Wireless Networks Instructor: Fatima Naseem Lecture # 03 Computer Engineering Department, University...
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Transcript of Wireless Networks Instructor: Fatima Naseem Lecture # 03 Computer Engineering Department, University...
Wireless Networks
Instructor: Fatima NaseemLecture # 03
Computer Engineering Department, University of Engineering and Technology, Taxila
Signal Encoding Techniques
Chapter 6
Digital Signaling
Data source g(t), which may be digital or analog, is encoded as a digital signal, x(t)
Form of x(t) depends on encoding technique that is chosen to optimize the use of transmission medium
Eg. It might be chosen to conserve the bandwidth or minimize errors
Analog Signaling
Carrier signal: a continuous constant frequency signal The frequency of the carrier signal is chosen to be
compatible with transmission medium Data transmitted using carrier signal by modulation
Modulation: process of encoding source data onto a carrier signal
with frequency fc Involve operation on one of the following three
parameters; frequency, amplitude, phase
Analog Signaling
Modulating signal: Input signal m(t) May be analog or digital
Modulated signal: Resulting signal s(t) s(t) bandlimited signal BW on spectrum related to fc, often centered at fc encoding technique is chosen to optimize some
characteristic of transmission
Reasons for Choosing Encoding Techniques
Digital data, digital signal Equipment less complex and less expensive than
digital-to-analog modulation equipment Analog data, digital signal
Permits use of modern digital transmission and switching equipment
Common to digitize voice signals prior to transmission to improve quality
For wireless transmission the resulting digital data must be modulated onto an analog carrier
Reasons for Choosing Encoding Techniques
Digital data, analog signal Some transmission media will only propagate analog
signals E.g., optical fiber and unguided media
Analog data, analog signal Analog data in electrical form can be transmitted easily
and cheaply Done with voice transmission over voice-grade lines Must be modulated on a higher frequency carrier
Signal Encoding Criteria Some important terms:
Digital signal sequence of discrete, discontinuous voltage pulses
Each pulse is a signal element Binary data are transmitted by encoding each
data bit into signal element A digital bit stream can be encoded onto an
analog signal as a sequence of signal elements, with each signal element being a pulse of constant freq, amp, phase
There might be one-to-one, one to many or many to one correspondence
Signal Encoding Criteria
Data signaling rate/data rate: bits per second data is transmitted Duration/length of a bit: amount of time taken by
the transmitter to emit a bit; for data rate R, bit duration is 1/R
Modulation rate: rate at which signal level is changed Expressed in baud/ signal elements per second Depends on nature of encoding technique
Signal Encoding Criteria
Tasks involved in interpreting digital signals at receiver:
Receiver must know timing of each bit i.e. when it starts and when it ends
Receiver must know whether the signal level for each bit position is high or low
For these tasks the sampling of each bit position is done in the middle of interval.
Signal Encoding Criteria
What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth
An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate
Factors Used to CompareEncoding Schemes
Signal spectrum With lack of high-frequency components, less bandwidth
required Transfer function of a channel is worse near band edges
Clocking Ease of determining beginning and end of each bit position
Factors Used to CompareEncoding Schemes
Signal interference and noise immunity Performance in the presence of noise
Cost and complexity The higher the signal rate to achieve a given data rate, the greater
the cost
Basic Encoding Techniques
Modulation involves operation on one of the three characteristics of carrier signal: Amp, Freq, Phase
Digital data to analog signal Amplitude-shift keying (ASK)
Amplitude difference of carrier frequency Frequency-shift keying (FSK)
Frequency difference near carrier frequency Phase-shift keying (PSK)
Phase of carrier signal shifted
Basic Encoding Techniques
Amplitude-Shift Keying
Two binary values represented by two amplitudes of the carrier frequency
One binary digit represented by presence of carrier, at constant amplitude
Other binary digit represented by absence of carrier
where the carrier signal is Acos(2πfct)
ts tfA c2cos
01binary 0binary
Amplitude-Shift Keying
Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber
Binary Frequency-Shift Keying (BFSK)
Two binary digits represented by two different frequencies near the carrier frequency
where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts
ts tfA 12cos tfA 22cos
1binary 0binary
Binary Frequency-Shift Keying (BFSK)
Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio
transmission Can be used at higher frequencies on LANs that
use coaxial cable
Multiple Frequency-Shift Keying (MFSK)
More than two frequencies are used More bandwidth efficient but more susceptible to error One signaling element represents more than one bit The transmitted signal for one signal element time is:
f i = f c + (2i – 1 – M)f d
f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L
L = number of bits per signal element
tfAts ii 2cos Mi 1
Multiple Frequency-Shift Keying (MFSK)
To match data rate of input bit stream, each output signal element is held for:
Ts=LT seconds where T is the bit period (data rate = 1/T)
So, one signal element encodes L bits
Multiple Frequency-Shift Keying (MFSK)
Total bandwidth required 2Mfd
Minimum frequency separation required 2fd=1/Ts
Therefore, modulator requires a bandwidth ofWd=2L/LT=M/Ts
Multiple Frequency-Shift Keying (MFSK)
Phase-Shift Keying (PSK)
Two-level PSK (BPSK) Uses two phases to represent binary digits
ts tfA c2cos tfA c2cos
1binary 0binary
tfA c2cos
tfA c2cos1binary 0binary
Anther formulation is if d(t) is a discrete function that takes on values +1 and -1,same results can be achieved
A d(t)d
ts
Phase-Shift Keying (PSK)
Differential PSK (DPSK) Phase shift with reference to previous bit
Binary 0 – signal burst of same phase as previous signal burst
Binary 1 – signal burst of opposite phase to previous signal burst
Phase-Shift Keying (PSK)
Four-level PSK (QPSK) For efficient use of bandwidth each element represents
more than one bit
ts
42cos
tfA c 11
4
32cos
tfA c
4
32cos
tfA c
42cos
tfA c
01
00
10
Phase-Shift Keying (PSK) Multilevel PSK
Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved
D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2L
L = number of bits per signal element
M
R
L
RD
2log
Performance
Bandwidth of modulated signal (BT) ASK, PSK BT=(1+r)R
FSK BT=2DF+(1+r)R
R = bit rate 0 < r < 1; related to how signal is filtered DF = f2-fc=fc-f1
Performance
Bandwidth of modulated signal (BT)
MPSK
MFSK
L = number of bits encoded per signal element M = number of different signal elements
RM
rR
L
rBT
2log
11
R
M
MrBT
2log
1
Quadrature Amplitude Modulation
QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same
carrier frequency
tftdtftdts cc 2sin2cos 21
Quadrature Amplitude Modulation
Any Questions?