ECE 4710: Lecture #13 1 Bit Synchronization Synchronization signals are clock-like signals necessary in Rx (or repeater) for detection (or regeneration)

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


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


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 f0 = R = 1/ Tb Must have good # of alternating 1’s and 0’s

1 1 0 1 0 0 1


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



Square law circuit rectifies polar NRZ to produce quasi unipolar RZ note periodic type waveform for alternating 1/0 sequences

1 1 0 1 0 0 1 0 0 1

1 1 0 1 0 0 1 0 0 1



Filtered signal is periodic and comparator generates high/low clock signal centered on Tb


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

1 1 0 1 0 0 1


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 = 23 = 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 = Bnull = R /

Filtered multi-level signals can provide narrowband digital signals (remember PCM BW??)

2

NRZ

sin)(

b

bmulti Tf

TfKfP



0 1 0 1 0 0 0 0 0

0 0 1 1 1 0 1 0 0 1 1 1



010

100

000

001

110

100

111


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

)Hz / bps(BR


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

)Hz / bps(1log2max

NS

BC


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

)bps/Hz(


Spectral Efficiency

Typical spectral efficiencies achieved by 2G wireless digital communication systems is 1.5-2 bps/Hz


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

)bps(1log2

N

SBBC

00 01 00 10 00 11 00 01

Ts1

0 1 0 1 0 1 0 1

Ts2

Ts1 = Ts2 but R1 = 2R2

vs.


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

)bps/Hz(1log2

NS

BC


Digital System Performance

Critical Performance Measures: Bit Error Rate (BER) Channel BW = Transmitted Signal BW Received S/N Signal Power Channel Data Rate (Rc)

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 Rc Ts must and BW » But coding will correct errors allowing weaker signal power for same BER

Documents

ECE 4710: Lecture #13 1 Bit Synchronization Synchronization signals are clock-like signals necessary in Rx (or repeater) for detection (or regeneration)