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Compare TDMA, CDMA, FDMA and SDMA
1 Answer
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Approach SDMA TDMA FDMA CDMA
Idea Segment spaced into cells or sectors. Segments sending time into disjoint time slots demand driven or fixed patterns. Segment the frequency band into disjoint subbands Spread the spectrum using orthogonal codes.
Terminals Only one terminal can be active in one cell or one sector. All terminals are active for short periods of time on same frequency. Every terminal has its own frequency uninterrupted All terminals can be active at the same place at the same moment uninterrupted.
Signal separation Cell structure, directed antennas Synchronization in time domain Filtering in the frequency domain. Code plus special receivers.
Transmission scheme Continuous Discontinuous Continuous Continuous
Cell capacity Depends on cell area Limited Limited No absolute limit on channel capacity but it is an interference limited system
Advantages Very simple, increases capacity per Established fully digital, flexible Simple, established, robust Flexible, less frequency planning needed, soft handover
Disadvantages Inflexible, antennas typically fixed Guard space needed (multipath propagation), synchronization difficult Inflexible, frequencies are scarce resource Complex receivers, needs more complicated power control for senders
Comment Only in combination with TDMA, FDMA or CDMA useful Standards in fixed networks, together with FDMA or SDMA used in many mobile networks Typically combined with TDMA and SDMA Still faces some problems, higher complexity, lowered expectations, will be integrated with TDMA or FDMA
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