[PPT] - TDMA, FDMA, and CDMA TDMA, FDMA, and CDMA Telecomunicazioni PowerPoint Presentation

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TDMA, FDMA, and CDMA TDMA, FDMA, and CDMA

Telecomunicazioni Undergraduate course in Electrical Engineering University of Rome La Sapienza Rome, Italy

2007-2008

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T Time ime D Division ivision M Multiple ultiple A Access (TDMA) ccess (TDMA)

Each user is allowed to transmit only within specified time intervals (Time Slots). Different users transmit in differents Time Slots. When users transmit, they occupy the whole frequency bandwidth (separation among users is performed in the time domain).

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TDMA : TDMA : Frame Frame Structure Structure

TDMA requires a centralized control node, whose primary function is to transmit a periodic reference reference burst burst that defines a frame and forces a measure of synchronization of all the users. The frame so-defined is divided into time slots, and each user is assigned a Time Slot in which to transmit its information.

TF TS Frame Time Slot

Reference Burst

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TDMA : Frame Structure TDMA : Frame Structure

User 1 User 2 User 3

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TDMA : TDMA : guard guard times times

Since there are significant delays between users, each user receives the reference burst with a different phase, and its traffic burst is transmitted with a correspondingly different phase within the time slot. There is therefore a need for guard guard times times to take account of this uncertainty. Each Time Slot is therefore longer than the period needed for the actual traffic burst, thereby avoiding the overlap of traffic burst even in the presence of these propagation delays.

misalignment misalignment

with guard time without guard time

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TDMA : TDMA : preamble preamble

Since each traffic burst is transmitted independently with an uncertain phase relaive to the reference burst, there is the need for a preamble preamble at the beginning of each traffic burst. The preamble allows the receiver to acquire on top of the coarse synchronization provided by the reference burst a fine estimate of timing and carrier phase.

preamble information

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TDMA: TDMA: reference reference transmitter transmitter scheme scheme S

STX SLOW IN FAST OUT TDMA coder Pulse Shaper Mod Code generator Digital signal BUFFER Carrier generator fP

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TDMA: TDMA: a case a case study study

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1

( )( ) ( ) (

)

=

k j k j

kT t a t s

Digital signal of user j Sequence of equally spaced binary antipodal symbols ak

(j) : k-th binary antipodal

symbol generated by user j T : time period between symbols User j

s(j)(t)

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TDMA: a case study TDMA: a case study

( )( )

t s j

SLOW IN FAST OUT BUFFER

( )( )

t s j

C

Compressed signal

The symbols of the original signal are organized in groups of Nbps

symbols. Each group is transmitted

in a single Time Slot of duration TS. Time Slots are organized in frames

f duration TF.

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1

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TDMA: a case study TDMA: a case study

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1

( )( ) ( )

( )

= +

=

m N 1 k F C j mN k j C

bps bps

mT kT t a t s

( )( ) ( ) (

)

=

k j k j

kT t a t s

TC : time interval between symbols after compression

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TDMA: a case TDMA: a case study study

TDMA coder Code generator TDMA Coded Signal

The position in time of each group is modified according to the TDMA code, which is assigned to the user. In other words, the TDMA code indicates which slot inside each frame must be

ccupied by the user.

( )

t s j

TDMA

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1

( )( )

t s j

C

from the buffer

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TDMA: a case TDMA: a case study study

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1

( )

( ) ( )

( )

= +

=

m N 1 k F S j m C j mN k j TDMA

bps bps

mT T c kT t a t s

cm

(j) : TDMA code assigned to

user j for the m-th frame

( )( ) ( )

( )

= +

=

m N 1 k F C j mN k j C

bps bps

mT kT t a t s

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S(j)

TX(t)

Pulse Shaper Mod Carrier generator fP

TDMA: a case study TDMA: a case study

Transmitted signal at Radio Frequencies

All users adopt the same carrier frequency fp for modulating the base- band signal

1 2 3 4 5 6 7 8 9 10 x 10-3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1 0.002 0.004 0.006 0.008 0.01 0.012 0.014

100
50

50 100 0.002 0.004 0.006 0.008 0.01 0.012 0.014

100
50

50 100

f r

m

t h e T D M A c

d

e r

( )

t s j

TDMA

( )( )

t s j

bb

Base-band signal

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TDMA: a case TDMA: a case study study

( )

( ) ( )

( )

= +

=

m N 1 k F S j m C j mN k j TDMA

bps bps

mT T c kT t a t s

( )( ) ( )

( )

j P j TDMA TX j TX

t f 2 sin ) t ( g t s P 2 t s

+
=

g0(t) : energy-normalized impulse response of the Pulse Shaper. It has unitary energy. PTX : transmitted power fP : carrier frequency ϕ(j) : istantaneous phase For the sake of simplifying the notation, let us consider the simple case of BPSK (in phase carrier modulation)

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TDMA: a case TDMA: a case study study

0.002 0.004 0.006 0.008 0.01 0.012 0.014

100
50

50 100 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

15
10
5

5 10 15 Time [s] A m p l i t u d e [ V ]

( )( )

t s j

RX

( )( )

t s j

TX

Received signal after propagation over a two-paths channel BEWARE! At risk for multi user interference!

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TDMA: a case TDMA: a case study study

.004 0.006 0.008 0.01 0.012 0.014 0.016 Time [s]

1 2 3 4 5 6 7 8 9 10 x 10

Received waveform

Front-end filtering Front-end filtering Demodulation Demodulation Sampling Sampling Threshold detection Threshold detection

Received binary antipodal signal

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F Frequency requency D Division ivision M Multiple ultiple A Access (FDMA) ccess (FDMA)

Each user transmits with no limitations in time, but using only a portion of the whole available frequency bandwidth. Different users are separated in the frequency domain.

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FDMA vs. TDMA FDMA vs. TDMA

Frequency division is very simple: all transmitters sharing the medium have output power spectra in non-overlapping bands.

Many of the problems experienced in TDMA due to different propagation delays are eliminated in FDMA.

The major disadvantage of FDMA is the relatively expensive and complicated bandpass filters required.

TDMA is realized primarily with much cheaper logic functions.

Another disadvantage of FDMA is the rather strict linearity requirement of the medium.

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FDMA: FDMA: reference reference scheme scheme S

STX Pulse Shaper Mod Code generator Digital signal Carrier generator

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FDMA: a case FDMA: a case study study

5 10 x 10 -3

1
0.5

0.5 1 Generated bit stream for each user 0.005 0.01 0.015

60
40
20

20 40 60 Signal after Pulse Shaping 0.005 0.01 0.015

60
40
20

20 40 60 Signal after FDMA coding

( )( )

t s j

( )( )

t s j

bb

( )

t s j

FDMA

Digital binary signal Base-band signal FDMA-coded signal

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FDMA: a case FDMA: a case study study

( )( ) ( ) (

)

=

k j k j

kT t a t s

( )( ) ( )( )

( )

t g t s t s

j j bb

=

( )

( ) ( )( )

( )

j j P j bb TX j FDMA

t f t c f 2 sin ) t ( s P 2 t s

+
+
=

Digital binary signal Base-band signal FDMA-coded signal Δf : frequency spacing between adjacent users c(j) : FDMA code assigned to user j STX

(j)(t)

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FDMA: a case FDMA: a case study study

Propagation Propagation Demodulation Demodulation ( (Decoding Decoding) ) Sampling Sampling Threshold Threshold detection detection

0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018

60
40
20

20 40 A m p l i t u d e [ V ] Received Signal after Demodulation (Decoding) Transmitted Received

4
2

2 4 6 8 10 12 14 16

6
4
2

2 4 x 10-3 Samples of the received waveform

0.005 0.01 0.015 0.02

8
6
4
2

2 4 6 8 Time [s] A m p l i t u d e [ V ]

4
2

2 4 6 8 10 12 14 16

0.5

0.5 1 1.5

Transmitted signal at RF Received base-band waveform Samples at the receiver

utput

Received binary stream

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TDMA + FDMA TDMA + FDMA FDMA TDMA + FDMA

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TDMA + FDMA in GSM900 standard TDMA + FDMA in GSM900 standard

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C Code

de D

Division ivision M Multiple ultiple A Access (CDMA) ccess (CDMA)

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CDMA: basic CDMA: basic principles principles

In CDMA each user is assigned a unique code sequence (spreading code), which it uses to encode its data signal. The receiver, knowing the code sequence of the user, decodes the received signal and recovers the original data. The bandwidth of the coded data signal is chosen to be much larger than the bandwidth of the original data signal, that is, the encoding process enlarges (spreads) the spectrum of the data signal.

CDMA is based on spread-spectrum modulation.

If multiple users transmit a spread-spectrum signal at the same time, the receiver will still be able to distinguish between users, provided that each user has a unique code that has a sufficiently low cross- correlation with the other codes.

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CDMA CDMA schemes schemes Direct Sequence CDMA (DS-CDMA)

The original data signal is multiplied directly by the high chip rate spreading code.

Frequency Hopping CDMA (FH-CDMA)

The carrier frequency at which the original data signal is transmitted is rapidly changed according to the spreading code.

Time Hopping CDMA (TH-CDMA)

The original data signal is not transmitted continuously. Instead, the signal is transmitted in short bursts where the times of the bursts are decided by the spreading code.

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x(t) s(t)

CODING

Cx

frequency

Band of the original signal band of the coded signal

frequency

Direct Sequence Direct Sequence Spread Spectrum Spread Spectrum

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Direct Sequence Direct Sequence Spread Spectrum Spread Spectrum

Original signal (band related to the bit rate) Spreading sequence composed by chips, with chip rate >> bit rate Coded signal (band related to the chip rate)

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Signal 1 Signal 2 Coded signal 1 Coded signal 2 Sum of coded signals 1 and 2

Direct Sequence Direct Sequence Spread Spectrum Spread Spectrum

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Received signal

code used for signal 1 multiplier

signal 1 decoded signal

Direct Sequence Direct Sequence Spread Spectrum Spread Spectrum

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In FH-SS, the transmitter spreads the spectrum by continuously jumping from one frequency channel to another A larger number of intervals leads to a better spreading Each user selectees the next frequency hop according to a code (FH code)

Frequency Hopping Spread Spectrum Frequency Hopping Spread Spectrum

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Frequency Hopping Spread Spectrum Frequency Hopping Spread Spectrum

Time-frequency occupation for a FH-SS signal

f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 f t Dwell time FH code period

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Frequency Hopping Spread Spectrum Frequency Hopping Spread Spectrum

FH-SS signal robustness to a interferers at constant frequency

f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 f t Interference limited a un dwell time Interferer at constant frequency

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Frequency Hopping Spread Spectrum Frequency Hopping Spread Spectrum

Coexistence of different FH-SS signals

f0 f1 f2 f3 f4 f5 f6 f7 f8 f9 f t Signal 2 Signal 1 If codes are well chosen (orthogonal) No interference!!

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CDMA : the CDMA : the partial correlation problem partial correlation problem

Partial Partial correlations correlations among encoded signals arise when no attempt is made to synchronize the transmitters sharing the channel, or when propagation delays cause misalignment even when transmitters are synchronized. Partial correlations impede the receiver to totally cancel the contributions of other users even in the presence of spreading codes having low cross-correlation. In presence of partial correlations, the received signal is therefore affected by Multi User Interference. The partial correlations can be reduced by proper choice of the spreading codes, but cannot be totally eliminated. CDMA system CDMA system capacity capacity is is thus thus tipically tipically limited limited by by the the interference interference from from

ther
ther

users users, , rather rather than than by by thermal thermal noise noise.

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CDMA : the CDMA : the near-far problem near-far problem

If all the users transmit at the same power level, then the received power is higher for transmitters closer to the receiving antenna. Thus, transmitters that are far from the receiving antenna are at a disadvantage with respect to interference from other users. This inequity can be compensated by using power control power control. Each transmitter can accept central control of its transmitted power, such that the power arriving at the common receiving antenna is the same for all transmitters. In other words, the nearby transmitters are assigned a lower transmit power level than the far away transmitters. Power control can be easily achieved in centralized access schemes (e.g. third generation cellular networks), but is a challenging issue in distributed systems.

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DS-CDMA: DS-CDMA: reference scheme reference scheme S

STX CDMA coder (multiplier) Pulse Shaper Mod Code generator Digital signal Carrier generator fP

Transmitter

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DS-CDMA: DS-CDMA: reference reference scheme scheme

Front-End filter and demodulator Multiplier Integrator Code generator Received signal

Receiver

SRX to the decisor

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5 10 x 10 -3

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1 Generated bit stream for each user 2 4 6 8

1
0.5

0.5 1 Assigned Codeword 0.005 0.01

1
0.8
0.6
0.4
0.2

0.2 0.4 0.6 0.8 1 Binary signal after coding for each user

DS-CDMA: a case DS-CDMA: a case study study

( )( )

t s j

( )[ ]

k c j

( )

t s j

DSCDMA

binary data signal Codeword DS-CDMA-coded signal

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DS-CDMA: a case DS-CDMA: a case study study

Digital binary signal

( )( ) ( ) (

)

=

k j k j

kT t a t s

( )

( ) ( )[ ] (

)

=
=

k N 1 m C j j k j DSCDMA

DS

kT mT t m c a t s

DS-CDMA-coded signal NDS : length of the codeword TC : chip time

( )( ) ( )

( )

j P j DSCDMA TX j TX

t f 2 sin ) t ( g t s P 2 t s

+
=

( )( ) ( )( ) ( )( ) ( ) ( ) ( )

( )

=
=
=

L 1 l j l j TX j l j j TX j

t s t h t s t s

RX

Transmitted signal Signal after propagation over a multipath channel Spreading Signal

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DS-CDMA: a case DS-CDMA: a case study study

0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

1

1 x 10-4 Received Signal after Demodulation A m p l i t u d e [ V 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

1

1 x 10-4 Received Signal after Code Multiplication A m p l i t u d e [ V ] 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

5

5 x 10-4 Received Signal after Integration A m p l i t u d e [ V ]

Received signal after Front-End filtering and demodulation Signal obtained by direct multiplication of the base- band signal with the spreading signal Received sequence after integration of the above samples