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Time Division Frames

Preamble. Information Message. Guard Time. Slot N. Slot 1. Slot 2. Slot 3. Synch Bits. Control, Signaling. Info. Bits (Training). Guard Time. Header. CRC. Time Division Frames. Frame (T f ). In TDD half the slots are for upstream traffic and half for downstream traffic

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Time Division Frames

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  1. Preamble Information Message Guard Time ... Slot N Slot 1 Slot 2 Slot 3 Synch Bits Control, Signaling Info. Bits (Training) Guard Time Header CRC Time Division Frames Frame (Tf) • In TDD half the slots are for upstream traffic and half for downstream traffic • This is generic structure: not all frames used in all systems, and order may vary

  2. Frame Details • Preamble contains address and sync information used by base and mobile • Guard times allow sync of receivers between different frames • Users are assigned a position in each frame (delay of Tf between bursts) • Superframes (frames of frames) may have additional control frames

  3. Slot Structure • Header:guard (ramp) time for receiver synch. between slots • Synch:Used to establish bit synch (also for equalizer training) • Control:Used for handshaking, control, and supervisory messages • Info. Bits: Coded or uncoded information bits, may include pilot symbols/sequences for channel measurement and equalizer training. • Guard Time: Prevents overlap at base of slots arriving from different terminals.

  4. Requirements • Equalizer requirements: adaptive equalizer must compensate for time-varying ISI. • Minimum N=t/Ts symbols for training. • For t=20msec and Rb=280 Kbps, N=6 minimum (GSM: N=26) • If Tf~Tc, need to retrain every frame (GSM: Tf=4.615 ms, Tc=fD-1=12.5ms for fD=80 Hz, retrains every frame). • Guard time requirements: must compensate for LOS propagation delay (R/c for R the cell radius) and delay spread t due to multipath (reverse link only). • No delay spread: Tg>R/c=3.3 msec for R=1Km. • Do not need guard time for LOS propagation delay if base station synchronizes to received (instead of transmitted) signal. • With delay spread t: Tg>R/c+t, but typically have a smaller guard time.

  5. Tail 3b Data 57b Flag 1b Flag 1b Equal. Train 26b Data 57b Tail 3b Guard 8.25ms GSM Slots • Multiframe has 26 frames (each frame is 4.615ms), with 24 for data and 2 for control. Each call in progress assigned a control channel. • Slot time is 577ms • 26b equalizer training designed to handle delay spread up to 20 msec. (equalizer design not part of spec.) • Guard time less than maximum t. • Flag bits distinguish voice from data • Transmission rate approx. 270 Kb/s

  6. Spread Spectrum MAC • Basic Features • signal spread by a code • synch. between pairs of users • compensation for near-far problem (in MAC channel) • compression and channel coding • Spreading Mechanisms • direct sequence multiplication • frequency hopping Note: spreading is 2nd modulation (after bits encoded into digital waveform, e.g. BPSK), and DS spreading codes are inherently digital.

  7. Linear Modulation. (PSK,QAM) d(t) s(t) Linear Demod. X X Channel Sci(t) Sci(t) SS Modulator SS Demodulator Direct Sequence • Chip time Tc is N times the symbol time Ts. • Bandwidth of s(t) is N+1 times that of d(t). • Channel introduces noise, ISI, and multiple access interference.

  8. sci(t) s(t) BPSK Example d(t) Tb Tc=Tb/10

  9. Code Properties • Good code designs have r(t)=d(t) and rij(t)=0 for all t. • Hard to get these properties simultaneously.

  10. Walsh-Hadamard Codes • For N chips/bit, can get N orthogonal codes • Bandwidth expansion factor is roughly N. • Roughly equivalent to TD or FD from a capacity standpoint • Multipath destroys code orthogonality.

  11. Semi-Orthogonal Codes • Maximal length feedback shift register sequences have good properties • In a long sequence, equal # of 1s and 0s. • No DC component • A run of length r chips of the same sign will occur 2-rl times in l chips. • Transitions at chip rate occur often. • The autocorrelation is small except when t is approximately zero • ISI rejection. • The cross correlation between any two sequences is small (roughly rij=G-1/2 , where G=Bss/Bs) • Minimizes MAC interference rejection

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