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CODE DIVISION MULTIPLE ACCESS DEMAND ASSIGNED MULTIPLE ACCESS
- 1. SATELLITE COMMUNICATION UNIT IV – SATELLITE ACCESS AND CODING METHODS KARPAGAM INSTITUTE OF TECHNOLOGY COIMBATORE - 105. DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
- 2. • In code-division multiple access (CDMA), the individual carriers may be present simultaneously within the same rf bandwidth, but each carrier carries a unique code waveform (in addition to the information signal) that allows it to be separated from all the others at the receiver. • The code signal may be applied as modulation in exactly the same way as the information signal so that the BPSK signal carries both the information signal p(t) and the code signal c(t). This method is referred to as direct-sequence spread spectrum (DS/SS) Code Division Multiple Access
- 3. Direct Sequence Spread Spectrum CDMA System
- 4. • The code signal c(t) carries a binary code that has special properties needed for successful implementation of CDMA. The binary symbols used in the codes are referred to as chips rather than bits to avoid confusion with the information bits that also will be present. Chip generation is controlled by a clock, and the chip rate, in chips per second, is given by the clock speed. Code Division Multiple Access
- 5. Generation of a 7 chip maximal sequence code
- 6. Acquisition and Tracking of Carrier in CDMA system Acquisition and Tracking
- 7. • The spectrum shows the power density (watts per hertz) in the signal. For constant carrier power, it follows that if a signal is forced to occupy a wider bandwidth, its spectrum density will be reduced. This is a key result in CDMA systems. In all direct-sequence spread-spectrum systems, the chip rate is very much greater than the information bit rate, or Rch. Spreading and Despreading
- 8. • The maximum number of channels in a CDMA system can be estimated as follows: It is assumed that the thermal noise is negligible compared with the noise resulting from the overlapping channels, and also for comparison purposes, it will be assumed that each channel introduces equal power PR into the receiver. CDMA Throughput
- 9. • The beamwidth for VSAT antennas is comparatively broad and therefore could be subject to interference from adjacent satellites. CDMA rejects interference. • Multipath interference can be avoided. • Synchronization is not required. • Degradation of the system (reduction in Eb/No) is gradual with an increase in number of users. • Disadvantage: Low throughput efficiency Advantages of CDMA
- 10. • Single hop satellite transmission network • Direct connection between any two nodes in the network • Supports full mesh, circuit switched remote to remote site connectivity • Network allocates bandwidth per call basis • Benefits • Provide Reliable digital communication connectivity • Once call completion, channels are returned to pool for another assignment. • No dedicated station to station links (Single channel per carrier) • Reduction in Satellite bandwidth Demand Assigned Multiple Access (DAMA)
- 11. • Network Signaling Channels • One outlink carriers • Three signaling returnlink carriers • One Monitor and control returnlink carrier • Multiple Traffic carriers • Time Division Multiplexed carrier from NMS to all channels. Provides system control, call setup, software downloads • Returnlinks are shared by entire network to support call requests and status. Aloha access scheme. Five signalling returnlinks support to busy hour calls. Demand Assigned Multiple Access (DAMA)
- 12. • Multiple Traffic carriers • Separate returnlink for monitor and control to monitor large networks. • NMS poll all idle channel units in the network. These units are tuned to outlink. Demand Assigned Multiple Access (DAMA) SCPC DAMA frequency plan
- 13. • Call setup procedure • Caller picks up the phone and dials the number to be called. • Channel unit verifies dialed digits and send a message to NMS • NMS logs the request and compare with routing table, compute BW. • Caller and channel unit tuned to assigned frequencies • Establish the channel units and handshake takes place • Called channel unit rings the called phone. • NMS has a complete record of the calls and channel available at all times. Demand Assigned Multiple Access (DAMA)