Packet radio telephone services

Details Packet radio telephone services Packet radio telephone services

1998-11-10

2002-11-05

Kizou, Hassan (Department: 2662)

Multiplex communications

Communication over free space

Portable address responsive receiver

C370S324000, C370S347000, C370S350000, C370S508000, C370S509000

Reexamination Certificate

active

06477151

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to packet switched radio telephone services and is applicable in particular, though not necessarily, to the General Packet Radio Service (GPRS).
BACKGROUND OF THE INVENTION
Current digital cellular telephone systems such as GSM (Global System for Mobile communications) were designed with an emphasis on voice communications. Data is normally transmitted between a mobile station (MS) and a base station subsystem (BSS) over the air interface using the so called “circuit switched” transmission mode in which a series of regularly spaced time slots on one frequency band are reserved for the duration of the call. For voice communications, where the stream of information to be transmitted is relatively continuous, the circuit switched transmission mode is reasonably efficient. However, during data calls, e.g., internet access or facsimile transmission, the data stream is “bursty” and the long term reservation of time slots in the circuit switched mode represents an uneconomic use of the air interface.
Given that the demand for data services with digital cellular telephone systems is increasing rapidly, a new GSM based service known as the General Packet Radio Service (GPRS) is currently being standardised by the European Telecommunications Standards Institute (ETSI) and is defined in overall terms in recommendation GSM 03.60. GPRS provides for the dynamic allocation of transmission capacity for data transmission. That is to say that time slots on a frequency band (or bands) are allocated to a particular MS to BSS link only when there is data to be transmitted. The unnecessary reservation of time slots when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS will therefore uses a basic channel structure similar to that defined for GSM. In GPRS, a given frequency band is divided in the time domain into multi-frames, each multi-frame consisting in turn of 52 TDMA (Time Division Multiple Access) frames. The length of a TDMA frame is 4.61 5 ms and each TDMA frame is in turn divided into eight consecutive slots of equal duration. This frame structure is illustrated in FIG.
1
and is relative to the transmission and reception time at the BSS. p In the conventional circuit switched transmission mode, when a call is initiated, two physical channels are defined for that call at the BSS by reserving two respective time slots, separated by two intervening slots, in each of a succession of TDMA frames. One of these channels provides a downlink channel for carrying user data from the BSS to the MS whilst the other provides the uplink channel for carrying user data from the MS to the BSS.
With the introduction of GPRS (the general architecture of a GSM/GPRS network is illustrated in
FIG. 2
) the fixed relationship between time slots allocated for uplink and downlink channels no longer applies. Time slots may be dynamically assigned to the uplink channel and the downlink channel for a given MS depending upon demand and capacity and MS multi-slot class. So, for example, in any given TDMA frame one time slot may be allocated to the downlink channel with two slots being allocated to the uplink channel. Also, there is no fixed time relationship between the uplink and the downlink allocated slots. Slot allocation is notified to the MS during a channel set-up stage.
When a MS first connects to a GPRS cellular network, the MS synchronises itself to the BSS using information carried by a synchronisation channel (SCH) transmitted by the BSS to all listening MSs. Synchronisation involves the initialisation of a modulo counter at the MS which has a 52 TDMA frame cycle. When a user data transmission channel (either uplink or downlink or both) is requested, the BSS allocates time slots for user data and notifies the MS of the allocation. Time slots are allocated in consecutive TDMA frames and may be reserved for a fixed number of frames or until the MS or the BSS chooses to release the channel. For data transmissions from the BSS to the MS, the transmission slots coincide with those defined by the modulo counter and the MS therefore knows when to “listen” for its allocated slots.
The correct synchronisation of the receiver at a MS is therefore easily achieved using the BSS broadcast synchronisation channel. Synchronisation of the transmitter of a MS is however somewhat more complex. As data transmitted from the MS (MS
TX
) to the BSS must arrive at the BSS in the allocated time slot (BSS
RX
), it is necessary to advance the transmission of data (by a timing advance value TAV relative to the time defined by the modulo counter) to take account of the propagation delay from the MS to the BSS (as illustrated in
FIG. 3
where slot number
2
is reserved to the MS for transmission). Moreover, as the MS may be moving rapidly relative to the BSS, it is necessary to recalculate the propagation delay at regular intervals and to provide the updated values to the MS.
It will be clear that a TAV is required when an uplink channel is established for transmitting user data from the MS to the BSS. However, a TAV is also required when a downlink channel is established as, even though user data is coming from the BSS to the MS, certain signalling data (e.g. acknowledgements) is going in the reverse direction (i.e. the uplink direction).
In the current GPRS recommendation, a MS transmits a “timing access burst” to the BSS on an uplink Packet Timing Advance Control Channel (PTCCH) channel once every eight multiframes. One access burst is transmitted for each channel allocated to the MS (uplink and downlink). The timing access burst is transmitted in a slot allocated to the MS for this purpose. This transmission is not advanced and so the BSS is able to determine the TAV by determining the time shift in the access burst relative to the time base of the BSS. The TAV for each channel allocated to a MS is transmitted to the MS (on a downlink PTCCH) and is updated once every eight multiframes, i.e. following receipt of each new corresponding timing access burst. This process is illustrated schematically in FIG.
4
.
FIG. 5
illustrates eight consecutive multiframes, n to n+7, each of which comprises 52 TDMA frames. The multiframe structure provides 12 radio blocks B
0
to B
11
, each radio block comprising 4 consecutive TDMA frames. The radio blocks are used for transmitting user data (and also some signalling information). In the current GPRS proposal, each slot in a TDMA frame may be simultaneously allocated to up to 16 different downlink channels or to 8 different uplink channels. In the case of a downlink channel, a MS must therefore listen during its allocated slot(s) in each TDMA frame (according to the time base defined by its modulo counter), and decode the received signal to determine if the signal is intended for it.
Each multiframe also contains 4 “idle” TDMA frames (numbered
0
to
31
in the 8 multiframe structure of FIG.
5
). The even numbered idle frames,
0
,
2
,
4
etc, are used to accommodate timing access bursts transmitted from the MSs to the BSS whilst the odd numbered idle frames,
1
,
3
,
5
etc, are used to accommodate TAVs transmitted from the BSS to the MSs. Considering the former, one time slot is able to accommodate one timing access burst. Given that 16 channels may be allocated to each time slot, with two idle frames per multiframe allocated for access bursts (e.g. idle frames
0
and
2
in multiframe n), it takes all eight of the multiframes shown in
FIG. 5
to convey the maximum possible number of timing access bursts.
Considering the transmission of TAVs from the BSS to the MSs, once calculated, TAVs for the 16 channels (assuming maximum allocation) allocated to a given time slot are coded and transmitted as a split packet. Thus, a packet carrying TAVs for the slot
0
allocated channels is transmitted in the first slot of each of four consecutive idle TDMA frames all

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