Wireless & Mobile Lecture # 18

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Wireless & Mobile Lecture # 18

  1. 1. CORDLESS SYSTEMS AND WIRELESS LOCAL LOOP Chapter No 11 Lecture # 18Lecture # 18
  2. 2. Cordless System Operating Environments  Residential – a single base station can provide in-house voice and data support  Office  A single base station can support a small office  Multiple base stations in a cellular configuration can support a larger office  Telepoint – a base station set up in a public place, such as an airport
  3. 3. Design Considerations for Cordless Standards  Modest range of handset from base station, so low-power designs are used  Inexpensive handset and base station, dictating simple technical approaches  Frequency flexibility is limited, so the system needs to be able to seek a low- interference channel whenever used
  4. 4. Time Division Duplex (TDD)  TDD also known as time-compression multiplexing (TCM)  Data transmitted in one direction at a time, with transmission between the two directions  Simple TDD  TDMA TDD
  5. 5. Simple TDD  Bit stream is divided into equal segments, compressed in time to a higher transmission rate, and transmitted in bursts  Effective bits transmitted per second: R = B/2(Tp+Tb+Tg)  R = effective data rate  B = size of block in bits  Tp = propagation delay  Tb = burst transmission time  Tg = guard time
  6. 6. Simple TDD  Actual data rate, A: A = B /Tb  Combined with previous equation:  The actual data rate is more than double the effective data rate seen by the two sides       + += b gp T TT RA 12
  7. 7. TDMA TDD  Wireless TDD typically used with TDMA  A number of users receive forward channel signals in turn and then transmit reverse channel signals in turn, all on same carrier frequency  Advantages of TDMA/TDD:  Improved ability to cope with fast fading  Improved capacity allocation
  8. 8. DECT Frame Format  Preamble (16 bits) – alert receiver  Sync (16 bits) – enable receiver to synchronize on beginning of time slot  A field (64 bits) – used for network control  B field (320 bits) – contains user data  X field (4 bits) – parity check bits  Guard (60 bits) – guard time, Tg
  9. 9. A Field Logical Control Channels  Q channel – used to broadcast general system information from base station to all terminals  P channel – provides paging from the base station to terminals  M channel – used by terminal to exchange medium access control messages with base station  N channel – provides handshaking protocol  C channel – provides call management for active connections
  10. 10. B Field  B field transmits data in two modes  Unprotected mode - used to transmit digitized voice  Protected mode - transmits nonvoice data traffic
  11. 11. DECT Protocol Architecture
  12. 12. DECT Protocol Architecture  Physical layer – data transmitted in TDMA-TDD frames over one of 10 RF carriers  Medium access control (MAC) layer – selects/ establishes/releases connections on physical channels; supports three services:  Broadcast  Connection oriented  Connectionless  Data link control layer – provides for the reliable transmission of messages using traditional data link control procedures

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