communications system. The base station transmits an indication to mobile stations in the area served by the base station that the subsequent transmission of the synchronization word will be suspended or altered and thus unreliable. In an alternative embodiment, the mobile station may send a pointer to the base station that the mobile station needs the second sync word, in which case the base station will continue to send the second sync word for use by the mobile station.

These embodiments allow the deployment of improved base stations, using more advanced directional antenna arrays, which can transmit different time slots of a TDMA frame period in different directions, adapted to the location of the designated receiver. Alternatively, adaptive power control is allowed, in which the transmit power level may be increased or decreased in time slots, depending on the distance of the designated receiver. These two changes affect the properties of the second sync word. With the illustrated embodiment, these advanced base stations may be adapted to transmit a second sync word with signal continuity to the previous time slot to maintain compatibility retrospectively with mobile station receivers that rely on the second sync word.

While a particular embodiment of the invention has been shown and described, modifications may be made. For example, any pre-specified data or symbols contained in the time slots may be used by the mobile station when the synchronization word transmitted in each time slot has been shown for use by the mobile station in demodulating and decoding transmissions from the base station. It is therefore intended to cover in the appended claims all such changes and modifications that are within the true spirit and scope of this invention.

Claims (25)

1. A method for operating a wireless communication system comprising at least one base station and at least one mobile station, the method comprising the steps of:

transmitting, at a base station, a first predefined data symbol and an unknown data symbol;

selectively transmitting, at the base station, a second predetermined data symbol;

at the base station, transmitting an indication when transmission of a second predefined data symbol is not guaranteed;

at the mobile station, the transmission from the base station is demodulated using at least a first predefined data symbol.

2. The method of claim 1, wherein decoding the transmission comprises the steps of:

decoding unknown data symbols using only the first predefined data symbols when the indication is transmitted; and

when no indication is transmitted, the unknown data symbol is decoded using at least one of the first predefined data symbol and the second predefined data symbol.

3. The method of claim 1, wherein the first predefined data symbol comprises a synchronization word for the current slot.

4. The method of claim 1, wherein the second predefined data symbol comprises a synchronization word for a subsequent slot.

5. The method of claim 1, wherein sending the indication comprises:

at the base station, the indication is sent when the base station suspends transmission of the synchronization word during a subsequent time slot.

6. The method of claim 1, wherein sending the indication comprises:

the indication is sent when the base station cannot guarantee that the second predefined data symbol is sent with any reliable timing, reliable phase, reliable amplitude and reliable directional continuity.

7. The method of claim 1, further comprising the steps of:

transmitting, at the mobile station, a pointer indicating that the mobile station expects to transmit a second predefined data symbol; and

in response to the pointer, the second predefined data symbol is reliably transmitted at the base station.

8. The method of claim 7, wherein the pointer comprises a predefined data pattern.

9. The method of claim 7, wherein the pointer comprises an operating mode pointer for the mobile station.

10. The method of claim 7, wherein the pointer comprises an identifier for the mobile station and further comprising the steps of:

determining, at the base station, transmission requirements for the mobile station in response to the identifier; and

the predefined second data symbols are transmitted in response to a transmission request for the mobile station.

11. The method of claim 10, wherein the transmission request is retrieved from a user database.

12. A mobile station operable in a wireless communication system and configured to receive first and second types of transmissions from a distant base station, the mobile station comprising:

means for receiving and demodulating a signal from a distant base station, the signal indicating whether the base station transmission is of a first type or a second type; and

responsive to the signal for decoding the communication signal received from the base station in an allocated time slot of a Time Division Multiple Access (TDMA) frame period by means of symbols transmitted from the base station in subsequent time slots when the indication indicates that the base station is a first type of transmission and decoding the communication signal in a time slot by means of symbols transmitted in subsequent unallocated time slots when the indication indicates that the base station is a second type of transmission.

13. A mobile station operable at a wireless communication station and configured to receive transmissions of a first type and a second type from a distant base station, the transmissions being arranged in a plurality of successive time slots, the mobile station comprising:

a receiver configured to receive a transmission and to generate data responsive thereto, the data including an indication having a first value when the transmission is of a first type and a second value when the transmission is of a second type; and

a circuit, coupled to the receiver, is configured to decode data using data from a current time slot when the indication has a first value and configured to decode data using data from the current time slot and a subsequent time slot when the indication has a second value.

14. The mobile station of claim 13, further comprising:

a transmitter for transmitting signals to a remote base station; and

circuitry configured to format data for transmission to a remote base station, the data including a pointer to the base station indicating whether the mobile station accepts transmissions of the first type.

15. A mobile station operable in a wireless communication system including one or more base stations that transmit during successive time slots, the mobile station comprising:

a decoder configured to identify an indication in a transmission from one or more base stations indicating whether a subsequent time slot may be relied upon for reception; and

a demodulator for demodulating transmissions from one or more base stations during a current time slot according to one of the first algorithm and the second algorithm in accordance with the indication.

16. The mobile station of claim 15, wherein the decoder is configured to recognize an indication in data received in the one or more base stations.

17. The mobile station of claim 16, wherein the decoder is configured to identify the indication from data received from the one or more base stations during initiation of bi-directional communication between the mobile station and the one or more base stations.

18. The mobile station of claim 15 wherein the decoder is configured to identify the indication in successive time slots.

19. The mobile station of claim 18, wherein the decoder is configured to measure signal characteristics of transmissions from one or more base stations during successive time slots and identify the indication by comparing the signal characteristics to a predefined threshold.

20. A base station configured to operate in a wireless communication system, the base station for transmitting signals to one or more mobile stations in a predefined geographical area, the mobile stations being of one of a first type and a second type, the base station comprising:

means for determining a type of a particular mobile station in wireless communication with a base station; and

means responsive to a particular type of mobile station for transmitting:

(a) radio signals to a particular mobile station during a current time slot and at least a portion of a subsequent time slot when the particular mobile station is of a first type; and

(b) when the particular mobile station is of the second type, the radio signal to the particular mobile station during the current time slot is adapted to the radio signal of the other mobile station type.

21. The base station of claim 20 wherein the means for determining comprises a circuit for determining the type of the particular mobile station based on transmissions on the mobile station.

22. The base station of claim 20, wherein the means for transmitting employs a wireless signal that is one of a change in power level of transmissions in subsequent time slots, a change in phase of transmissions in subsequent time slots, a change in timing of transmissions in subsequent time slots, a change in direction of transmissions in subsequent time slots, a change in modulation of transmissions, and a change in burst format of transmissions in subsequent time slots.

23. A Time Division Multiple Access (TDMA) radio communication system, comprising:

at least one base station transmitting a wireless signal during successive time slots, the time slots including a current time slot and a subsequent time slot, the wireless signal including an indication of whether the subsequent time slot transmission is warranted;

at least one mobile station receives radio signals in accordance with the indication.

24. The TDMA radio communication system of claim 23 wherein at least one of said mobile stations comprises a circuit for decoding said radio signal using information in said current time slot and information in said subsequent time slot when said indication indicates that transmission of said subsequent time slot is guaranteed.

25. The TDMA radio communication system of claim 23 wherein at least one of said base stations comprises a circuit for receiving a signal from a particular one of said at least one mobile station indicating a type of said particular mobile station, said mobile station being one of a type requiring a guaranteed transmission of a subsequent time slot and a type not requiring a guaranteed transmission of a subsequent time slot.