Wireless CommuniCation unit 3

unit iiiWireless transCeiVers

Structure of a wireless communication link

• Information source

– The information source provides an analog source signal

and feeds it into the source ADC, where the signal is

converted into a stream of digital data at a certain

sampling rate and resolution.

• Source coder

– This reduces the amount of source data to be transmitted.

– Datas are encrypted in order to prevent unauthorized

listening

– GSM speech coder reduces the source data rate from 64

kbit/s to 13 kbit/s

• Channel coder

– Channel coder adds redundancy in order to

protect data against transmission errors

– GSM channel coding increases the data rate from

13 to 22.8 kbit/s

• Baseband modulator

– The baseband modulator converts the raw data

bits to complex transmit symbols in the baseband.

• Multiple access

– In this a suitable multiple access scheme is chosen

– In GSM, multiple access increases the data rate

from 22.8 to 182.4 kbit/s (8 · 22.8) for the

standard case of eight users.

• Propagation channel

– This will be mostly our air-interface.

• Diversity combiner

– For negating the multipath propagation we use

diversity techniques for reception.

– It receives the time delayed signals and combine them

to form an effective output

• Separation

– In this we separate the multiple accessed signals.

– For GSM each of the 8 user’s data will be separated.

• Equalizer

– Equalization compensates for intersymbol

interference (ISI)

• Channel decoder

– This is used to detect or correct the errors

introduced by the channel.

– All the redundant bits added during channel

encoding will be removed.

• Source Decoder

– The source decoder reconstructs the source signal

from the rules of source coding

– The data are transferred to the DAC, which

converts the transmitted information into an

analog signal and hands it over to the information

sink.

Binary FrequenCy shiFt Keying

Binary Frequency Shift Keying

• BFSK uses a pair of discrete frequencies to

transmit binary data.

• The frequency of a constant amplitude carrier

signal is switched between two values

corresponding to a binary 1 or 0

Method 1

VCONRZ Encoder

Binary data

FSK signal

Method 2

0 1 0 1

Spectrum and Bandwidth

• FSK signal consists of discrete frequencycomponents at fc, fc + n π f, fc-n π f

• BFSK = 2Δf+ 2B

– B is the bandwidth of the digital baseband signal.

Coherent Receiver

Regenerated Carrier

• The carrier used in the transmitter was regenerated

by the Carrier Recovery circuit.

• Then they are multiplied with the incoming FSK

signal.

• Then the high frequency components are filtered out

and only the low frequency message is passed

though.

• Threshold comparator will decide whether the

received signal is logic 1 or a 0

Non-Coherent Receiver

• Extracting the same Carrier from the FSK signal will

not be accurate , and it take lot of effort to do that.

• To avoid this we use Non Coherent detection, where

we don’t need any information about the original

carrier.

• The receiver consists of a pair of matched filters,

upper filter is matched to the FSK signal of frequency

fH and the lower filter is matched to the frequency fL

• Depending on the magnitude of the envelope

detector output, the comparator decides

whether the data bit was a 1 or 0.

qudrature phase shiFt Keying

qpsK

• 2 bits are combined in a single symbol.

• It is represented by carriers with 4 differentphases.

• QPSK has twice the bandwidth efficiency ofBPSK.

Es – Amplitude of digital symbol

Ts - duration of symbol

Tb- duration of a single bit

• For QPSK Ts = 2 Tb

Phasor diagram

00 -135(225)

01 +135

10 -45 (315)

11 +45

QPSK Transmitter Block

NRZ Encoder

Data Signal

I- Channel

Q- Channel

• The input to the system is a binary message

stream has bit rate Rb

• The NRZ Encoder will convert the unipolar

message to bipolar bit sequence.

• The serial to parallel convertor will split the

stream of bits into two separate data streams.

• They are provided to the I-channel and Q-

Channel.

• The bit stream ml (t) is called the "even" stream

and mQ (t) is called the "odd" stream

• The oscillator produces a high frequency carrier.

• The 90o phase shifter will produce an exactly out

of phase signal to that of the carrier.

• The two binary sequences are separately

modulated by two carriers

• The two modulated signals are summed to

produce a QPSK signal.

• The filter at the output of the modulator confines

the power spectrum of the QPSK signal within the

allocated band.

• This prevents spill-over of signal energy into

adjacent channels and also removes out-of-band

spurious signals generated during the modulation

process

QPSK- Waveforms

Receiver

• A carrier recovery is used to estimate the

frequency and phase of a received

signal's carrier.

• The incoming signal is split into two parts, and

each part is coherently demodulated using the in-

phase and quadrature carriers.

• The decision making device is used to regenerate

digital signals. It uses a threshold level to

distinguish between “0” & “1”

oFFset qpsK

Offset QPSK

• In QPSK phase shift of π radians will cause the

amplitude to fluctuations.

• This will lead to generation of side lobes and

spectral widening.

• To reduce this 180o phase shift we use O-QPSK

• In this we have only 90o shifts.

• This is achieved by delaying one channel by Tb sec

• In QPSK the even and odd bit streams occur at the

same time instants.

• But in OQPSK signaling, the even and odd bit streams,

mI (t) and mQ(t) are offset in their relative alignment

by one bit period (half-symbol period).

• Because of this at any given time only one of the two

bit streams can change values.

• So the maximum phase shift of the transmitted signal

at any given time is limited to ±90°

QPSK

O-QPSK

π/4 qpsK

π/4 QPSK

• In π/4 QPSK the maximum phase change is

limited to ±135°.

• So it has less amplitude fluctuations than QPSK.

• It can be detected non coherently .

• The phase shift between successive symbols is an

integer multiple of π /4 radians

π/4 QPSK Generator

where

p(t-kTs-Ts/2) represents pulse shape

MINIMUM SHIFT KEYING

Intro….

• QPSK results in larger side lobes due to the phase

change of 900 or 1800. .

• So to reduce this we use MSK, where the peak

deviation is ¼ of bit rate.

• MSK is closely related to OQPSK, where we

replace rectangular pulses by sinusoidal pulses.

• Modulation index of MSK is 0.5

MSK Transmitter

aI(t)

aQ(t)

• MSK uses two frequencies which are separated by 1/4T.

• The carrier frequency is choose to be the multiple of 1/4T

fc - 1/4T & fc + 1/4T.

• The filtered signal is then multiplied with the odd and

even data sequences to form MSK signal.

• SMSK(t) = aI(t)cos(π/2T)t. cos2πfct + aQ(t)sin( π/2T)t . sin2πfct

aI(t) = In-phase bit sequence (Even)

aQ(t) = Qudrature sequence (odd)

let bk(t) = aI(t) . aQ(t)

after simplification

SMSK(t) = cos[2 π fct + bk(t) π t/2T+ φk ]

aI(t) & aQ(t) = +1 for Binary “1” & Φk = 0

aI(t) & aQ(t) = -1 for Binary “0” & Φk = π

Also bk(t) = 0 if aI(t) & aQ(t) are opposite

bk(t) = 1 if aI(t) & aQ(t) are same

MSK Receiver

Phasor Diagram

GAUSSIAN MINIMUM SHIFT KEYING

• In GMSK the side lobe levels of the spectrum

are further reduced.

• GMSK will minimize bandwidth, improve

spectral performance, and easy for detection.

• GMSK has excellent power efficiency , so it is

used for GSM applications.

• But GMSK is affected by ISI.

GMSK Transmitter

FM TransmitterGaussian

Filter

Amplifier

• An unfiltered binary data stream will produce

an RF spectrum of considerable bandwidth.

• A Gaussian pulse-shaping filter smoothes the

phase trajectory of input NRZ code.

Gaussian Filter response

• The impulse response of the filter is

BT- bandwidth-bit duration product is

As the BT value decreases the GMSK spectrumbecomes more compact and the ISI increases .

BitRatefBT dB3−=

– For BT=0.3 the adjacent symbols will interfere with each other more than for BT=0.5

Pulse Shaping

Input NRZ Output from filter

• Input: Binary NRZ Signal

• Each binary pulse goes through a Gaussian LPF

• The filter smoothes the phase trajectory of the binary pulses

and stabilizes the instantaneous frequency variations.

• The power spectrum of MSK & GMSK are equivalent.

Bit Error Rate• The bit error probability is a function of BT.

• Where γ is a constant related to BT

0.68 for GMSK with BT = 0.25

0.85 for simple MSK BT= ∞

• Eb/N0 is the energy per bit to noise power

spectral density ratio.

• It is a normalized signal-to-noise

ratio (SNR) measure, also known as the

"SNR per bit“

• N0 = Noise Spectral Density

oooooo

MSK Vs GMSK

Receiver