Lecture 3

Communication basics.

(2 hours)

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Contents• Spread spectrum

– FHSS

– DSSS

– CDMA

– OFDM

• Multiple Access techniques– FDMA

– TDMA

– CDMA

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Spread Spectrum• Transmit analog or digital data as analog

signals.

• Spread the signal over a wider bandwidth to avoid jamming and frequency interception.

• This technique is used for military and intelligence applications. Also used in wireless and cordless networks.

• Three techniques are commonly used FHSS, DSSS and CDMA.

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Spread Spectrum - Frequency Hopping FHSS

• Broadcast the signal over a random series of radio frequencies, hopping at fixed interval.

• The receiver should hop at those frequencies to demodulate the signal.

• s(t) = A cos (2 (f0 + fi +(bi+1)f/2) t )– f is the frequency separation, bi is equal to 1 for

binary 1 and -1 for binary 0. The frequencies fi are random, at a hop equals to the bit duration.

– The ith bit interval has a frequency f0+fi , if it is 0 and f0+fi+f if it is 1.

• Using MFSK, implementation … next

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Example with M=4 and K=2, two bits random sequence (PN)

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Tc

Ts

T |

00

01

10

11

00

01

10

11

00

01

10

11

00

01

10

11

01000010011111001101

0001101100

00

01

10

11

PN

Input data

Slow FHSS

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00 11 01 10 00 10 00 11 10 00 10 11 11 01 00 01 10 11 01 10

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

11

10

01

00

11

10

01

00

11

10

01

00

11

10

01

00

T

Ts

Tc

00

01

10

11

PN

Input data

Fast FHSS

Spread Spectrum - Direct Sequence DSSS

• A digital random signal is generated (1, 0) as PN (pseudonoise) or a chip code.

• This signal is XORed with the data signal - at a rate of 4 times, higher rate exist, to generate a wider spectrum, and modulated using BPSK.

• Any jamming signal will be filtered out by the receiver without affecting the data.

• (Data Random) Random = Data

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Example 4

Ran

Data

DataRan

DSSS

Spread Spectrum-Code Division Multiple Access CDMA

• Given a data signal of bit rate R. We assign to each bit a unique user code of k Chips according to a fixed pattern.

• The new transmission has a chip data rate of kR bits per second with data bit 1 corresponding to the user code, and the data bit 0 to the inverse of the user code.

• Chip Pattern = (d1, d2, …, dn), User sent code = (c1, c2, …, cn), the receiver performs the decoding function

– f = (d1*c1+ d2*c2 + …+ dn*cn)

– if f = n, means correct bit 1 received

– if f = -n, means correct bit 0 received otherwise incorrect code, unwanted user or error in transmission.

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Walsh Codes• Set of Walsh codes of length n consists of the n

rows of an n ´ n Walsh matrix:

– W1 = (-1)

• n = dimension of the matrix

– Every row is orthogonal to every other row and to the logical not of every other row

– Requires tight synchronization

• Cross correlation between different shifts of Walsh sequences is not zero

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nn

nn

nWW

WWW2

Example

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1111

1111

1111

1111

2

11

11

1

)1(

22

22

42

11

11

22

1

WW

WWWW

n

WW

WWWW

n

W

n

n

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User A chip code (-1, -1, -1, -1)

User B chip code (-1, +1, -1, +1)

Case 1: A transmits bit 1 and B transmits bit 1, we have

A + B = (-1, -1, -1, -1) + (-1, +1, -1, +1)

= (-2, 0, -2, 0).

The receiver filters A by multiplying, inner product operation, the

received signals by the chip code of A.

f = (-2, 0, -2, 0)*(-1, -1, -1, -1)

= -2*-1 –1*0 –2*-1 +0*-1 = 2+2 = 4. Thus confirming that A is

sending a bit 1.

Case 2: A transmits bit 0 and B transmits bit 1, we have

A’ + B = (1, 1, 1, 1)+(-1, +1, -1, +1)

= (0, 2, 0, 2, )

f = (0, 2, 0, 2, )*(-1, -1, -1, -1)

= 0*-1 + 2*-1 + 0*-1 + 2*-1 = -2 –2 = -4.

Thus conforming that A transmits bit 0.

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A

B

CDMA

1

0

1

0

0

-2

+2

OFDM• Orthogonal Frequency Division Multiplexing.

• Inter Symbol Interferences (ISI) occur if the symbol timeis smaller than the channel delay spread, this is always the case for higher transmission rates.

• The idea is to divide the wideband incoming data stream into L narrow band streams (LTs >> )

• 1 symbol is send during a time Ts, or L symbols are sent during a time LTs produces the same rate of transmission.

• Implementation of OFDM uses Circular convolution and the DFT.

• IFFT/FFT algorithms with circular convolution create ISI-free channel.

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+

.

.

.

Rbps

R/L bps

R/L bps

R/L bps

Cos(2fc)

Cos(2fc + f)

Cos(2fc + (L-1)f)

S(t)

. . .

fc fc+ f fc+ (L-1)f

B

B/L

L RF Radios

Quiz1. How does FSSS work?

2. What is the most popular technique used for 3rd

generation mobile phones?

3. What is the technique used for 4th generation mobile phones?

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