Global System Mobile, GSM, 2G
7.4 Background for the Choice of Radio Parameters
Controller
Controller
Controller
Controller
Tx, f0...fn TRX 1
TRX 2
TRX 3
TRX 4
Tx, f0...fn
Tx, f0...fn
Tx, f0...fn
Hybrid combiner
Hybrid combiner
Figure 7.16 RF synthesizer frequency hopping.
and not routed by the bus in contrast to baseband hopping. The transmitter tunes to the correct frequency at transmission of each burst, see Figure 7.16.
The advantage is that the number of frequencies that can be used for hopping is not dependent on the number of transmitters. It is possible to hop over a lot of frequen- cies even if only a few transceivers are installed. The gain from frequency hopping can thereby be increased. This concept is often called fractional loading. A disadvantage with synthesizer hopping is that wideband hybrid combiners have to be used. This type of combiner has approximately 3 dB loss making more than two combiners in a cascade impractical.
82 Introduction to Mobile Network Engineering
• Short burst length: smaller than correlation time of channel; that is, delay spread of the channel.
These factors result in the following design constraints and trade-offs:
• The equalizer window size of 16μs limits equalizing range to four radio symbols and a minimum symbol length of 4μs, thus limiting the information rate to 250 kbaud.
• The burst length is limited by the period during which the impulse response of the channel is stable. In a worst-case scenario on a high-speed train, the impulse response is stationary over an interval of ∼0.25 ms (for high-speed trains). With a training sequence in the middle of the burst, the maximum burst length is∼0.5 ms.
• The design constraints for the length of TDMA frame are shown in Figure 7.17. As observed, the major impact to frame length comes from the switching time of fre- quency synthesizer. The mobile has to be able to transmit, receive and listen at vari- able frequencies in line with the duplex arrangement, frequency hopping pattern and neighbour cell frequency list. At the time of GSM development, the switching time of frequency synthesizer was about 1 ms, together with requirements for MAHO, that is, listening and measuring the signal level of neighbour cells; this limits the minimum length of TDMA frame to>4 ms.
Apparently, a limitation coming from the switching time of synthesizer was an issue of a commercial nature rather than a technical one: two synthesizers could be deployed in the mobile terminal, one active at the current frequency, another one settled for the next channel to transmit or receive.
• Given the burst duration constraints,∼0.5 ms and∼4 ms for frame length, we have another limitation for the maximum number of time slots per TDMA frame; that is, eight time slots per TDMA frame.
• Another design parameter is a total transmission delay due to the overall data pro- cessing and interleaving procedure in particular. The depth of interleaving should be greater than the fade duration but not too large in order to not extend a burst
Transmit
Receive
Measure 0.5 ms
0.5 ms 0.5 ms
1 ms
1 ms
1 ms
Transmit
4.5 ms
Figure 7.17 TDMA frame length constraints.
transmission delay. The result is a reasonable compromise between total transmission delay due to TDMA formatting and interleaving of each burst over eight time frames
∼40 ms and requirements for listening and measuring the neighbour cells during idle slot periods for MAHO preparation.
7.4.1 Guard Period, Timing Advance
To prevent data bursts from different terminals overlapping in the input to the base-station receivers, a guard period (GP) with a time of 8.25 bits∼31μs has been introduced. The GP copes with variations in the two-way propagation time to terminals at different distances from the base. The guard period corresponds to a two-way propagation path of about 4.5 km. This is considerably less than the maximum specified range (cell radius) of 35 km. Therefore, to prevent data bursts from different terminals overlapping in the input to the base-station receivers, the base-station instructs the terminals to insert a suitable delay between received and transmitted data bursts.
The delay is adjusted such that a transmitted burst from the terminal reaches the base-station receiver at the right position relative to the time slot structure. Regardless of how far the terminal is away from the base, the bursts arriving to the base receiver must arrive close to the middle of the allocated time slot. The closer a terminal to the base station, the greater the delay inserted. This is illustrated in Figure 7.18.
The timing advance parameter is transmitted as a six-bit number providing 64 tim- ing advance steps each corresponding to the period of one bit, namely 4.69μs. This means that the system may apply a maximum timing advance of 63×4.69μs=232μs, which corresponds to a round trip propagation distance of approximately 70 km and a maximum distance from the base station to the cell boundary of 35 km.
The timing advance values might be used as one of the parameters determining when handover between cells should take place. It could also be used for used to calculate the distance between the terminals and the base, which will be an important added value service.
to MS #1
τ
τ BS transmits
MS#1 receives
from MS#1 MS#1 transmits
3 × time slot - 2τ
BS receives
time MS#1 receives
and transmits
Figure 7.18 Timing alignment.
84 Introduction to Mobile Network Engineering