 or -4.5kHz and the modulation does not diminish to zero calls are stored, batched and then transmitted. Some systems trans- during the change. Three common types of digital modulation for a mit calls twice or even three times depending on service provider. binary number are shown in Figure 45.4. In NRZ level a one is Transmission in these zones can be sequential or simultaneous. represented by one level and a zero by the other level. In NRZ mark, Often only one frequency is employed but BT, who claim to cover a one is represented by a change in level and a zero by no change in 97% of the UK population with an estimated half a million sub- level. In biphase level a one and a zero are represented by a scribers, have 2 frequency channels and 40 zones. Since zones are positive/negative pulse sequence, each pulse being half a symbol adjacent, with varying degrees of signal overlap and CO channel wide. PRE AMBLE 1st BATCH 2nd BATCH 1.125s 1.0625s 1.0625s 1 FRAME 2 CODEWORDS 1 BATCH SYNCHRONISATION CODEWORD (S) + 8 FRAMES 20 - 21 22 31 BIT No. ADDRESS ADDRESS BITS PARITY CHECK BITS CODEWORD MESSAGE MESSAGE BITS PARITY CHECK BITS CODEWORD Figure 45.3 RPC no. 1 POCSAG code format Paging receiver design 45/11 Figure 45.4 Three common types of modulation used for digital paging codes: (a) NRZ level; (b) NRZ mark (differential encoding); (c) Bi-Phase level (Manchester code) 45.7.5 Swedish Mobilsoekring system (MBS) network at 280MHz and use a 1.4v zinc air cell of 700mAh capacity. Many of these more ambitious physical formats have compromised This nation wide system was launched by the Swedish PTT in late sensitivity when actually worn on the body. 1978 and uses the existing broadcast FM infrastructure. It is a cost Paging receiver designs must match world wide requirements in effective system for thinly populated countries; 8 million people in the range 25MHz to 900MHz. 449,750 sq. km. Use is made of one of the stereo broadcast pro- grammes in the 87MHz to 104MHz band, which covers most of 45.8.1 Receiver specifications Sw