Radio aspects

Date: 2015-10-07; view: 400.

CT2/CAI digital specification

These specifications are described in Swain (1990), Holmes and Swain (1990) and Evans (1940). The FDMA/TDD principle of operation of CT2/CA1 is shown in Figure 49. 1. Duplex transmission is provided by transmitting in time interleaved burst mode on the same carrier frequency for both directions. This technique was appropriate for a low cost instrument since both ends are identical. Furthermore the technique only required a single block of radio spectrum rather than the duplex bands favoured by other mobile radio systems. Hence the band 864-868MHz was allocated to the service to support forty 100 kHz duplex channels.

By adopting the CCITT standard 32kbit/s ADPCM speech coding algorithm it was possible to contain the transmitted symbol rate to 72kbit/s with some allowance for framing and signalling bits. In practice the corresponding B-channel capacity is 32kbit/s and that for the D-channel 1 kbit/s or 2 kbit/s depending on manufacturer's choice.

By adopting a common air interface the radio interface is stand­ardised in terms of:

1. Physical parameters of the radio link e. g. modulation, frame rate, data rate.

2. System and user signalling.

3. Speech transmission.

With only forty channels available to support the high traffic demands it was clear that the benefit of dynamic channel selection would be needed. Thus both handset and base are required to operate on any of the forty channel pairs through selection of the channel with the lowest co-channel and/or adjacent channel interference. This technique is more than adequate to meet the traffic capacity needs of residential and small business operations, but other techniques need to be exercised to raise the effective capacity to meet the requirements of larger business systems.

To increase the frequency re-use ability of the system in large buildings cordless business systems will need to use the multi-cell coverage techniques employed by cellular mobile systems. However, in a building the cells will be used on each floor consequently the cell structure has three dimensions to it and allowance must be made for signals passing through the floors as well horizontally through the walls of the building.

To ensure good and even coverage both antennae and radiating cables may be used to provide service in a large building complex as in Figures 49. 2 and 49.3 (Holmes and Swain, 1990).

A typical example for CT2/CAI coverage from an antenna is shown in Figure 49.4 (Swain, 1984) where measured signal levels and computed bit error ratios are indicated.

The detail parameters of the radio interface are given in ETSI (1991) but it specifies two level frequency shift keying with Gaus­sian filter shaping and a frequency deviation of 14. 4kHz to 25. 2kHz above carrier frequency to represent a binary 1 and equal shifts below the carrier frequency for binary 0. This represents a modula­tion index range of 0. 4 to 0. 7. As previously noted time division duplex transmission is used with 1ms for each transmit and receive packet. A dead time between transmit and receive bursts has been created to allow transmitters to ramp up and ramp down their power in a way designed to limit spectral splatter across adjacent channels. This period also permits oscillators to settle between bursts.

Exercise 1 Learn the words and word combinations

Exercise 2 Read the text

Exercise 3 Give the Russian equivalents for the following English ones:

Exercise 4 Answer the following questions: