《通信原理》课程教学资源（PPT课件讲稿，英文版）3.9 Time-Division Multiplexing

3.9 Time-Division Multiplexing Definition Time-division multiplexing (Tdm) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel

1 3.9 Time-Division Multiplexing • Definition. Time-division multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel

3.9 Time-Division Multiplexing Analog input signal Channel I ansmitter (from source 1) i i sampler Channel2 PAM TDM (from source 2) PCM Quantizer encoder Channel 3 (from source 3) --------. Synchronization Channel Receiver TDM PCM Channel 1 plus noise Decoder PAM Channel 2 Channel 3 Output analog signals Figure 3-35 Three-channel TDM PCM system

2 3.9 Time-Division Multiplexing

3.9 Time-Division Multiplexing in the transmitter the pulse width of the TDM PAM: the pulse width of the TDM PCM:

3 3.9 Time-Division Multiplexing • in the transmitter the pulse width of the TDM PAM: the pulse width of the TDM PCM: 3 Ts Ts

3.9 Time-Division Multiplexing At the receiver the decommutator has to be synchronized with the incoming waveform so that the PAM Samples corresponding to source l will appear on the channel l output. This is called frame synchronization, which is very important for TDM system Low-pass filters are used to reconstruct the analog signals from the Pam samples Because of isi resulting from poor channel filtering, PCM samples from one channel may appear on another channel. often called crosstalk, even though perfect bit and frame synchronization were maintained

4 3.9 Time-Division Multiplexing • At the receiver the decommutator has to be synchronized with the incoming waveform so that the PAM samples corresponding to source 1 will appear on the channel 1 output. This is called frame synchronization, which is very important for TDM system. • Low-pass filters are used to reconstruct the analog signals from the PAM samples. • Because of ISI resulting from poor channel filtering, PCM samples from one channel may appear on another channel, often called crosstalk, even though perfect bit and frame synchronization were maintained

3.9 Time-Division Multiplexing Frame synchronization -grouping data yncnronous Word synchronization (start up signaling) system e Digital bit synchronization -regenerating data system Asynchronous system Synchronous system: all devices have same clock, by data signal with clock, by separate clocking line) Asynchronous system: receiver clock is started aperiodically and no synchronization with a master clock

5 3.9 Time-Division Multiplexing • Synchronous system: all devices have same clock, by data signal with clock, by separate clocking line) • Asynchronous system: receiver clock is started aperiodically and no synchronization with a master clock. Digital system Synchronous system Asynchronous system Frame synchronization -- grouping data Word synchronization (start up signaling) bit synchronization -- regenerating data

3.9 Time-Division Multiplexing (Frame Synchronization Methods for frame sync (1) Sending a frame sync signal from the transmitter over a separate channel (2) Deriving the frame sync from the TDM signal itself. TDM frame sync format … Channel s1s2…sk| Channel Channel… Channel s1s2|… n data 1 data2 data n dat Information sync words Information sync words words words Frame Fig 3-36 TDM sync format

6 3.9 Time-Division Multiplexing (Frame Synchronization) • Methods for frame sync: • (1) Sending a frame sync signal from the transmitter over a separate channel • (2) Deriving the frame sync from the TDM signal itself. • TDM frame sync format Channel N data … s1 s2 … sk Channel 1 data Channel 2 data … Channel N data s1 s2 … Information words sync words Information words sync words Frame Fig.3-36 TDM sync format

3.9 Time-Division Multiplexing Frame synchronization word: A segmented bits data stream which obeys same rules. Usually, it should be unique in the data stream, or at least, the appear probability is very small in information words Sw----the sync word, usually it appears in the TDm data stream periodly. So if we locate the sw, then we could separate Ai from data stream, at same time, it can be located at other a: anda

7 3.9 Time-Division Multiplexing • Frame synchronization word: A segmented bits data stream which obeys same rules. Usually, it should be unique in the data stream, or at least, the appear probability is very small in information words • SW ----the sync word, usually it appears in the TDM data stream periodly. So if we locate the sw, then we could separate Ai from data stream, at same time, it can be located at other Ai and Aj

3.9 Time-Division Multiplexing (principle of frame synchronizer circuit TDM input Amplifer Sampler and Regenerated tdM data e z(t)and decision filter circuit Frame synchronizer hift register Bit Stage Stage Stage synchronizer 12 Coincident detector is a Coincident detector Frame sync K-input p T AND gate. Figure 3. 37 Frame synchronizer with TDM receiver format end 8

8 3.9 Time-Division Multiplexing (principle of frame synchronizer circuit ) • Coincident detector is a K-input AND gate. Amplifer and filter Sampler and decision circuit TDM input Bit synchronizer Stage 1 Tb Ts Stage 2 Stage 3 Stage i … Frame synchronizer shift register 2 s 3 s i s Coincident detector Figure 3.37 Frame synchronizer with TDM receiver format end Z(t) Regenerated TDM data Frame sync 1 s

3.9 Time-Division Multiplexing In some situation when there is word in receiver data stream happening to match the bits in the sync word, False sync output pulses will occur. Its probability is: P=2 -K Erampleif P=4*10-5 is the allowed probability of false sync, then a K=15-bit sync word is required. That is, a 15-stage shift register is needed for the frame synchronizer in the receiver e. Since the output of the coincident detector is a digitized crosscorrelation of the sync word with the passing K-bit word stored in the shift register. The sync words autocorrelation function must have the properties: Rs(O)=l and r(k=0 for k not equal to 0.9

9 3.9 Time-Division Multiplexing • In some situation, when there is word in receiver data stream happening to match the bits in the sync word, False sync output pulses will occur. Its probability is: = 2 (3 -90) -K Pf • Since the output of the coincident detector is a digitized crosscorrelation of the sync word with the passing K-bit word stored in the shift register. The sync word’s autocorrelation function must have the properties: Rs (0)=1 and R(K)=0 for k not equal to 0. • Exampleif Pf=4*10-5 is the allowed probability of false sync, then a K=15-bit sync word is required. That is, a 15-stage shift register is needed for the frame synchronizer in the receiver

3.9 Time-Division Multiplexing coincident: at each clock, a new binary bit of TDM data stream is clocked into the shift register, then, the data in it is corss-correlated with the expected unique sync word. If there is no difference, the coincident detector will output a binary l with Tb width, or will output a binary 0 with Tb width. → Received SW A1 A2 A3 SW TdM data k bits k bits k bits k bits bits Frame sync output 3k-bits information( known) sync Impuise sync impulse 10

10 3.9 Time-Division Multiplexing • coincident: at each clock, a new binary bit of TDM data stream is clocked into the shift register, then, the data in it is corss-correlated with the expected unique sync word. If there is no difference, the coincident detector will output a binary 1 with Tb width, or will output a binary 0 with Tb width. SW A1 A2 A3 SW k bits k bits k bits k bits k bits 3k-bits information (known) sync impulse sync impulse Received TDM data Frame sync output