Digital CW Techniques

8/12/2019 Digital CW Techniques

1/39


2/39

Desirable features Low bit error rate at low received S/N Performs well in multipath conditions

Occupies a minimum of BW

Easy and cost-effective to implement

None of existing techniques satisfy all of these

requirements

Depending upon application some of the abovefeatures are prioritized over other and accordingly

modulation technique is selected

Digital CW Modulation Techniques


3/39

Bandwidth efficiency (R/B; max=log2(1+S/N)) Ability to accommodate data within a limited bandwidth. (may be

greater than 1)

In general increasing data rate implies decreasing pulse width i.e.increase in BW of signal.

Power efficiency (Eb/N0) Ability to keep the minimum BER at low power levels (may be greater

than 1)

With decrease in power, the BER increases but rate of increasedepends upon modulation tech. (The distance between valid symbols in

constellation diagram governs BER)

Phase characteristics If phase change is smooth then side lobe level is low thus ICI is

reduced.

Nonlinear amplifiers can be used which reduces implementationcomplexity, the cost and the power consumption (enhances battery life).


4/39

Phase Shift Keying (PSK)

Binary Phase Shift Keying (BPSK) demonstrates better

performance than ASK and FSK.

PSK can be expanded to a M-ary scheme, employing multiple

phases and amplitudes as different states.

Filtering can be employed to avoid spectral spreading.

Baseband

Data

Binary PSK modulatedsignal

s1 s1s0 s0

where s0 = -A cos wct and s1 = A cos wct


5/39

The Constellation Diagram

1 2

1

2 2cos 2 , cos 2 ; ; 0

energy per bit; bit period

For this signal set, there is a single basic signal

2cos 2 ; 0

b bBPSK c c b

b b

b b

c b

b

BPSK

E ES s t f t s t f t t T

T T

E T

t f t t T T

S E

1 1

,b b

t E t

-Eb Eb

Q

I

Constellation diagram

Distance = 2Eb= 2Es


6/39


7/39

Types of QPSK

Conventional QPSK has transitions through zero (ie. 180ophase transition). Highly linear amplifier required.

In Offset QPSK, the transitions on the I and Q channels are

staggered. Phase transitions are therefore limited to 90o

.In/4-QPSK the set of constellation points are toggled eachsymbol, so transitions through zero cannot occur. Thisscheme produces the lowest envelope variations.

All QPSK schemes require linear power amplifiers.


8/39

Amplitude Shift Keying (ASK)

Pulse shaping can be employed to remove spectral spreading.

ASK demonstrates poor performance, as it is heavily affectedby noise and interference.

Long string of zeros causes synchronization loss.

Baseband

Data

ASK modulated

signal

A cos wct A cos wct0 0


9/39

M-ary Phase and Amplitude Modulation

Amplitude and phase shift keying can be combined to transmit several bits persymbol (in this case M=4). These modulation schemes are often referred to aslinear, as they require linear amplification.

16QAM has the largest distance between points, but requires very linearamplification. 16PSK has less stringent linearity requirements, but has lessspacing between constellation points, and is therefore more affected by noise.

M-ary schemes are more bandwidth efficient, but more susceptible to noise.

16 PSK 16 APSK16 QAM


10/39

Frequency Shift Keying (FSK)

Bandwidth occupancy of FSK is dependant on the spacing of the two

symbols.

If either f0or f1are chosen such that there is no-integer number of

periods in symbol period Tb then discontinuities in the phase will result.

FSK can be expanded to a M-ary scheme, employing multiplefrequencies as different states (which should be orthogonal).

BasebandData

FSK modulated

signal

f1 f1f0 f0

where f0 = A cos(wc-Dw)t and f1 = A cos(wc+Dw)t


11/39


12/39

GMSK Signals

1-2

0T 2TT T

Time

GMSK Pulse Shapes and ISI

MSKGMSK,

BT=0.5

GMSK

BT=0.3

The Gaussian pre-modulation filtersmoothes the phase trajectory of the MSK

signal thus limiting the instantaneous

frequency variations. The result is an FM

modulated signal with a much narrower

bandwidth.

This bandwidth reduction does not comefor free since the pre-modulation filter

smears the individual pulses in pulse

train. As a consequence of this smearing

in time, adjacent pulses interfere with

each other generating what is commonlycalled inter-symbol interference or ISI.

In the applications where GMSK is used,the trade-off between power efficiency

and bandwidth efficiency is well worth thecost.


13/39


14/39

Coherent Detection An estimate of the channel phase and attenuation

is recovered. It is then possible to reproduce thetransmitted signal and demodulate.

Requires a replica carrier wave of the samefrequency and phase at the receiver.

The received signal and replica carrier are cross-

correlated using information contained in theiramplitudes and phases.

Also known as synchronous detection


15/39

Optimum binary detection (a) parallel matched filters (b) correlation detector

Figure 14.2-3


16/39

Non-Coherent Detection

Requires no reference wave; does notexploit phase reference information(envelope detection)

Differential Phase Shift Keying (DPSK) Frequency Shift Keying (FSK)Amplitude Shift Keying (ASK) Non coherent detection is less complex than

coherent detection (easier to implement), buthas worse performance.


17/39

Noncoherent detection of bianry FSK

Figure 14.3-5


18/39


19/39

Optimum Signal selectionThe simplex signal set, which form a M-

dimensional pyramid having centre of gravity atorigin, is the optimum signal set as it maximizes

the distance thus minimizes error

Orthogonal signal set is easy to generate and

decode and approximates the simplex set forlarge value of M

The transmission BW for orthogonal

signals>=M/2D

The threshold effect can be observed for M>>1

i.e. above it a slight increase in S/N causes huge

reduction in Pe.

The errorless transmission at any data rate is

possible for M=infinity for very small S/N0.


20/39

Hard Vs. Soft decision decoding

In error coded transmission

Bitwise evaluation of all received bits of a vector thenhamming distance with valid vectors. (hard decision taken)

Euclidean distance of received vector as a whole (basedon the soft decision)

The representation ofdemodulator o/p (m) >1 bit

corresponds to soft

decision decoding.

2dB gain with 3 bit and 2.2

dB with infinite bitrepresentation.


21/39

Trellis coded modulation


22/39

8-ary PSK encoder for TCM (a) 4-state,

(b) 8 state,

Figure 14.5-3

2, 1m m

2, 2m m


23/39

Partitioning of an 8-PSK signal set,Ungerbroeck, 1982Figure 14.5-4


24/39

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Trellis diagram for the encoder of Fig. 14.5-3a,

Ungerbroeck, 1982

Figure 14.5-4


25/39

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Two possible error events for encoder of Fig. 14.5-3a.

Figure 14.5-6transmitted sequence

most likely error event

error event


26/39

Spread Spectrum Technology

Low power high bandwidth transmission.

Better performance in noisy condition.

Anti jamming feature

Security, etc.


27/39

Types of SS techniques

Direct Sequence Spread Spectrum (DS-SS)e.g. CDMA

Frequency Hopping Spread Spectrum (FH-SS)

Time hopping Spread Spectrum (TH-SS)

Hybrid (DS/FH, DS/TH, FH/TH,DS/FH/TH)


28/39

Direct Sequence Spread Spectrum (DS-SS)

This figure shows BPSK-DS

transmitter and receiver (multiplicationcan be realized by RF-mixers)

DS-CDMA is used in WCDMA, cdma2000 and IS-95 systems

2

22

av av

AP A P

spreading


29/39

Effect of narrowband noise signal

Effect of wideband Interference


30/39

Basic principle of CDMA

Assumptions

Polar line coding is used. All the users are synchronized All the users produce same power level at the base

station. Base station has a copy of all chipcodes of its

network

All the codes should be either PN-sequence or

orthogonal codes (preferable) i.e.Cx*Cy = +1 if x= y

= -1 if x= complement (y)

= 0 otherwise.


31/39

Algorithm

A unique codeword of same length

{Cx=(c1,c2,c3)} is assigned to each user. A user transmits its codeword for transmittinglogic 1 and its compliment for 0.

The base station receives the algebraic sumof chips {D=(d1,d2,)} transmitted by all theactive users.

The BS calculates Sx = Cx(i)*D(i) for allusers.

If it is above +ve (-ve) threshold user x hastransmitted logic 1 (0)


32/39

Problem:

There are three users A,B,C in a CDMA networkwith corresponding chip sequences Ca = {1,-1,-

1,1,-1,1}, Cb = {1,1,-1,-1,1,1}, Cc = {1,1,-1,1,1,-1}.

If both the thresholds are set at 0 volts, find the

data decoded by BS when(i) A transmits logic 1.

(ii) A and B transmit logic 1

(iii) A and C transmit logic 1.


33/39

PN - sequences

Uniform distribution and independence.

Period = 2n1 bits; n= length of shift register

To identify a PN both algorithm (interconnection oftaps) and seed (starting point) must be known.

Seed can change the starting and ending point ofa PN sequence but the contents remains thesame. (this property is used for cell identificationin CDMA mobile systems.)

Walsh codes are known to be orthogonal codes.


34/39

Frequency Hopping Transmitter and Receiver

May be slow (two or more

symbols are tx at same fc) or

fast (two or more frequencies

are used to transmit single

symbol)

d

BW W

d

BW W

s

BW W

sBW W

2 frequenciesk

2 level modulationL

Hopping frequencies aredetermined by the code.

This method is applied inBlueTooth

Frequency Hopping Spread Spectrum (FH SS) (ex: tx


35/39

Frequency Hopping Spread Spectrum (FH-SS) (ex: tx

of two symbols/chip)

2 levelsL

2 slotsk

:chip duration

: bit duration

: symbol duration

c

b

s

T

T

T

2 ( data modulator BW)

2 ( total FH spectral width)

L

d d

k

s d

W f

W W

b

T

2L

4-level FSK modulation

Hopped frequency

slot determined by

hopping code


36/39

B th th d fi t d i ilit


37/39

Both methods were first used in military comm, FH can be advantageous because the

hopping spancan be very large (makeseavesdroppingdifficult)

DS can be advantageous because spectraldensitycan be much smaller than

background noise density (transmission isunnoticed)

By using hybrid systemssome benefits can becombined: The system can have a lowprobability of interception and negligible near-far effect at the same time. (Differentially

coherent modulationis applicable)


38/39

FDMA, TDMA and CDMA compared

TDMA and FDMA principle: TDMA allocates a time instantfor a user

FDMA allocates a frequency bandfor a user

CDMA allocates a codefor user

CDMA-system can be synchronousorasynchronous:

Synchronous CDMA difficult to apply in multipath

channels that destroy code orthogonality

Therefore, in wireless CDMA-systems as in IS-95,cdma2000, WCDMA and IEEE 802.11 users

are asynchronous

d


39/39

FDMA, TDMA and CDMA

yield conceptually the same

capacity

However, in wireless

communications CDMA has

improved capacity due to

statistical multiplexinggraceful degradation

Performance can still beimproved by adaptive antennas,

multiuser detection, FEC, and

l i di

Digital CW Techniques

Documents

Transcript of Digital CW Techniques