It 802 d_Mobile Communications_part 2

IT 802DMobile Communications

Part 2

Debasis Das

Some DefinitionsBase Station: A fixed station that

communicates with mobile stations. Located on the center or the edge of a coverage region it has transmitter, a set of allocated radio channels, receiver antenna all on a tower.

Control channel: Radio channel for call set up, call request, call initiation and other beacon or control purposes.

Froward channel: radio channel for transmission from the base stations to the mobile stations.

Handoff: Process of transferring a mobile station from one channel or base station to another.

04/08/2023 Debasis Das Mallabhum Institute of Technology 2

Definitions-1 Mobile Station: Stations in use while in

motion in a mobile cellular system. Could be installed in a vehicle or a personal unit.

Mobile Switching center: Switching center that coordinates the routing of calls in a large service area in a cellular radio system. The MSC connects the cellular base stations and the mobiles to the PSTN. MSCs are also known as mobile telephone switching office or MTSO.

Page: A brief message broadcast over the entire service area service area by many base stations.


Definitions-2Reverse Channel: Channel for transmission from the

mobile stations to the base station.Roamer: A mobile station operating in an area other

than the area where services have been subscribedSubscriber: A user paying subscription charges for

using the mobile communication services.Transceiver: A device capable of simultaneously

transmitting and receiving radio signals.Simplex: One way communication systemHalf Duplex: System that communicates the same

radio channel for forward as well the reverse communication, one at a time.

Full Duplex: Systems that can communicate both ways simultaneously.04/08/2023 Debasis Das Mallabhum Institute of Technology 4

Cellular System


MSC/MTSO PSTN

Call Set Up-15% of all channel allocations over a given service area

are control channels.All mobiles scan the forward control channels to

determine the strongest one and hangs onto that until the signal drops altogether, when it scans again for a strong signal.

When a mobile is called the MSC send dispatches the request to all base stations.

Mobile Identification number(MIN) of the called number is broadcast as a page over all forward control channels.

The called mobile responds by identifying itself over a reverse control channel.

The base station relays the ack inform MSC.04/08/2023 Debasis Das Mallabhum Institute of Technology 6

Call Set up-2MSC instructs the base station to move the call to an

unused voice channel within the channel. Typically 10 to 60 voice channels and one control channel are used at a base station.

Base station asks the mobile to switch to unused voice forward and reverse channels.

Data message, called an alert, is sent over the forward voice channel is sent to make the mobile ring.

When the call is in progress the MSC adjusts the transmitted power of the mobile and the switches channel, as needed, to maintain call quality as the subscriber moves through the various cells. Special control signaling is used to voice channels so that the mobile unit may be controlled by the base station and the MSC during call progress.


Call Set Up-3When a mobile originates a call, a call

initiation request is sent on a reverse control channel

The mobile also transmits its telephone number(MIN), the ESN, electronic serial number and the telephone number of the called party.

The mobile also transmits a service class mark (SCM) which is an indicator of what the maximum power the mobile can handle.

The base station forwards the data to the MSC which in turn makes the connection and asks the base station to switch to unused pair of frequencies


RoamingThe mobile is registered as a “roamer” in the

new service area.Every roamer camps on a FCC.Every few minutes the MSC asks over all

FCCs all unregistered roamers to provide MIN/ESN information and register via the RCC.

On registration MSC requests billing status from the HLR, home location register

When status is valid, all call in and out are recorded in the home location for billing


Reference Model


NetworkNetwork

Data LinkData Link

PhysicalPhysical

Application

Transport

Network

Data Link

Physical Physical

Data Link

Network

Transport

Application

Wireless

Base station

End system End system

Layer Functionality-1Physical: data bits converted to signals for

the medium, For wireless frequency selection, carrier generation, signal detection, data modulation, encryption.

Data Link: accessing the medium, multiplexing data streams, correction of transmission errors, synchronization, responsible for reliable point to point and point to multi point connections.

Network: routing of packets, establishing two entity connections, (addressing, routing, device location, handover, etc.)04/08/2023 Debasis Das Mallabhum Institute of Technology 11

Layer Functionality-2Transport: establish end to end connection,

QoS, flow and congestion control,

Application: service location, support for

multi-media applications, adaptive

applications taking care of variations in

transmission media, wireless to web access.


Medium Access Control


Need for Specialized MACThe well known CSMA/CD does not work

It is the transmitter that is trying to detect

collision

It would not detect collisions at receiver on

wireless medium


Hidden & Exposed Terminals


A CB

A is hidden from CC is hidden from A

Hidden terminals can cause collisions, while exposed terminals cause delays

Near and Far Terminals


A B C

B being near C, will drown out signal from A at C

SDMAEach user operates from a different space

with different frequenciesWith mobiles MAXC layer has to decide

which base station may be the best for a mobile station

Decides which base station is best based on which frequencies, time slot or code is the available

SDMA is always used along with some other scheme.

Cells with Sectorized antennas could be a SDMA scheme


FDMA


124

3

2

1

915 to 890.2 MHz, uplinks

Downlinks are from 96o MHzTo 935.2 MHz, 200 KHz deviation per channel, corresponding channel is automatically allocated

TDMASeveral schemes possible

In a fixed scheme, receivers have fixed time

slot, not flexible

Dynamic schemes will need MAC addressing

as receiver needs to be identified


TDMA Schemes


Fixed TDM


1 2 311

12

1 2 311

12

Up linksDownlinks

417 micro Sec

Fixed RDM CharacteristicsFixed allocation of time slots to a channel.MAC is simple, crucial thing is to detect the

right time slot at the right momentFixed pattern can be assigned by a base

station.Many digital system use this scheme; IS-34,

IS-136, GSM, DECT, PACS, PHS Like most fixed allocation , inefficient use of

bandwidth.


Classical AlohaNeither coordinates medium access, nor does

it resolve contentions on MAC.

When simultaneous transmissions occur,

collisions happen and data is destroyed.

Higher levels need to take care of this

situation.

Maximum throughput is achieved for 18%

load04/08/2023 Debasis Das Mallabhum Institute of Technology 23

Slotted AlohaTransmissions are permitted only at fixed

times( beginning of a time slot).

All stations need to be synchronized

Reduces chances of collision

Throughput increase from 18% to 36%


Carrier Sense Multiple AccessTransmit only when there is no carrier(no

transmission).Probability of collision reduces.Hidden terminals can still cause collisions.Persistent schemes allow transmission with a

probability p if the medium is quiet.Back off times can be made proportional to

waiting time to introduce some fairness.CSMA/CA and EY-NMPA9as used in

HIPERLAN1)04/08/2023 Debasis Das Mallabhum Institute of Technology 25

Demand Assigned Multiple Access

Reservation is contention based. No data loss during actual data transmission.DAMA is an explicit reservation schemeNeeds periodic synchronization


Aloha Reserved

Packet Reservation Multiple Access(PRMA)

Fixed number of slots form a frameA frame can be reservedBase station transmits status of the slots in a

frame to all mobile stationsAttempts to use the free slots can be Aloha

styleOnce reservation succeeds, all future slots

are for use by the station that got the reservation, until relinquished

Guaranteed data rates can be ensured.04/08/2023 Debasis Das Mallabhum Institute of Technology 27

Reservation TDMAN mini slots are followed by Nxk data slotsEach station is assigned a mini slot for

reservation of up to k slotsEach station gets a guaranteed data rate and

fixed delayThe free slots can be used by any station in a

round robin scheme or Aloha accessBest effort traffic can be sent but without any

guarantees.


Multiple Access with Collision Avoidance(MACA)

Is not affected by hidden terminalsSenders send “Request to Send(RTS) to the

recipientRecipient sends a CTSRTS collisions can still occur, these packets

being small, probabilities are lower.Hidden terminal do not cause a problem as

the receiver decides which data to accept.Exposed terminals also do not cause

problems04/08/2023 Debasis Das Mallabhum Institute of Technology 29

PollingOne master stations queries others if they

have data to be transmitted.

Raise hands and ask question type polling

A contention phase followed by polling of the

stations that have data to deliver


Inhibit Sense Multiple AccessAlso known as Digital sense multiple access

Base station send out a busy/idle indicator

Any station can transmit when busy tone is

not present

Not receiving positive ACK indicates a

collision.

Back off and retransmissions are used in case

of collisions.04/08/2023 Debasis Das Mallabhum Institute of Technology 31

CDMA Schemes


CDMA RequirementsThe code, chipping sequence, should have

very high autocorrelationThe codes should be orthogonalPower adjustments need to be madeOften at a rate of 1000 times per secondIs a problem when the members in the

network can be arbitrary, as in ad-hoc networks


Spread Aloha Multiple Access(SAMA)

In the normal course stations access the medium in the narrowband spectrum

In case of collisions stations use the same chipping code with a slight phase shift, which brings down the auto-correlation

Probability of collision is quite low when number of transmitters are low


Comparisons


Approach

SDMA TDMA FDMA CDMA

Idea Segment space into sectors

disjoint time slots,

Disjoint sub bands

Use orthogonal codes

Terminals One active terminal

All can be active

All can be active

All terminals

Signal Separation

Cell structure, directed antennas

Time sync Filtering in frequency domain

Code + special receivers

Advantages simple Well established, digital

Simple, robust

Flexible, soft handover

Disadvantages

inflexible Guard spaces, sync

Inflexible, scarce resource

Complex receivers, power control

Comments In combination

Standard in fixed networks

In combination

Used in many 3G systems

It 802 d_Mobile Communications_part 2

Education

Transcript of It 802 d_Mobile Communications_part 2