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Broadcast Channels

Broadcast Channels. Synchronization easy. Interference signals follow same path as desired signal (no near-far problem) Complexity/power at transmitter less restricted than at receiver. Multiple Access Channels. Synchronization overhead required.

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Broadcast Channels

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  1. Broadcast Channels • Synchronization easy. • Interference signals follow same path as desired signal (no near-far problem) • Complexity/power at transmitter less restricted than at receiver.

  2. Multiple Access Channels • Synchronization overhead required. • Interference signals follow different paths than desired signal (near-far problem) • Complexity/power at receiver less restricted than at transmitter. • MAC channel is limiting factor in cellular systems with symmetric traffic.

  3. DuplexingSeparation of uplink and downlink traffic • Frequency Division Duplexing-FDD: uplink and downlink traffic sent in different frequency bands. • No synchronization issues. • Uplink and downlink channels may fade indepently. • Duplexor required to separate signals • Time Division Duplexity- TDD: uplink and downlink traffic sent on different timeslots. • Simple duplex equipment • requires synchronization • need guard bands to prevent overlap • transmission in one direction can be used to measure channel in other direction. • Flexible bandwidth allocation between uplink and downlink.

  4. Code Division Duplexing • Separate uplink and downlink using orthogonal or semiorthogonal codes • Semi-orthogonal codes have insurmountable near-far problems. • Orthogonal codes destroyed by multipath channel • Not used in any existing systems.

  5. Frequency Division Total system bandwidth divided into orthogonal channels assigned to different users. • Advantages • Narrowband channels (no ISI) • Low complexity • Allows cts. time transmission and channel estimation. • Disadvantages • Multiple radios at base station • Handoff complicated by cts. transmission • Dedicated channels (idle ones wasted) • Difficult to allocated multiple channels per user FD alone not used in current digital systems

  6. Time Division Time divided into orthogonal slots, with different timeslots assigned to different users. • Advantages • Common radio eqmt. for all users at the base station. • Noncontinuous transmission facilitates handoff and reduces power consumption. • Easy to allocated multiple channels/user • No duplexor required • Disadvantages • Synchronization required • Multipath destroys slot orthogonality • Typically require ISI mitigation • Idle channels may be wasted • Short transmissions make equalization and dynamic resource allocation hard.

  7. Code Division Orthogonal or semi-orthogonal codes used to modulate each users signal. Code properties used to separate users at the receiver. • Advantages • No hard limit to # of users in system (soft capacity - system is interference-limited) • Interference reduction techniques increase capacity • Synchronization not required • Can allocated multiple “channels”/user using multicode or multirate techniques. • Disadvantages • Complexity • Near-far problem

  8. Examples • AMPS FDMA/FDD • GSM (EDGE) TDMA/FDD • IS-54 and IS-136 TDMA/FDD • JDC TDMA/FDD • IS-95 CDMA/FDD • IMT-2000 CDMA/FDD

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