EELE 5490, Fall, 2009EELE 5490, Fall, 2009

Wireless CommunicationsWireless Communications

Ali S. Afana

Department of Electrical Engineering

Class 10

Dec. 20h, 2009

                                                           

Duplexing

It is sharing the media between two parties.

If the communication between two parties is one way, the it is called simplex communication.

If the communication between two parties is two- way, then it is called duplex communication.

Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver.

Duplex communication achieved by: – Time Division (TDD)

– Frequency Division (FDD)

– Some other method like a random access method

                                                           

Duplexing

Usually the two parties that want to communication in a duplex manner (both send and receive) are: – A mobile station

– A base station

Two famous methods for duplexing in cellular systems are: – TDD: Time Division Duplex

– FDD: Frequency Division Duplex

                                                           

Duplexing - FDD

A duplex channel consists of two simplex channel with different carrier frequencies– Forward band: carries traffic

from base to mobile– Reverse band: carries traffic

from mobile to base

Reverse Channel

Forward Channel

frequencyfc,Rfc,,F

Frequency separation

Frequency separation should be carefully decidedFrequency separation is constant

B MF

RBase Station

MobileStation

                                                           

Duplexing - TDD A single radio channel (carrier frequency) is

shared in time in a deterministic manner. – The time is slotted with fixed slot length

(sec)– Some slots are used for forward channel

(traffic from base to mobile)– Some slots are used for reverse channel

(traffic from mobile to base)

B M

Base Station

MobileStation

F R R R R

0 1 2 3 4 5 6 7 …

….

Reverse Channel

Forward Channel

timeTi

Time separation

Ti+1

channel

Slot number

F F F

                                                           

Duplexing – TDD versus FDD

FDD– FDD is used in radio systems that can allocate individual radio

frequencies for each user. For example analog systems: AMPS

– In FDD channels are allocated by a base station. – A channel for a mobile is allocated dynamically– All channels that a base station will use are allocated usually statically. – More suitable for wide-area cellular networks: GSM, AMPS all use FDD

TDD – Can only be used in digital wireless systems (digital modulation). – Requires rigid timing and synchronization– Mostly used in short-range and fixed wireless systems so that

propagation delay between base and mobile do not change much with respect to location of the mobile.

Such as cordless phones…

                                                           

Multiple Access - Coordinated

We will look now sharing the media by more than two users.

Three major multiple access schemes– Time Division Multiple Access (TDMA)

Could be used in narrowband or wideband systems– Frequency Division Multiple Access (FDMA)

Usually used narrowband systems– Code Division Multiple Access

Used in wideband systems.

                                                           

Narrow- and Wideband Systems

Narrowband System– The channel bandwidth (frequency band allocated for the

channel is small) More precisely, the channel bandwidth is large

compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel)

AMPS is a narrowband system (channel bandwidth is 30kHz in one-way)

Wideband Systems– The channel bandwidth is large

More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel.

A large number of users can access the same channel (frequency band) at the same time.

                                                           

Narrow- and Wideband Systems

Narrowband Systems– Could be employing one of the following multiple access

and duplexing schems FDMA/FDD TDMA/FDD TDMA/TDD

Wideband systems– Could be employing of the following multiple access and

duplexing schemes TDMA/FDD TDMA/TDD CDMA/FDD

CDMA/TDD

                                                           

Cellular Systems and MAC

Cellular System Multiple Access Technique

AMPS FDMA/FDD

GSM TDMA/FDD

USDC (IS-54 and IS-136) TDMA/FDD

PDC TDMA/FDD

CT2 Cordless Phone FDMA/TDD

DECT Cordless Phone FDMA/TDD

US IS-95 CDMA/FDD

W-CDMA CDMA/FDD

CDMA/TDD

cdma2000 CDMA/FDD

CDMA/TDD

                                                           

Frequency Division Multiple Access

Individual radio channels are assigned to individual users

Each user is allocated a frequency band (channel)– During this time, no

other user can share the channel

Base station allocates channels to the users

B

M M M…

f1,F f2,F fN,F

fN,Rf2,Rf1,R

                                                           

Features of FDMA

An FDMA channel carries one phone circuit at a time

If channel allocated to a user is idle, then it is not used by someone else: waste of resource.

Mobile and base can transmit and receive simultaneously

Bandwidth of FDMA channels are relatively low.

Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)

Lower complexity systems that TDMA systems.

                                                           

Capacity of FDMA Systems

…

Frequency spectrum allocated for FDMA system

Guard Band

Guard Band

channel

c

guardt

B

BBN

2

Bt : Total spectrum allocationBguard: Guard band allocated at the edge of the spectrum bandBc : Bandwidth of a channel

AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz channel bandwidth (simplex): Number of channels is 416.

                                                           

Time Division Multiple Access

The allocated radio spectrum for the system is divided into time slots– In each slot a user can transmit or receive

– A user occupiess a cyclically repeating slots.

– A channel is logically defined as a particular time slot that repeats with some period.

TDMA systems buffer the data, until its turn (time slot) comes to transmit. – This is called buffer-and-burst method.

– Leaky bucket

Requires digital modulation

                                                           

TDMA Concept

1 2 3 … N 1 2 3 …. N …

1 2 3 … N 1 2 3 …. N …

Downstream Traffic: Forward Channels: (from base to mobiles)

Upstream Traffic: Reverse Channels: (from mobile to base)

Logical forward channel to a mobile

Base station broadcasts to mobiles on each slot

A mobile transmits to the base station in its allocated slotLogical reverse channel from a mobile

Upstream and downstream traffic uses of the two different carrier frequencies.

                                                           

TDMA Frames

Multiple, fixed number of slots are put together into a frame.

A frame repeats.

In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels.

In TDMA/FDD: a different carrier frequency is used for a reverse or forward

– Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa).

– Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame.

                                                           

General Frame and Time Slot Structure in TDMA Systems

Preamble Information Trail Bits

Slot 1 Slot 2 …Slot 3 Slot N

Guard Bits

Sync Bits

CRCInformation

One TDMA Frame

One TDMA Slot

A Frame repeats in time

Control Bits

                                                           

A TDMA Frame

Preamble contains address and synchronization info to identify base station and mobiles to each other

Guard times are used to allow synchronization of the receivers between different slots and frames– Different mobiles may have different propagation delays to a base

station because of different distances.

                                                           

Efficiency of a Frame/TDMA-System

Each frame contains overhead bits and data bits.– Efficiency of frame is defined as the percentage of data

(information) bits to the total frame size in bits.

xRTb

xb

befficiency

fT

T

OHf

%100)1(

bT: total number of bits in a frameTf: frame duration (seconds)bOH: number of overhead bits

Number of channels in a TDMA cell:

c

guardtot

B

BBmN

)2(

m: maximum number of TDMA users supported in a radio channel

                                                           

TDMA

TDMA Efficiency

– GSM: 30% overhead

– DECT: 30% overhead

– IS-54: 20% overhead.

TDMA is usually combined with FDMA

– Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.

– There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels)

For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA.

                                                           

Contemporary TDMA SystemsGSM

(Europa)

IS-54

(USA)

PDC

(Japan)

DECT

(European Cordless)

Bit Rate 270.8 Kbps 48.6 Kbps 42 Kbps 1.152 Mbps

Bandwidth 200 KHz 30 KHz 25 KHz 1.728 MHz

Time Slot 0.577 ms 6.7 ms 6.7 ms 0.417 ms

Upstream slots

per frame

8 3 3 12

Duplexing FDD FDD FDD TDD

Efficiency of Time Slots 73 % 80 % 80 % 67 %

Modulation GMSK /4 DQPSK /4 DQPSK GMSK

Adaptive Equalized Mandatory Mandatory Optional None

                                                           

Features of TDMA

Enables the sharing of a single radio channel among N users

Requires high data-rate per radio channel to support N users simultaneously.

– High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread parameter)

Transmission occurs in bursts (not continues)– Enables power saving by going to sleep modes in unrelated slots

– Discontinues transmission also enables mobile assisted handoff

Requires synchronization of the receivers. – Need guard bits, sync bits. large overhead per slot.

Allocation of slots to mobile users should not be uniform. – It may depend on the traffic requirement of mobiles.

– This brings extra flexibility and efficiency compared to FDMA systems.

                                                           

Questions?Questions?