Burst arriving at BTS correctly in TS3 MS moves
towards BTS TX delay reduces
MS moves away from BTS TX delay increases
BTS MS
Variation in transmission path
length BTS monitors
position of burst in timeslot
Figure 7
Transmission Timing
3.2 Burst Mode Transmission
A user can transmit only for a short period of time, i.e. during the allocated timeslot of 577 µs, every frame period, which is every 4.62 ms.
In order that the information can be transmitted coherently in such a short period of time, it is formatted into a suitable structure for burst transmission.
There are four types of burst transmission used in GSM. These, shown in Figure 8, are:
• Frequency Correction Burst (FCB)
• Synchronization Burst (SB)
• Access Burst (AB)
• Normal Burst (NB)
The FCB is transmitted downlink only in the main control channel. It provides the MS with two functions: it identifies the presence and position of the main control channel and it allows the MS to make any internal adjustments to its frequency source.
The SB is transmitted downlink only in the main control channel, and carries information which allows the MS to become bit-, slot- and frame-synchronized with the network.
The AB is transmitted uplink by an MS wishing to access the network. The significant part of this burst is its very long guard period. This allows the mobile to make unsynchronized access attempts from up to 35 km away, without causing interference.
This is the most common structure used in both the uplink and downlink. It is used for carrying speech or data (traffic), or control information. The type of content is indicated by the Fast Associated Control Channel (FACCH) flags.
Tail
a) Frequency Correction Burst (FCB)
b) Synchronization Burst (SB)
c) Access Burst (AB)
3.3 Tail Bits and Guard Period
If data is only transmitted in a designated timeslot, it is necessary to consider what the transmitter in the mobile equipment is doing.
The mobile must not be radiating any power when it is not transmitting. When its designated timeslot arrives, it must increase its output power to the required level, and maintain that level for the duration of the ‘useful information’ in the burst. At the end of the ‘useful information’ it reduces its power to zero again.
The tail bits are used to indicate start and stop points of the burst.
The guard period is to allow for mobility and power ramping.
Tail
a) Frequency Correction Burst (FCB)
b) Synchronization Burst (SB)
c) Access Burst (AB)
3.4 Extended Range
The maximum timing advance of 63 bits imposes a theoretical limit on cell size of about 35 km from the base station. Mobiles at greater range would supply bursts which would arrive so late they encroached on the next timeslot. This theoretical limit can pose problems in providing coverage in specific areas, for example offshore coverage from a site on the coast.
This can be overcome by allocating two or more successive timeslots to the mobile, thus breaking the theoretical 35 km limit. It is not actually possible to allocate more than one timeslot to a GSM mobile. However, leaving the next timeslot unallocated has the same effect in this case. Bursts from a mobile beyond the the 35 km limit will drift into the next timeslot. At a range of about 120 km, the burst would arrive such that it lies completely within the next timeslot. See Figure 9.
This is the principle of extended-range cell operation. Note that traffic capacity is halved, because each channel uses two timeslots. A single carrier can thus handle four channels in an extended-range cell.
In practice, the ability to achieve the theoretical maximum range will depend on power budget considerations as well as timing effects.
Time at BTS TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7
Burst arriving from a mobile at 35 km using maximum timing advance (63 bits)
Burst arriving from a mobile at > 35 km using maximum timing advance
Burst arriving from a mobile at ≈ 120 km using maximum timing advance
Note: The MS is assigned to TS0
Figure 9
Extended Range Principle
4.1 Introduction
The logical channel describes the current utilization of a physical channel for either GSM or GPRS operation. A physical channel is used for transmitting and receiving traffic information (speech or user data), or control information (e.g. call set-up information). If it is currently transmitting or receiving traffic information, then
‘logically’ the physical channel is a traffic channel. However, the same physical channel (timeslot) can be used to transmit or receive control information. In that case the physical channel becomes a ‘logical’ control channel.
4.2 Traffic Channels
The Full-rate Traffic Channel (TCH/F) supports encoded/protected speech at a gross rate of 22.8 kbit/s, or Forward Error Correction (FEC) coded data at 9.6, 4.8 or 2.4 kbit/s (TCH/F9.6/F4.8 or /F2.4). The Half-rate Traffic Channel (TCH/H) will support second-generation speech vocoders at a gross rate of 11.4 kbit/s, or FEC-coded data at 4.8 or 2.4 kbit/s.
A full-rate channel requires one physical channel, whereas two half-rate channels can be supported by a single physical channel.
4.2.1 Packet Data Traffic Channel (PDTCH)
The PDTCH is allocated for data transfer. It is temporarily dedicated to one MS or a group of MSs in the Point-to-Multipoint – Multicast (PTM-M) case. In multislot operation, one MS may use multiple PDTCHs in parallel for individual packet transfer. The PDTCH can be found on both uplink and downlink.