m Ary Signalling

1999 BG Mobasseri 1

M-ary signaling

Binary communications sends one of only 2 levels; 0 or 1

There is another way: combine several bits into symbols

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

Combining two bits at a time gives rise to 4 symbols; a 4-ary signaling

1999 BG Mobasseri 2

A few definitions

We used to work with bit length Tb. Now we have a new parameter which we call symbol length,T

1 10

T

Tb

1999 BG Mobasseri 3

Bit length-symbol length relationship

When we combine n bits into one symbol; the following relationships hold

T=nTb- symbol length

n=logM bits/symbolT=TbxlogM- symbol length

All logarithms are base 2

1999 BG Mobasseri 4

Example

If 8 bits are combined into one symbol, the resulting symbol is 8 times wider

Using n=8, we have M=28=256 symbols to pick from

Symbol length T=nTb=8Tb

1999 BG Mobasseri 5

Defining baud

When we combine n bits into one symbol, numerical data rate goes down by a factor of n

We define baud as the number of symbols/sec Symbol rate is a fraction of bit rate

R=symbol rate=Rb/n=Rb/logM For 8-level signaling, baud rate is 1/3 of bit

rate

1999 BG Mobasseri 6

Why M-ary?

Remember Nyquist bandwidth? It takes a minimum of R/2 Hz to transmit R pulses/sec.

If we can reduce the pulse rate, required bandwidth goes down too

M-ary does just that. It takes Rb bits/sec and turns it into Rb/logM pulses sec.

1999 BG Mobasseri 7

Issues in transmitting 9600 bits/sec

Want to transmit 9600 bits/sec. Options:• Nyquist’s minimum bandwidth:9600/2=4800 Hz• Full roll off raised cosine:9600 Hz

None of them fit inside the 4 KHz wide phone lines

Go to a 16 - level signaling, M=16. Pulse rate is reduced to

R=Rb/logM=9600/4=2400 Hz

1999 BG Mobasseri 8

Using 16-level signaling

Go to a 16-level signaling, M=16. Pulse rate is then cut down to

R=Rb/logM=9600/4=2400 pulses/sec To accommodate 2400 pulses /sec, we have

several options. Using sinc we need only 1200 Hz. Full roll-off needs 2400Hz

Both fit within the 4 KHz phone line bandwidth

1999 BG Mobasseri 9

Bandwidth efficiency

Bandwidth efficiency is defined as the number of bits that can be transmitted within 1 Hz of bandwidth

=Rb/BT bits/sec/Hz

In binary communication using sincs, BT=Rb/2--> =2 bits/sec/Hz

1999 BG Mobasseri 10

M-ary bandwidth efficiency

In M-ary signaling , pulse rate is given by R=Rb/logM. Full roll-off raised cosine bandwidth is BT=R= Rb/logM.

Bandwidth efficiency is then given by =Rb/BT=logM bits/sec/Hz

For M=2, binary we have 1 bit/sec/Hz. For M=16, we have 4 bits/sec/Hz


M-ary bandwidth

Summarizing, M-ary and binary bandwidth are related by

BM-ary=Bbinary/logM Clearly , M-ary bandwidth is reduced by a

factor of logM compared to the binary bandwidth


8-ary bandwidth

Let the bit rate be 9600 bits/sec. Binary bandwidth is nominally equal to the bit rate, 9600 Hz

We then go to 8-level modulation (3 bits/symbol) M-ary bandwidth is given by

BM-ary=Bbinary/logM=9600/log8=3200 Hz


Bandwidth efficiency numbers

Here are some numbersn(bits/symbol) M(levels) (bits/sec/Hz)

1 2 12 4 23 8 34 16 48 256 8


Symbol energy vs. bit energy

Each symbol is made up of n bits. It is not therefore surprising for a symbol to have n times the energy of a bit

E(symbol)=nEb

Eb

E


QPSKquadrature phase shift keying

This is a 4 level modulation. Every two bits is combined and mapped to

one of 4 phases of an RF signal These phases are 45o,135o,225o,315o

si(t) 2E

Tcos 2fct (2i 1)

4

, i 1,2,3,4

0

, 0 t T

Symbol energy

Symbol width


QPSK constellation

45o

0001

11 10

√E

1 t 2T

cos2fct

2 t 2

Tsin 2fct

Basis functions S=[0.7 √E,- 0.7 √E]


QPSK decision regions

0001

11 10

Decision regions re color-coded


QPSK error rate

Symbol error rate for QPSK is given by

This brings up the distinction between symbol error and bit error. They are not the same!

Pe erfc(E

2No)


Symbol error

Symbol error occurs when received vector is assigned to the wrong partition in the constellation

When s1 is mistaken for s2, 00 is mistaken for 11

0011s1s2


Symbol error vs. bit error

When a symbol error occurs, we might suffer more than one bit error such as mistaking 00 for 11.

It is however unlikely to have more than one bit error when a symbol error occurs

10 10 11 1000

11 10 11 1000

10 symbols = 20 bits

Sym.error=1/10Bit error=1/20


Interpreting symbol error

Numerically, symbol error is larger than bit error but in fact they are describing the same situation; 1 error in 20 bits

In general, if Pe is symbol error

PelogM

BERPe


Symbol error and bit error for QPSK

We saw that symbol error for QPSK was

Assuming no more than 1 bit error for each symbol error, BER is half of symbol error

Remember symbol energy E=2Eb

Pe erfc(E

2No)

BER1

2erfc(

E

2No)


QPSK vs. BPSK

Let’s compare the two based on BER and bandwidth

BER BandwidthBPSK QPSK BPSK QPSK

1

2erfc

EbNo

1

2erfc

EbNo

Rb Rb/2

EQUAL


M-phase PSK (MPSK)

If you combine 3 bits into one symbol, we have to realize 23=8 states. We can accomplish this with a single RF pulse taking 8 different phases 45o apart

si(t) 2E

Tcos 2fct (i 1)

4

, i 1,...,8

0

,0 t T


8-PSK constellation

Distribute 8 phasors uniformly around a circle of radius √E

45o

Decision region


Symbol error for MPSK

We can have M phases around the circle separated by 2π/M radians.

It can be shown that symbol error probability is approximately given by

Pe erfcE

Nosin

M

,M 4


Quadrature Amplitude Modulation (QAM)

MPSK was a phase modulation scheme. All amplitudes are the same

QAM is described by a constellation consisting of combination of phase and amplitudes

The rule governing bits-to-symbols are the same, i.e. n bits are mapped to M=2n symbols


16-QAM constellation using Gray coding

16-QAM has the following constellation Note gray codingwhere adjacent symbolsdiffer by only 1 bit

0010001100010000

1010

1110

0110

1011

1111

0111

1001

1101

0101

1000

1100

0100


Vector representation of 16-QAM

There are 16 vectors, each defined by a pair of coordinates. The following 4x4 matrix describes the 16-QAM constellation

[ai ,bi ]

3,3 1,3 1,3 3,3 3,1 1,1 1,1 3,1 3, 1 1, 1 1, 1 3, 1 3, 3 1, 3 1, 3 3, 3


What is energy per symbol in QAM?

We had no trouble defining energy per symbol E for MPSK. For QAM, there is no single symbol energy. There are many

We therefore need to define average symbol energy Eavg

Eavg 1

Mai

2 bi2

i1

M


Eavg for 16-QAM

Using the [ai,bi] matrix and using E=ai^2+bi^2 we get one energy per signal

E

18 10 10 18

10 2 2 10

10 2 2 10

18 10 10 18

Eavg=10


Symbol error for M-ary QAM

With the definition of energy in mind, symbol error is approximated by

Pe 2 1 1

M

erfc

2Eavg2 M 1 No


Familiar constellations

Here are a few golden oldies

V.22 600 baud1200 bps

V.22 bis600 baud2400 bps

V.32 bis2400 baud9600 bps


M-ary FSK

Using M tones, instead of M phases/amplitudes is a fundamentally different way of M-ary modulation

The idea is to use M RF pulses. The frequencies chosen must be orthogonal

si t 2E

Tcos 2fit ,0 t T

i 1,...,M


MFSK constellation:3-dimensions

MFSK is different from MPSK in that each signal sits on an orthogonal axis(basis)

s1

s2

s3

1

2

3i t 2

Tcos 2fit ,

0 t Ti 1,...,M

s1=[√E ,0, 0]s2=[0,√E, 0]s3=[0,0,√E]

√E

√E

√E


Orthogonal signals:How many dimensions, how many

signals?

We just saw that in a 3 dimensional space, we can have no more than 3 orthogonal signals

Equivalently, 3 orthogonal signals don’t need more than 3 dimensions because each can sit on one dimension

Therefore, number of dimensions is always less than or equal to number of signals


How to pick the tones?

Orthogonal FSK requires tones that are orthogonal.

Two carrier frequencies separated by integer multiples of period are orthogonal


Example

Take two tones one at f1 the other at f2. T must cover one or more periods for the integral to be zero

2 cos 2f1t cos 2f2t dt cos2 f1 f2 dt0

T

averages to zero

0

T

cos2 f1 f2 dt0

T

averages to zero if T=i/(f1-f2); i=integer

Take f1=1000 and T=1/1000. Thenif f2=2000 , the two are orthogonal so will f2=3000,4000 etc


MFSK symbol error

Here is the error expression with the usual notations

Pe 1

2M 1 erfc

E

2No


Spectrum of M-ary signals

So far Eb/No, i.e. power, has been our main concern. The flip side of the coin is bandwidth.

Frequently the two move in opposite directions Let’s first look at binary modulation bandwidth


BPSK bandwidth

Remember BPSK was obtained from a polar signal by carrier modulation

We know the bandwidth of polar NRZ using square pulses was BT=Rb.

It doesn’t take much to realize that carrier modulation doubles this bandwidth


Illustrating BPSK bandwidth

The expression for baseband BPSK (polar) bandwidth is

SB(f)=2Ebsinc2(Tbf)

BT=2Rbf1/Tb

BPSK

fc+/Tbfc-/Tbfc

2/Tb=2Rb


BFSK as a sum of two RF streams

BFSK can be thought of superposition of two unipolar signals, one at f1 and the other at f2

0 1000 2000 3000 4000 5000 6000 7000 8000-1

-0.5

0

0.5

1

0 1000 2000 3000 4000 5000 6000 7000 8000-1

-0.5

0

0.5

1

0 1000 2000 3000 4000 5000 6000 7000 8000-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1BFSK for 1 0 0 1 0 1 1

+


Modeling of BFSK bandwidth

Each stream is just a carrier modulated unipolar signal. Each has a sinc spectrum

f1 f2

1/Tb=Rb

fc

fc=(f1+f2)/2

f

BT=2 f+2Rb

f= (f2-f1)/2


Example: 1200 bps bandwidth

The old 1200 bps standard used BFSK modulation using 1200 Hz for mark and 2200 Hz for space. What is the bandwidth?

UseBT=2f+2Rb

f=(f2-f1)/2=(2200-1200)/2=500 Hz

BT=2x500+2x1200=3400 Hz

This is more than BPSK of 2Rb=2400 Hz


Sunde’s FSK

We might have to pick tones f1 and f2 that are not orthogonal. In such a case there will be a finite correlation between the tones

2

Tbcos(2f1t)

0

Tb

cos(2f2t)dt

1 2 3 2(f2-f1)Tb

Good points,zero correlation


Picking the 2nd zero crossing:Sunde’s FSK

If we pick the second zc term (the first term puts the tones too close) we get

2(f2-f1)Tb=2--> f=1/2Tb=Rb/2

remember f is (f2-f1)/2 Sunde’s FSK bandwidth is then given by

BT=2f+2Rb=Rb+2Rb=3Rb

The practical bandwidth is a lot smaller


Sunde’s FSK bandwidth

Due to sidelobe cancellation, practical bandwidth is just BT=2f=Rb

f1 f2

1/Tb=Rb

fc

fc=(f1+f2)/2

f

BT=2 f+2Rb

f= (f2-f1)/2

f


BFSK example

A BFSK system operates at the 3rd zero crossing of -Tb plane. If the bit rate is 1 Mbps, what is the frequency separation of the tones?

The 3rd zc is for 2(f2-f1)Tb=3. Recalling that f=(f2-f1)/2 then f =0.75/Tb

Then f =0.75/Tb=0.75x106=750 KHz

And BT=2(f +Rb)=2(0.75+1)106=3.5 MHz


Point to remember

FSK is not a particularly bandwidth-friendly modulation. In this example, to transmit 1 Mbps, we needed 3.5 MHz.

Of course, it is working at the 3rd zero crossing that is responsible

Original Sunde’s FSK requires BT=Rb=1 MHz

Bandwidth of MPSK modulation


MPSK bandwidth review

In MPSK we used pulses that are log2M times wider tan binary hence bandwidth goes down by the same factor.

T=symbol width=Tblog2M For example, in a 16-phase modulation, M=16,

T=4Tb.

Bqpsk=Bbpsk/log2M= Bbpsk/4


MPSK bandwidth

MPSK spectrum is given by

SB(f)=(2Eblog2M)sinc2(Tbflog2M)

f/Rb

Notice normalized frequency

1/logM

Set to 1 for zero crossing BWTbflog2M=1-->f=1/ Tbflog2M=Rb/log2M

BT= Rb/log2M


Bandwidth after carrier modulation

What we just saw is MPSK bandwidth in baseband

A true MPSK is carrier modulated. This will only double the bandwidth. Therefore,

Bmpsk=2Rb/log2M


QPSK bandwidth

QPSK is a special case of MPSK with M=4 phases. It’s baseband spectrum is given by

SB(f)=2Esinc2(2Tbf)

f/Rb0.5

B=0.5Rb-->half of BPSK

1

After modulation:Bqpsk=Rb


Some numbers

Take a 9600 bits/sec data stream Using BPSK: B=2Rb=19,200 Hz (too much for

4KHz analog phone lines) QPSK: B=19200/log24=9600Hz, still high

Use 8PSK:B= 19200/log28=6400Hz

Use 16PSK:B=19200/ log216=4800 Hz. This may barely fit


MPSK vs.BPSK

Let’s say we fix BER at some level. How do bandwidth and power levels compare?

M Bm-ary/Bbinary (Avg.power)M/(Avg.power)bin4 0.5 0.34 dB8 1/3 3.91 dB16 1/4 8.52 dB32 1/5 13.52 dB Lesson: By going to multiphase modulation, we save

bandwidth but have to pay in increased power, But why?


Power-bandwidth tradeoff

The goal is to keep BER fixed as we increase M. Consider an 8PSK set.

What happens if you go to 16PSK? Signals get closer hence higher BER

Solution: go to a larger circle-->higher energy


Additional comparisons

Take a 28.8 Kb/sec data rate and let’s compare the required bandwidths• BPSK: BT=2(Rb)=57.6 KHz

• BFSK: BT = Rb =28.8 KHz ...Sunde’s FSK

• QPSK: BT=half of BPSK=28.8 KHz

• 16-PSK: BT=quarter of BPSK=14.4 KHz

• 64-PSK: BT=1/6 of BPSK=9.6 KHz


Power-limited systems

Modulations that are power-limited achieve their goals with minimum expenditure of power at the expense of bandwidth. Examples are MFSK and other orthogonal signaling


Bandwidth-limited systems

Modulations that achieve error rates at a minimum expenditure of bandwidth but possibly at the expense of too high a power are bandwidth-limited

Examples are variations of MPSK and many QAM

Check BER rate curves for BFSK and BPSK/QAM cases


Bandwidth efficiency index

A while back we defined the following ratio as a bandwidth efficiency measure in bits/sec/HZ

=Rb/BT bits/sec/Hz Every digital modulation has its own


for MPSK

At a bit rate of Rb, BPSK bandwidth is 2Rb When we go to MPSK, bandwidth goes down

by a factor of log2M

BT=2Rb/ log2M Then

=Rb/BT= log2M/2 bits/sec/Hz


Some numbers

Let’s evaluate vs. M for MPSKM 2 4 8 16 32 64 .5 1 1.5 2 2.5 3 Notice that bits/sec/Hz goes up by a factor of 6

from M=2 and M=64 The price we pay is that if power level is fixed

(constellation radius fixed) BER will go up. We need more power to keep BER the same


Defining MFSK:

In MFSK we transmit one of M frequencies for every symbol duration T

These frequencies must be orthogonal. One way to do that is to space them 1/2T apart. They could also be spaced 1/T apart. Following The textbook we choose the former (this corresponds to using the first zero crossing of correlation curve)


MFSK bandwidth

Symbol duration in MFSK is M times longer than binary

T=Tblog2M symbol length Each pair of tones are separated by 1/2T. If

there are M of them, BT=M/2T=M/2Tblog2M

-->BT=MRb/2log2M


Contrast with MPSK

Variation of bandwidth with M differs drastically compared to MPSK

MPSK MFSKBT=2Rb/log2M BT=MRb/2log2M

As M goes up, MFSK eats up more bandwidth but MPSK save bandwidth


MFSK bandwidth efficiency

Let’s compute ’s for MFSK=Rb/M=2log2M/M bits/sec/Hz…MFSK

M 2 4 8 16 32 64 1 1 .75 .5 .3 .18 Notice bandwidth efficiency drop. We are

sending fewer and fewer bits per 1 Hz of bandwidth

COMPARISON OF DIGITAL MODULATIONS*

*B. Sklar, “ Defining, Designing and Evaluating Digital Communication Systems,”IEEE Communication Magazine, vol. 31, no.11, November 1993, pp. 92-101


Notations

M 2m # of symbols

m = log2 M bits/symbol

R =mTs

log2 MTs

bits/ sec

Ts symbol duration

Rs symbol rate

Tb 1R

Tsm

1mRs

bit length

Bandwidth efficiency measure

RW

log2 MWTs

1WTb


Bandwidth-limited Systems

There are situations where bandwidth is at a premium, therefore, we need modulations with large R/W.

Hence we need standards with large time-bandwidth product

The GSM standard uses Gaussian minimum shift keying(GMSK) with WTb=0.3


Case of MPSK

In MPSK, symbols are m times as wide as binary.

Nyquist bandwidth is W=Rs/2=1/2Ts. However, the bandpass bandwidth is twice that, W=1/Ts

Then

RW

log2 MWTs

log2 M bits/sec/Hz


Cost of Bandwidth Efficiency

As M increases, modulation becomes more bandwidth efficient.

Let’s fix BER. To maintain this BER while increasing M requires an increase in Eb/No.


Power-Limited Systems

There are cases that bandwidth is available but power is limited

In these cases as M goes up, the bandwidth increases but required power levels to meet a specified BER remains stable


Case of MFSK

MFSK is an orthogonal modulation scheme. Nyquist bandwidth is M-times the binary case

because of using M orthogonal frequencies, W=M/Ts=MRs

Then

RW

log2 MWTs

log2 MM

bits/sec/Hz


Select an Appropriate Modulation

We have a channel of 4KHz with an available S/No=53 dB-Hz

Required data rate R=9600 bits/sec. Required BER=10-5. Choose a modulation scheme to meet these

requirements


Minimum Number of Phases

To conserve power, we should pick the minimum number of phases that still meets the 4KHz bandwidth

A 9600 bits/sec if encoded as 8-PSK results in 3200 symbols/sec needing 3200Hz

So, M=8


What is the required Eb/No?

SNo

EbRNo

EbNoR

EbNo

(dB) SNo

(dB Hz) R(dB bits / sec

13.2dB


Is BER met? Yes

The symbol error probability in 8-PSK is

Solve for Es/No

Solve for PE

PE M 2Q2EsNo

sinM

BERPE

log2 M

2.2 10 5

37.3 10 6

EsNo

log2 M EbN0

3 20.89 62.67


Power-limited uncoded system

Same bit rate and BER Available bandwidth W=45 KHz Available S/No=48-dBHz Choose a modulation scheme that yields the

required performance


Binary vs. M-ary Model

M-ary ModulatorR bits/s

Rs R

log2 Msymbols / s

M-ary demodulator

SNo

EbNoR

EsNo

Rs


Choice of Modulation

With R=9600 bits/sec and W=45 KHz, the channel is not bandwidth limited

Let’s find the available Eb/No

EbNo

(dB) SNodB Hz R(dB bit / s)

EbNo

(dB) 48dB Hz

(10 log 9600)dB bits / s

8.2dB


Choose MFSK

We have a lot of bandwidth but little power ->orthogonal modulation(MFSK)

The larger the M, the more power efficiency but more bandwidth is needed

Pick the largest M without going beyond the 45 KHz bandwidth.


MFSK Parameters

From Table 1, M=16 for an MFSK modulation requires a bandwidth of 38.4 KHz for 9600 bits/sec data rate

We also wanted to have a BER<10^-5. Question is if this is met for a 16FSK modulation.


16-FSK

Again from Table 1, to achieve BER of 10^-5 we need Eb/No of 8.1dB.

We solved for the available Eb/No and that came to 8.2dB


Symbol error for MFSK

For noncoherent orthogonal MFSK, symbol error probability is

PE M M 12

exp Es

2No

Es Eb log2 M


BER for MFSK

We found out that Eb/No=8.2dB or 6.61 Relating Es/No and Eb/No

BER and symbol error are related by

EsNo

log2 M EbNo

PB 2m 1

2m 1PE


Example

Let’s look at the 16FSK case. With 16 levels, we are talking about m=4 bits per symbol. Therefore,

With Es/No=26.44, symbol error prob. PE=1.4x10^-5-->PB=7.3x10^-6

PB 23

24 1PE

815PE


Summary

Given:• R=9600 bits/s• BER=10^-5• Channel bandwith=45

KHz

• Eb/No=8.2dB

Solution• 16-FSK• required bw=38.4khz

• required Eb/No=8.1dB

m Ary Signalling

Documents

Transcript of m Ary Signalling