1

Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance between the carrier frequencies

special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit is doubled

depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen

the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM

2

Advanced Frequency Shift Keying

If Even and Odd bits are both zero : - f2 is inverted.

If Even bit is 1 and odd bit is zero: - Lower frequency f1 is inverted.

If Even bit is zero and the odd bit is 1: - f1 is taken without phase change, as is.

If Both even and odd bits are 1 : - frequency f2 is taken as is.

3

Example of MSK

data

even bits

odd bits

1 1 1 1 000

t

low frequency

highfrequency

MSKsignal

biteven 0 1 0 1

odd 0 0 1 1

signal h n n hvalue - - + +

h: high frequencyn: low frequency+: original signal-: inverted signal

No phase shifts!

4

Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK more complex

Often also transmission of relative, not absolute phase shift: DQPSK – - Differential QPSK (IS-136, PHS)

11 10 00 01

Q

I

11

01

10

00

A

t

5

Quadrature Amplitude Modulation

Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation

it is possible to code n bits using one symbol 2n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to

comparable PSK schemes

Example: 16-QAM (4 bits = 1 symbol)Symbols 0011 and 0001 have the

same phase φ, but different amplitudes. 0000 & 1000 have different phases but the same amplitude.

0000

0001

0011

1000

I

0010

φ

a

6

Hierarchical Modulation

DVB-T modulates two separate data streams onto a single DVB-T stream

High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers QPSK, 16 QAM, 64QAM Example: 64QAM

good reception: resolve the entire 64QAM constellation

poor reception, mobile reception: resolve only QPSK portion

6 bit per QAM symbol, 2 most significant determine QPSK

HP service coded in QPSK (2 bit), LP uses remaining 4 bit

I

Q

10

00000010 010101

7

Spread spectrum technology

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using a special code

protection against narrow band interference

protection against narrowband interference Side effects:

coexistence of several signals without dynamic coordination tap-proof

Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal

signal

f f

power

power

8

Spread spectrum technology

MERITS OF SPREAD SPECTRUMBecause Spread Spectrum signals are noise-like, they are hard to detect.

Spread Spectrum signals are harder to jam (interfere with) than narrowband signals Spread Spectrum transmitters use similar transmit power levels to narrow band transmitters. Because Spread Spectrum signals are so wide, they transmit at a much lower spectral power density, measured in Watts per Hertz, than narrowband transmitters.

power

9

Spread spectrum technology

MERITS OF SPREAD SPECTRUM spread spectrum signals are hard to exploit or

spoof. Signal exploitation is the ability of an enemy

(or a non-network member) to listen in to a network and use information from the network without being a valid network member or participant.

Spoofing is the act of falsely or maliciously introducing misleading or false traffic or messages to a network.

power

10

Spreading and Despreading – Resistance to narrow band interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signalbroadband interferencenarrowband interference

dP/df

11

Spreading and Despreading – Resistance to narrow band interference

(i) Original signal to be transmitted.

(ii) The sender spreads the signal and converts the narrow-band signal to broadband (Power level can be much lower without losing data)

(iii) During transmission, narrow and broadband noise gets added.

(iv) The receiver despreads the given signal, narrow band interference is spread, leaving the broadband as it is.

(v) Receiver applies a band pass filter cutting off left & right of narrow band signal.

12

Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

narrowband channels

spread spectrum channels

13

Problem with narrow band transmission (FDM)

In the figure above,Frequencies 1,2 and 5 have reasonably

good quality of service. Frequencies 3 & 4 are of very narrow

band and they can get corrupted.Spread Spectrum can help in such a

situation.

14

How to fix the narrow band interference problem?

22

22

2

frequency

channelquality

1

spreadspectrum

15

Solution….

All narrow band signals are spread into broadband signals using the same frequency range

To separate the Channels, CDM is used.Each channel is allocated its own code

which the receivers know.Because of secret code, spread

spectrum acts as a security protection.

16

DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the

signal Advantages

reduces frequency selective fading

in cellular networks base stations can use the

same frequency range several base stations can

detect and recover the signal soft handover

Disadvantages precise power control necessary

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period

17

Bandwidth of the resulting signal

tc = Chip Period

tb = Bit Period

Spreading factor s -= tb/tc

s*original bandwidth is the new bandwidth.

It determines the BW of the resulting signal

18

Spreading Factor

Most civil applications need a spreading factor of 10 to 100.

Military applications use a speeding factor of around 10,000.

19

Barker Codes

- 10110111000 (802.11 wireless LANS)- 11- 110- 1110- 11101- 1110010- 1111100110101

20

Barker Codes

Let the code to be transmitted be 110.

Let the Chip Barker Code be

10110111000

Hence the transmitted code is:

11111111111 11111111111 00000000000 XOR

10110111000 10110111000 10110111000

- 01001000111 01001000111 10110111000

21

Barker Codes

At the Receiver : The transmitted signal is XORed with the same chip sequence.

01001000111 01001000111 10110111000

XOR 10110111000 10110111000 10110111000

Resulting in :

11111111111 11111111111 00000000000

This is the original signal 110

22

DSSS (Direct Sequence Spread Spectrum) II

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator

23

FHSS (Frequency Hopping Spread Spectrum) I

Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random

number sequence Two versions

Fast Hopping: several frequencies per user bit

Slow Hopping: several user bits per frequency

Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time

Disadvantages not as robust as DSSS simpler to detect

24

FHSS (Frequency Hopping Spread Spectrum) I

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time

25

FHSS

Total available BW is split into many channels of smaller BWs

Transmitter & Receiver stay in one of these channels and hop to another channel

The implementation is a combination of FDM & TDM Hopping sequence defines the transition sequence

amongst channels Dwell Time : Time spent on a channel with certain

frequency The bandwidth of a frequency hopping signal is simply

w times the number of frequency slots available, where w is the bandwidth of each hop channel.

26

FHSS

In Slow Hopping FHSS, one frequency is used for several bit periods

In Fast Hopping FHSS, transmitter changes frequency several times during single bit transmission.

Slow Hopping FHSS are not as immune to narrow band interference as the fast hopping FHSS.

27

FHSS Transmitter and Receiver

modulator

user data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

28

SPECTRAL ANALYSIS

29

Distinctions

Spreading is simpler in FHSSFHSS uses only a portion of the BW at

any given time.DSSS are more resistant to fading and

multi-path effects.DSSS signals are had to detect in the

absence of the knowledge of spreading code.

30

CELLULAR SYSTEMS

Mobile systems using cells employ SDMA Base station covers a certain area and

is called a cell.

Coverage(Radii) of a cell :- A few metres in buildings- Hundreds of metres in cities- Tens of kms on country side.

31

Cell structure

Implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station

Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

32

Frequency planning I

Frequency reuse only with a certain distance between the base stations

Standard model using 7 frequencies:

Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells

Dynamic frequency assignment: base station chooses frequencies depending on the

frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements

f4

f5

f1

f3

f2

f6

f7

f3

f2

f4

f5

f1

33

Frequency planning II

f1

f2

f3

f2

f1

f1

f2

f3

f2

f3

f1

f2

f1

f3f3

f3f3

f3

f4

f5

f1

f3

f2

f6

f7

f3

f2

f4

f5

f1

f3

f5f6

f7f2

f2

f1f1 f1

f2

f3

f2

f3

f2

f3h1

h2

h3g1

g2

g3

h1

h2

h3g1

g2

g3g1

g2

g3

3 cell cluster

7 cell cluster

3 cell clusterwith 3 sector antennas

34

Cell breathing CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells

35

Cell

A single cell in an analog cell-phone system uses one-seventh of the available duplex voice channels .

A cell-phone carrier typically gets 832 radio frequencies to use in a city.

Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels -- more on this later.)

The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in the figure above, both of the purple cells can reuse the same 56 frequencies. The same frequencies can be reused extensively across the city.

36

Cell

Each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. - Analog cellular systems(1G)

Digital system(2G) using : TDMA : can carry three times as many calls as an analog system, so each cell has about 168 channels available.

37

Cell Coverage

38

Borrowing Channel Allocation (BCA)

Fixed allocation of frequency channels do not optimize

channel usage, if the traffic pattern is varying. Cells with higher traffic pattern can borrow frequencies

from its neighbors (which do not carry heavy traffic) This scheme is termed BCA.

39

Dynamic Channel Allocation (DCA)

Channels (frequencies) will be allocated dynamically.

Frequencies can only be borrowed.The borrowed frequency can be blocked

for the surrounding cells, to reduce interference