Abstract:
The invention is a novel slot structure and method of transmitting data at a controllable power level. The slot comprises a header, a DATA field adjacent the header and a ramp interval adjacent the DATA field. The method consists of transmitting the entire slot at a sufficiently high power when the DATA field is occupied by data traffic or control information but to lower the power during transmission of the DATA field when the slot is a null slot. In the latter case, a gradual decrease of the transmitted power is effected during the first few symbols of the DATA field, whereas the power is brought back to a higher level during the ramp interval. The new power level may be higher or lower than the initial power level, depending on the destination mobile unit associated with the following slot.

Description:
FIELD OF THE INVENTION 
     The present invention relates to cellular communications networks and, in particular, to a novel downlink slot structure and method of power control for use in a TDMA packet data network. 
     BACKGROUND OF THE INVENTION 
     In order to enable multiple users of a cellular network to share the same geographic region, or “cell”, a time-division multiple-access (TDMA) standard was developed by members of the Telecommunications Industries Association, which standard came to be known as IS-136 and which is incorporated by reference herein. According to IS-136, time is divided into frames of 40 milliseconds (ms) and each frame is further divided into slots of 6.67 ms each. In the version of IS-136 currently implemented by most wireless service providers, a circuit is established between a base station and a mobile unit by using two or more slots out of each frame. 
     This circuit-switched characteristic of most current IS-136-compliant cellular systems works well when the only type of information to be communicated is voice information and when bandwidth is abundant. However, a growing number of communications applications currently in use today involve the transfer of data packets, common examples being Web browsing, file transfers, electronic commerce and electronic mail. As opposed to voice exchanges, packet transfers are sporadic, asynchronous and have varying bandwidth demands. It is therefore highly inefficient to establish a dedicated circuit for channelling a packet transfer between a base station and a mobile unit when the circuit may service the occasional data burst but otherwise remains idle. 
     Thus, it would be desirable to apply the TDMA concept to a packet-based architecture, although without excessively deviating from the current TDMA standard. To this end, members of the TIA are in the process of developing a new TDMA standard, known as GPRS-136. GPRS-136 is a packet delivery service that is essentially based on the GPRS (general packet radio service) protocol as defined by the ETSI (European Telecommunnications Standards Institute) but specifically adapted to operate on an IS-136 air interface. Therefore, GPRS-136 continues to provide slots of 6.67 ms and frames of 6 slots each, as in conventional IS-136. 
     Nevertheless, while certain similarities between GPRS-136 and IS-136 are present for reasons of compatibility, GPRS-136 also allows the configuration of new slot structures which differ significantly from the traditional IS-136 model. This permits the design of slot structures that counter various disadvantages of current TDMA systems which are compounded by the packet-based nature of today&#39;s traffic mix, while remaining within the broad scope of the accepted IS-136 air interface standard. 
     For instance, co-channel interference is a phenomenon known to affect a mobile unit when it enters a geographic region in which the signals from multiple base stations are strong enough to interfere with each other. In a packet-based system, out of the many mobile units communicating with base stations that contribute to the co-channel interference, it is likely that one or more will be idle and will not require the transmission of packets at any given time instant. Co-channel interference could therefore conceivably be mitigated by reducing the power emitted by a base station while in communication with idle mobile units. 
     However, if the current IS-136 slot structure and in particular the digital control channel (or DCCH) were adopted for packet transmission, it would be extremely impractical to achieve a reduction in co-channel interference. The reason for this is that the DCCH slot structure contains control fields that are distributed among the data fields of the slot, but these same control fields comprise information that must be properly received by the mobile unit, even if the data fields contain no valid data, e.g., when the mobile unit is idle. 
     Thus, a base station wishing to reduce co-channel interference should execute fast and frequent power ramps to keep the transmitted power at a high level for the control fields, while possibly lowering the power during the (fragmented) data fields of the slot. Unfortunately, an ordinary base station transmitter cannot provide such rapid variations in output power as are called for by the nature of the currently used IS-136 slot structure. 
     Clearly, there is a need for a new slot structure which is better suited to a packet-switched communications architecture and, specifically, which possesses advantageous properties to allow the transmitted power to be reduced during the transmission of null slots, thereby to reduce co-channel interference. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to mitigate or obviate one or more disadvantages of the prior art. 
     Therefore, in accordance with a first broad aspect, the invention provides a novel slot structure for use in a TDMA communications system. The slot comprises a header, a data field adjacent the header and a ramp interval adjacent the data field. 
     The invention may be summarized according to another broad aspect as a method of transmitting data at a controllable power level. The method comprises arranging the data into slots having the just described structure and then choosing one of two power curves for transmitting a given slot. If the slot is a null slot, then the header is sent at a high power and the power is ramped down to a low power during the first few symbols of the data field. The remainder of the data field is transmitted at this low power until the power is ramped back up again during the ramp interval. 
     Because the header is separate from the data field, it becomes practical to implement the power control mechanism using currently available base station transmitters, with the result that power is conserved at the base station transmitter and co-channel interference is mitigated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiment of the present invention will now be described with reference to the following figures, in which: 
     FIG. 1 illustrates a sequence of frames containing slots grouped into TDMA blocks associated with respective superframe phases; 
     FIG. 2A shows the structure of a slot in accordance with the preferred embodiment of the present invention; 
     FIG. 2B shows variations in the instantaneous power level emitted by a base station transmitter when the DATA field in the slot of FIG. 2A contains a data burst; 
     FIG. 2C shows variations in the instantaneous power level emitted by a base station transmitter when the DATA field in the slot of FIG. 2A contains a pause between bursts of data; and 
     FIG. 3 depicts the detailed structure of the DATA field in the slot of FIG. 2A when modulated using 8-PSK. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is partly described in TIA contribution TR45.3.5/98.07.15R2, incorporated by reference herein, which proposes a novel format for a physical layer packet data slot for both the uplink (mobile unit to base station) and the downlink (base station to mobile unit); the invention described herein is directed specifically to the downlink slot structure. 
     Referring to FIG. 1, there is depicted a sequence of frames, each consisting of six inventive slots as will be described in further detail hereinbelow. For each frame, the slots are identified as A, B, C, D, E and F. The triplet of slots A, B and C and the triplet of slots D, E and F each forms what is known to persons skilled in the art as a TDMA block. 
     The use of a single slot per TDMA block to transmit a packet-based service to one or more mobile units defines what may be termed a “full-rate” forward packet channel (FPCH). However, depending on the bandwidth required by the packet-based service, it may be desirable to use two or all three slots per TDMA block in the case of a “double-rate” or “triple-rate” FPCH, respectively. 
     Associated with each TDMA block is a so-called “superframe phase” between 0 and 31, which serves to identify the position of the TDMA block within a larger number of blocks forming a superframe. This preliminary discussion relating to superframes and superframe phases will prove useful later on when examining the contents of a specific field within the inventive slot structure. 
     Referring now to FIG. 2A, there is shown a downlink slot  200  having a structure in accordance with the preferred embodiment of the present invention. The inventive slot  200  preferably has a length of 162 symbols and is broken down into a fourteen-symbol SYNC field  210 , a six-symbol coded superframe phase (CSFP) field  220 , a twelve-symbol packet channel feedback (PCF) field  230 , a 128-symbol DATA field  250  and a ramp interval  260  having a length of two symbol intervals. In order to remain within the realm of IS-136, the slot  200  preferably has a total duration of 6.67 milliseconds (ms). 
     The ramp interval  260  affords a transition period between the DATA field  250  and the header of the subsequent slot in a sequence of slots such as that shown in FIG.  1 . The ramp interval  260  need not be modulated according to any particular scheme since it contains neither data nor control information. While the ramp interval  260  is preferably two symbols in duration, it may span any suitable integer number of symbols. 
     Digital modulation is used for modulating each symbol in the header  270  (consisting of the SYNC, CSFP and PCF fields) and in the DATA field  250 . In the preferred embodiment of the present invention, the header  270  is modulated using π/4-shifted differential quaternary phase shift keying (π/4-DQPSK), while the modulation scheme used for the symbols in the DATA field  250  is selected to be either π/4-DQPSK or octonary PSK (8-PSK). The selected modulation scheme used for the DATA field  250  is indicated in the CSFP field  220 , to be described later in further detail. 
     If π/4-DQPSK is used for modulating the DATA field  250 , then there are two bits of information per symbol while in the case of 8-PSK the number of bits per symbol is three. Depending on the modulation scheme, therefore, the DATA field  250  may carry either 256 bits (if π/4-DQPSK is used) or 384 bits (when 8-PSK is employed). Since 8-PSK is more prone to errors than is π/4-DQPSK, an acceptable error rate for 8-PSK can be maintained by preferably inserting pilot fields within the DATA field  250 . 
     Reference is now made to FIG. 3, which specifically depicts the DATA field  250  when it is modulated using 8-PSK, consisting of a first three-symbol pilot field P 1  placed 17 symbols after the start of the DATA field  250 , a second three-symbol pilot field P 2  located  33  symbols after pilot field P 1 , a third three-symbol pilot field P 3  found  33  symbols after pilot field P 2  and a fourth three-symbol pilot field P 4  at the edge of the DATA field  250 , separated from pilot field P 3  by  33  symbols. While detection accuracy and synchronization is enhanced by the presence of four evenly spaced pilot fields of known value, the number of available data bits in the DATA field  250  is reduced to 348 from the previously mentioned 384. 
     Although the DATA field  250  usually carries only genuine data traffic (in which case the FPCH is known as a packet traffic channel—PTCH), the DATA field  250  may also be used to carry control information some of the time and data traffic the rest of the time (in which case the FPCH is known as a packet control channel—PCCH). In either case, the slot structure of FIG. 2A remains unchanged. 
     With continued reference to FIG. 2A, the 14 symbols contained in the SYNC field  210  form one of a number of known combinations, thereby to identify the slot position within a frame and hence to enable accurate timing and carrier recovery at the mobile unit. The SYNC field  210  must be accurately received at the mobile unit and therefore it must always be transmitted at a sufficiently high power by the base station. 
     Likewise, the CSFP field  220  is also crucial and must be transmitted at a sufficiently high power to be received with accuracy by the mobile unit. As mentioned above, a first purpose of the CSFP field  220  is to indicate the modulation scheme used to modulate the DATA field  250 . In addition, as suggested by its acronym, the CSFP field  220  also serves to identify the superframe phase, discussed above, associated with the contents of the DATA field  250 . 
     A third purpose of the CSFP field  220  in the time slot is to specify whether the data in the DATA field of the slot is in an incremental redundancy mode or in a fixed coding mode. This allows ficed coded slots to be interspersed between incremental redundancy slots. 
     Preferably, the CSFP field  220  is encoded using a bit pattern that allows facilitated error detection and correction by the mobile unit, most preferably a shortened Hamming code with inverted parity bits. Also, it is a desirable feature of the CSFP field  220  that it be located after and adjacent the SYNC field  210 , since correct detection of the SYNC symbols at the mobile unit will increase the probability of correct detection of the CSFP symbols in the presence of strong multipath propagation through the air interface. 
     Finally, the PCF field  230  is an optional control field and contains control information which is used in support of reservation- or contention-based access channel operation. For instance, the PCF field  230  may indicate to a mobile unit the identity of the next slot which is destined for that mobile unit. If it is used, the PCF field must be transmitted at sufficient power to be accurately received by the mobile unit or units for which the slot is destined. 
     Due to the arrangement of the control fields as a contiguous header  270  separate from the DATA field  250  and to the presence of the ramp interval  260  after the DATA field  250 , reduction of co-channel interference during pauses becomes feasible because the power curve to be applied by an ordinary base station transmitter is drastically simplified. Specifically, FIG. 2B shows a power curve  280  of the instantaneous transmitted base station power to be applied in the case where the information in the DATA field  250  is a legitimate burst of data or control information, while FIG. 2C shows a power curve  290  corresponding to the case where the information in the DATA field  250  is a pause, i.e., when the slot  200  is a null slot. 
     In FIG. 2B, power curve  280  is seen to be at a relatively high level L, during the header  270  of the slot  200 , thus enabling the mobile unit to properly receive the control information in the SYNC field  210 , the CSFP field  220  and the optional PCF field  230 . Level L 1  can either be the maximum output power level of the base station transmitter or, in a more sophisticated system, level L 1  may be the power level just high enough to keep the bit error rate below a certain threshold value. It is noted that the emitted base station power remains at level L 1  throughout the DATA field  250 , since the DATA field in this case contains a burst which, just like the information in the header  270 , must be received with low probability of error by the mobile unit. 
     Continuing along power curve  280 , the power level is decreased during the ramp interval  260  until it reaches a level L 2  at the beginning of the next slot. Of course, the slot immediately following slot  200  may be destined for a different mobile unit which may be closer to (or further from) the base station than the first mobile unit, and therefore the power level L 2  required to yield a sufficiently low bit error rate may be lower, higher or the same as level L 1 . 
     In FIG. 2C, power curve  290  similarly begins at a relatively high level L 1 , but remains at this level only during the header portion  270  of slot  200 . In this case, the information in the DATA field  250  is a pause, i.e., the slot  200  is a null slot, and therefore the transmitted power is allowed to be significantly reduced during the DATA field  250 . Accordingly, the power curve is seen to drop from level L 1  to level L MIN , preferably several decibels below level L 1 , during the time span T F  immediately following the header  270 . 
     The “fall interval” T F  is implementation-dependent and preferably spans the duration of one to three symbol intervals. Although the power curve  290  is shown to behave in a linear fashion during the fall interval T F , it should be understood that the exact trajectory followed while dropping to level L MIN  is not limited to this type of linear behaviour. 
     Still with reference to FIG. 2C, the emitted base station power remains at level L MIN  until the ramp interval  260 , at which point the power curve  290  is seen to rise to level L 2 . Again, depending on the destination of the next slot and as explained with reference to FIG. 2B, level L 2  may be higher or lower than level L 1 . It is to be understood that the shape of the power curve  290  during the ramp interval  260  need not be linear and may have smoother or sharper edges. 
     It is noted from FIG. 2C that the base station transmitter is made to function in what may be termed as “quasi-discontinuous transmission” (Q-DTX) mode when transmitting a null slot. This leads to considerable power savings at the base station transmitter and advantageously reduces the amount of co-channel interference induced in the signals received by mobile units inside and outside the corresponding cell. 
     While the preferred embodiment of the present invention has been described and illustrated, it will be apparent to one skilled in the art that numerous modifications and variations are possible. For example, a different number of symbols may populate the SYNC, CSFP, PCF, DATA and pilot fields and the ramp interval  260  may be of a different duration. Also, as indicated, it is possible for the DATA field to transport control information in addition to data per se. Finally, modulation schemes other than π/4-DQPSK or 8-PSK may be used by the base station to encode the symbols in the various fields of the inventive slot. 
     In view of the many further conceivable variations of the present invention, its scope is only to be limited by the claims appended hereto.