Post on 17-Nov-2014
description
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.
04/08/2023 Debasis Das Mallabhum Institute of Technology 3
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 5
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.
04/08/2023 Debasis Das Mallabhum Institute of Technology 7
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 8
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 9
Reference Model
04/08/2023 Debasis Das Mallabhum Institute of Technology 10
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.
04/08/2023 Debasis Das Mallabhum Institute of Technology 12
Medium Access Control
04/08/2023 Debasis Das Mallabhum Institute of Technology 13
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 14
Hidden & Exposed Terminals
04/08/2023 Debasis Das Mallabhum Institute of Technology 15
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 16
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 17
FDMA
04/08/2023 Debasis Das Mallabhum Institute of Technology 18
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 19
TDMA Schemes
04/08/2023 Debasis Das Mallabhum Institute of Technology 20
Fixed TDM
04/08/2023 Debasis Das Mallabhum Institute of Technology 21
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.
04/08/2023 Debasis Das Mallabhum Institute of Technology 22
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%
04/08/2023 Debasis Das Mallabhum Institute of Technology 24
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 26
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 N8k 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.
04/08/2023 Debasis Das Mallabhum Institute of Technology 28
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 30
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 32
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 33
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
04/08/2023 Debasis Das Mallabhum Institute of Technology 34
Comparisons
04/08/2023 Debasis Das Mallabhum Institute of Technology 35
Approach
SDMA TDMA FDMA CDMA
Idea Segment space into sectors
disjoint time slots,
Disjoint sub bands band
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, sunc
Inflexible, scarce resource
Complex receivers, power control
Comments In combination
Standard in fixed networks
In combination
Used in many 3G systems