- •49 Cordless telecommunications
- •49. 1 Introduction
- •49. 1. 1 Analogue technology
- •49. 1. 2 H. F. /V. H. F. Analogue ct
- •49. 1.3 European 900 mHz analogue standard
- •49. 2 Digital technology
- •49. 2. 1 Areas of application
- •49. 2. 2 Main service principles
- •49. 2. 3 Traffic capacity
- •49. 2. 4 Digital cordless standards
- •49. 3 Ct2/cai digital specification
- •49. 3. 1 Radio aspects
- •49. 3. 2 Frame multiplex structure
- •49. 3. 3 Protocol structure
- •49.3. 4 Transmission plan and digital codec
- •49. 3. 5 Implementation
- •49. 4 Dect digital specification
- •49. 4. 1 Dect protocol architecture
- •49. 4. 2 Intel-working Units (iwu)
- •49. 4. 3 Spectrum resource
- •49. 4. 4 Detail radio aspects
- •49. 4. 5 Radio operational features
- •49. 4. 6 Frame structure
- •49. 4. 7 Transmission plan and digital codec
- •49. 4. 8 Implementation
49. 4. 4 Detail radio aspects
The above applications place a number of requirements on the basic design of the radio system. The following are indicative of this process:
1. Ability to handover between channels on the same base station and between channels on different base stations without disturbance to speech or data.
Ability to offer variable bit rate channels to data and ISDN options.
Ability to cater for the traffic requirements of high capacity business communications systems.
The result of putting all these requirements into practice, including those of Section 49.2, produced an FDMA/TDMA/TDD system having ten carriers each conveying 12 time division duplex 32kbit/s channels, (see Figures 49. 10 and 49. 11. To achieve the variable bit rate capability DECT is able to concatenate individual 32kbit/s timeslots to build up larger capacities. These concatenated timeslots need not be adjacent or on the same carrier frequency. Potentially the maximum throughput is, therefore, 12 x 32 = 384kbit/s bothway. Handover is another key specified feature of DECT and has been designed to support handovers in 10ms to 15ms.
Other radio parameters are given in Table 49. 2.
Each frame consists of 5ms alternate periods of transmission and reception und it is a general requirement that all base and portable terminals shall be able to operate on all timeslots.
This technical specification indicates that in cluttered, multi-path, environments outside buildings time dispersion may be a problem.
Antenna diversity located at the base station is considered the first line of defence in such instances.
49. 4. 5 Radio operational features
The fixed part (FP) has an active channel for paging and synchronisation. An incoming call is paged on all FP and the PP responds on a channel chosen by the PP.
If channel quality deteriorates, PP moves out of range or due to increased interference, then handover is initiated to a more appropriate FP. A base controller synchronises this re-routeing of calls. PP periodically scan all channels to update their list of free channels to ease handover.
Link quality is assessed from channel bit error ratios and received signal strength.
For handover the PP requests a second channel (perhaps on another FP) to he modulated with the wanted signal. The PP then decides when to switch channels and then informs the controller which channel to release.
49. 4. 6 Frame structure
Figure 19.12 (ETSI. I992b) shows the multiplex burst as it would appear for a normal telephony call and shows also the relationship between the PHL and MAC layer responsibilities. The 48 bits of the A field support the associated signalling channel requirements (C) the broadcast beacon transmission (Q) and the paging channel (P). The remaining 16 bits of the A field is a cyclic redundancy code (CRC) check used to protect the data.
The В field is used to transport the traffic information (1) and offers 320 hits per burst which is equivalent to 32kbit/s. The four X bits (which optionally can he extended to eight) are provided to detect collisions between bursts emanating from independent, and hence un-synchronised, systems. It should be noted of course that other multiplex structures are used in DECT particularly in the channel set up phase.