How To Understand The Theory Of Time Division Duplexing

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Dr. Francis CM Lau, Associate Professor, EIE, PolyU 1

Multiple Access Techniques

Dr. Francis LAU

Content

• Introduction

• Frequency Division Multiple Access

• Time Division Multiple Access

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Introduction

• multiple access

– techniques allowing users to share simultaneously a finite amount of radio spectrum

• duplexing

– two-way communications to occur simultaneously

Introduction

• frequency division duplexing (FDD)

– frequency separation between each forward and reverse channel is constant throughout the system, regardless of the particular channel being used

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Introduction

• time division duplexing (TDD)

Introduction

• narrowband systems

– signal bandwidth is comparable to the coherence bandwidth of the channel

– radio spectrum divided into a large number of narrowband channels

– usually operated using FDD

• frequency separation as large as possible to minimize interference

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Introduction

• wideband systems

– signal bandwidth is much larger than the coherence bandwidth of the channel

• what type of fading occurs?

– TDMA allocates time slots to many users on the channel and allows only one user to access the channel at any one time

– CDMA allows all users to access the channel at the same time

– work with both FDD and TDD

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Frequency Division Multiple

Access (FDMA)

FDMA

• each user is assigned a unique frequency

band or channel

• no other user can share the same channel

during the period of the call

• in FDD systems, a channel consists of a

frequency pair is assigned

– one frequency for forward channel – one frequency for reverse channel

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FDMA

• each FDMA channel has a relatively narrow

bandwidth because only one user is being supported • symbol time is large compared to the average delay

spread

– what type of fading occurs?

• lower complexity and lower data rate compared with TDMA

• fewer bits needed for overhead compared with TDMA

• …

FDMA

• nonlinear effects

– many channels share the same antenna at the base station

– power amplifiers or power combiners are nonlinear when operating at or near saturation for maximum power efficiency

– nonlinearities cause intermodulation (IM) which can interfere with other channels

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Example 9.1

• Find the intermodulation frequencies

generated if a base station transmits two

carrier frequencies at 1930MHz and

1932MHz that are amplified by a saturated

clipping amplifier. If the mobile radio band

is allocated from 1920MHz to 1940MHz,

designate the IM frequencies that lies inside

and outside the band.

Solution 9.1

• Intermodulation distortion products occurs at frequencies mf₁+nf₂for all integer values of m and n. Some of the possible IM frequencies that are produced by a nonlinear device are

– (2n+1)f₁–2nf₂, (2n+2)f₁–(2n+1)f₂, (2n+1)f₂–2nf₁, (2n+2)f₂–(2n+1)f₁

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Time Division Multiple Access

(TDMA)

TDMA

• divide the radio spectrum into time slots

• only one user is allowed to either transmit

or receive in each time slot

• N time slots comprise a frame

• data transmitted in a buffer-and-burst

method

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TDMA

• mobile assisted handoff (MAHO) can be

performed by a subscriber by listening on an idle slot in the TDMA frame

• possible to allocate different number of time slots per frame to different users (e.g. GPRS)

• higher transmission rate gives rise to a signal bandwidth larger than the coherence bandwidth of the channel

– What type of fading occurs?

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TDMA

• Efficiency

– frame efficiency: percentage of bits per frame that contain transmitted data

• information rate/transmission rate

Example 9.3

• Consider GSM, which is a TDMA/FDD

system that uses 25MHz for the forward

link, which is broken into radio channels of

200kHz. If 8 speech channels are supported

on a single radio channel, and if no guard

band is assumed, find the number of

simultaneously users that can be

accommodated in GSM.

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Solution 9.3

• number of simultaneously users that can be

accommodated in GSM

N = (25MHz/200kHz) x 8 = 1000

Example 9.4

• If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data are transmitted at 270.833 kbps in the channel, find

• (a) the time duration of a bit, • (b) the time duration of a slot, • (c) the time duration of a frame,

• (d) how long must a user occupying a single time slot wait between two successive transmissions.

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Solution 9.4

• If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data are transmitted at 270.833 kbps in the channel, find

• (a) the time duration of a bit T_b= 1/270.833 kbps = 3.692 µs • (b) the time duration of a slot T_s= 156.25 T_b= 0.577ms • (c) the time duration of a frame T_f= 8 T_s= 4.615 ms • (d) a user needs to wait one frame duration, i.e., 4.615 ms,

between two successive transmissions

Example 9.5

• If a normal GSM time slot consists of six

trailing bits, 8.25 guard bits, 26 training

bits, and two traffic bursts of 58 bits of data,

find the frame efficiency

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Solution 9.5

• If a normal GSM time slot consists of six

trailing bits, 8.25 guard bits, 26 training

bits, and two traffic bursts of 58 bits of data,

find the frame efficiency

• no. of data bits per time slot = 2 x 58 = 116

• equivalent no. of bits per time slot = 2 x 58

+ 6 + 8.25 + 26 = 156.25

• frame efficiency = 116/156.25 = 74.24%

Code Division Multiple Access

(CDMA)

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CDMA

• BPSK spreading accomplished by

multiplying s

_d

(t) by a function c(t)= ±1

representing the spreading waveform

BPSK DS-SS transmitter transmitted signal Phase modulator )] ( cos[ 2P ω0t+θd t )] ( cos[ ) ( 2Pct ω₀t+θ_d t Binary data t Pcos 0 2 ω c(t) Phase modulator )] ( cos[ 2P ω0t+θd t )] ( cos[ ) ( 2Pct ω₀t+θ_d t Binary data t Pcos 0 2 ω c(t)

CDMA

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BPSK DS-SS

(two-sided psd of a BPSK carrier)

Power spectral density of data-modulated carrier

{

f f T f f T

}

PT f S_d sinc [( ) sinc [( ) 2 1 ) ( 2 ₀ 0 2 ₋ ₊ ₊ =

BPSK DS-SS

psd of data- and spreading code-modulated carrier

• s_t(t) is also a BPSK carrier with T replaced by T_c

• T_c= T/3 ⇒ bandwidth of the transmitted signal spread by a factor of 3 ⇒ level of the psd reduced by a factor of 3

spreading code chip

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BPSK DS-SS

• despreading: re-modulation or correlation of the received signal with the delayed spreading waveform BPSK DS-SS receiver Bandpass filter Data phase demodulator Despreading mixer Estimated data φ + − θ + ω − )cos[ ( ) ( 2Pct Td 0t d t Td + interference ) ˆ (t Td c − ₂_P_c₍_t₋_T_d₎_c₍_t₋_Tˆ_d₎_cos[_ω₀_t₊_θ_d₍_t₋_T_d₎₊_φ signal component distortionless channel interference and/or Gaussian noise receiver's best estimate of the transmission delay transmission delay ] ]

Spreading Codes

• pseudorandom (PN) codes

• m-sequence

• Gold codes

• Walsh Codes

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Hadamard matrix M

_n

• n x n matrix

– n = even integer

• elements are

• one row of the matrix contains all ones

• other rows contain n/2 no. of “+1” and n/2

no. of “– 1”

• any row differs from the other row in

exactly n/2 positions

1 ±

Hadamard matrix M

_n       − = 1 1 1 1 2 M _      = n n n n n M M M M M₂             − − − − − − − − − − =             − − − − − − = 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ; 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 4 M M

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Walsh-Hadamard Codes

• rows of the Hadamard matrix used as code

words

• mutually orthogonal

– e.g. row 1 and 2, 1.1+1.(–1)+1.1+1.(–1) = 0 – breaks down in the presence of multipath

            − − − − − − = 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 M code 1 code 2 code 3 code 4

CDMA

• narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal • all users use the same carrier frequency and may

transmit simultaneously, TDD or FDD may be used

• unlike FDMA or TDMA, CDMA has a soft

capacity limit

– increasing the no. of users raises the noise level, more errors occur

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CDMA

• near-far problem

– mitigated using power control

• spread spectrum bandwidth much greater than the coherence bandwidth

– what type of fading?

• frequency reuse factor in CDMA cellular system is 1

– all cells use the same spectrum • soft handoff

• …

Summary

• multiple access techniques

– frequency division multiple access – time division multiple access – code division multiple access

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Summary

• Reading

– Rappaport T. S., Wireless Communications:

Principles and Practice, Prentice Hall PTR,

Sections 9.1-9.4.2, 2002.

• Problems

– Rappaport T. S., Wireless Communications:

Principles and Practice, Prentice Hall PTR,