ECE 4710: Lecture #13 1 Bit Synchronization  Synchronization signals are clock-like signals necessary in Rx (or repeater) for detection (or regeneration) of the data from a corrupted input signal  Must have precise frequency and phase relationship with respect to received input signal  Frequency  appropriate sampling rate  Phase  sample at maximum eye opening in ~ bit center »Phase at Rx is random (unknown) due to propagation delay in channel

ECE 4710: Lecture #13 2 Synchronization  Digital communications can have up to three types of synchronization signals  Bit synch  distinguish between different bit intervals  Frame synch  distinguish between groups of data »Time Division Multiplexing (e.g. combine voice, video, and data)  Carrier synch  coherent detection of bandpass signals »Required for certain modulation methods where absolute phase of signal must be measured »Can be used to improve S/N by ~3 dB even when absolute phase is not needed  Synch signals derived from: 1) Distorted (attenuated) RF signal at Rx 2) Separate channel  more expensive and less BW efficient

ECE 4710: Lecture #13 3 PSD Bit Synchronization  Most often derived from distorted Rx signal  More expensive for synch on separate channel  Type and complexity of bit synchronizer depends on line code properties  Unipolar RZ code:  Bit synchronizer is easy since PSD has periodic (sinusoidal) component at f = R !!  Pass signal through narrowband bandpass filter tuned to f 0 = R = 1/ T b  Must have good # of alternating 1’s and 0’s

ECE 4710: Lecture #13 4  Polar NRZ line code  Bit synchronizer requires square-law detector prior to bandpass filter  Square law detector or full-wave rectifier (diode circuit) used to convert Polar NRZ  ~Unipolar RZ »Must filter Polar NRZ prior to rectification Bit Synchronizer Circuit

ECE 4710: Lecture #13 5 Bit Synchronizer Circuit Square law circuit rectifies polar NRZ to produce quasi unipolar RZ  note periodic type waveform for alternating 1/0 sequences

ECE 4710: Lecture #13 6 Bit Synchronizer Circuit Filtered signal is periodic and comparator generates high/low clock signal centered on T b

ECE 4710: Lecture #13 7 Bit Synchronization  Unipolar, polar, and bipolar bit synchronizers will work only when there are sufficient # of alternating 1’s and 0’s  Loss of synchronization prevented by  Scrambling of data  bit interleaving to break up long strings and produce alternating 1’s and 0’s  Manchester line code »Zero crossing for each 1 or 0 bit »Clock signal easy to generate and independent of long strings »Disadvantage is 2  BW compared to unipolar & polar NRZ codes

ECE 4710: Lecture #13 8 Multi-Level Polar NRZ  Multi-level signals provide reduced bandwidth compared to binary signaling or increased R  Binary to multi-level conversion using -bit converter with L = 2 levels  e.g. 3-bit converter gives L = 2 3 = 8 levels  For binary data rate R (bps) then symbol rate is D = R /  PSD for multi-level signal is  K is some constant and FNBW = B null = R /  Filtered multi-level signals can provide narrowband digital signals (remember PCM BW??)

ECE 4710: Lecture #13 9 Multi-Level Polar NRZ

ECE 4710: Lecture #13 10 Multi-Level Polar NRZ

ECE 4710: Lecture #13 11 Spectral Efficiency  Spectral Efficiency : number of bits per second (bps) supported by each Hz of signal BW  **VERY** important measure for digital communication systems  especially wireless  Limited BW  must have high spectral efficiency to support large number of users  Cost for BW  more than $70B has been spent in U.S. by companies for wireless cellular spectrum

ECE 4710: Lecture #13 12 Spectral Efficiency  Communication engineer must choose signaling technique that  Has high spectral efficiency  Low system costs (Tx/Rx)  Meet S/N and BER requirements  Maximum possible spectral efficiency is limited by channel noise if BER is small  Shannon’s bound  Maximum theoretical bound  Never actually attained in practice

ECE 4710: Lecture #13 13 Spectral Efficiency  Spectral efficiencies approaching upper bound normally use 1) error correction coding, 2) multi- level signaling, and 3) pulse shaping filters  Spectral efficiencies for multi-level polar NRZ  cannot, in general, be increased to large number b/c S/N limitations will limit correct discrimination between multi-level amplitudes  BER will increase to unacceptable levels

ECE 4710: Lecture #13 14 Spectral Efficiency Typical spectral efficiencies achieved by 2G wireless digital communication systems is bps/Hz

ECE 4710: Lecture #13 15 Channel Capacity  Capacity, C, is  S/N   Higher signal power means larger channel capacity???  Larger S/N makes it easier to correctly differentiate (detect) multiple states per digital symbol in presence of noise  higher data rate for same symbol period & bandwidth T s T s2 T s1 = T s2 but R 1 = 2R 2 vs.

ECE 4710: Lecture #13 16 Channel Capacity  Shannon’s capacity formula  Use multi-level signal to decrease BW  required S/N increases to maintain same capacity for same BER  User error coding to lower S/N requirement for same BER  required bandwidth increases to handle additional coding bits while maintaining same capacity (data rate)  BW for S/N tradeoff is ** fundamental ** for all communication systems

ECE 4710: Lecture #13 17 Digital System Performance  Critical Performance Measures:  Bit Error Rate (BER)  Channel BW = Transmitted Signal BW  Received S/N  Signal Power  Channel Data Rate ( R c )  Desire high data rate with small signal BW, low signal power, and low BER  Trade BW for S/N improvement  Error Coding  add coding bits to data stream but keep same data rate »For same R c  T s must  and  BW  »But coding will correct errors allowing weaker signal power for same BER