Cellular Basics


Earlier systems: single central transmitter

Cover wide area

Single channel per user

25kHz for sufficient audio quality and guard interval

40 users in 1MHz, 400 users for 100MHz

Modern systems have millions of subscribers


Several transmitters, each having only certain coverage area

Cell==coverage area

Reuse same spectrum in many transmitters


Often shown as hexagonal shapes

In reality, very irregular boundaries

Signal strength decreases gradually=>no exact cell edges

Some cell areas may overlap

Allocate different spectrum to adjacent cells

Can overlap without causing interference

S
R
=
3 3
2
R
2


Cells with different spectrum grouped together as cluster

Often clusters of size 7

Theoretical Network Planning
Honeycomb (hexagonal) cell structure
Cluster: set of different frequencies used in group of cells
Cluster is repeated by linear shift i steps along one direction j steps in the other direction
How many different frequencies does a cluster contain?


With these coordinates, an array of cells can be laid out so that the center of every cell falls on a point specified by a pair of integer coordinates.

Reuse Distance
Distance between cell centers = 3
× Cell Radius
Reuse distance distance between the centers of two co-channel cells
2 2
R = i + j + 2ij p cos
3
3 R where
R is Cell Radius and
R u is Reuse Distance cos( p
/3) = 1/2

Cluster Radius
Radius of a cluster
R u
3

C : number of channels needed for (i,j) grid is proportional to surface area of cluster
Surface area of one hexagonal cell is
S
R
=
3 3
2
R
2
Surface area of a (hexagonal) cluster of C cells is
S
R u
= CS =
3 3
2
{ }
3
2
Combining these two expressions gives

We have seen and also
R =
2 2 i + j + ij 3 R
Thus:
2
C = i + j + ij with integer i and j .

·
Chose C to ensure acceptable link quality at cell boundary
Cluster size C = i
2+ ij + j
2 = 1, 3, 4, 7, 9, ...
·
C = 1 i = 1, j = 0 } Cluster size for CDMA net
C = 3 i = 1, j = 1
C = 4 i = 2, j = 0
C = 7 i = 2, j = 1 } Usual cluster sizes for TDMA
C = 9 i = 3, j = 0 } cellular telephone nets
C = 12 i = 2, j = 2

One frequency can be (re)used in all cells of the same color

•High spectrum efficiency many users per cell
Small cluster size gives much bandwidth per cell
• High performance
Little interference
Large cluster sizes

•Increased user capacity
•Increased number of handovers per call
•Increased complexity in locating the subscriber
•Lower power consumption in mobile terminal:
· Longer talk time,
· Safer operation
•Different propagation environment, shorter delay spreads
•Different cell layout,
· lower path loss exponent, more interference
· cells follow street pattern
· more difficult to predict and plan
· more flexible, self-organizing system needed


Macrocells

10km, sparsely populated area

Microcells

1km, densely populated area

Picocell

200m, particular buildings, streets



Fixed frequency assignment: permanent

 certain frequencies are assigned to a certain cell problem: different traffic load in different cells
Dynamic frequency assignment: temporary


 base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements






Add new channels
Dynamic channel allocation – frequencies can be taken from adjacent cells by congested cells
Cell splitting – cells in areas of high usage can be split into smaller cells
Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels (typical: 3)
Microcells – antennas move to buildings, hills, and lamp posts
24


Use directional antennas

Collocate cell antenna at the cell edges

Reduce cost


Maintain call while moving


Base Station Controller (BSC)

Control each base station

Manage hand-off of a call from one base station to other

Mobile Switching Center(MSC)

Manages setup and tear down of calls to and from mobile subscribers

Home Location Register (HLR)

HLR subscriber database including location


Base Transceiver
Station (BTS)

Antenna Tower

Radio transceivers

Power Supply

Link to BSC (land lines or microwave)


Make a call originated from mobile handset

Allocate resources (channel)

Receive a call

Locate cell of the subscriber

After the telephone is switched on

Contact base station

Register to use a network


Authenticate (e.g. for billing)

Authentication Center (AuC)

Store my location

HLR for “home” subscribers

VLR for “visiting”/roaming subscribers

Mobile communicates with the network to update status/location

Network keeps last known location


Network should send a notification to a mobile

Network send to the area where mobile is located

Mobile listen to a “paging” channel

Examine each message on the paging channel and compares number with his own

Respond if match


Always listening to the paging channel drains the battery

Divide paging channel into 10 subgroups according to a last digit of mobile phone number

Mobile has to listen only 1/10 of time

Longer call setup time


Respond to call /paging channel message

Initiate a call

“Access” message

Request a channel/slot/resources for further communications

Slotted ALOHA


Mobile monitor signal strength

Network knows about availability of channels

Mobile monitors strength of signal from current and adjacent cells and sends this information to network

When signal drops below certain level, network reserved new channel at adjacent cell

Mobile switch channel, network shuts down old channel

36
Signal strength due to BS i
Signal strength due to BS j
P i
(x) P j
(x)
E
P min
BS i
X
1
X
3
MS
X
5
X th
X
4
X
2
BS j
• By looking at the variation of signal strength from either base station it is possible to decide on the optimum area where handoff can take place.


Hard handoff



A hard handoff is a “break before make” connection.
MS is linked to no more than one BS at any given time.
Hard handoff is primarily used in FDMA and TDMA.

Soft handoff

It isn't a “ break before make ” transition.

The call can be carried on both cells simultaneously.

Soft handoff is used in CDMA.


Decision-making process of handoff may be
centralized or decentralized

Three different kinds of handoff decisions

Network-Controlled Handoff

Mobile-Assisted Handoff

Mobile-Controlled Handoff


Network Management Systems

Service Delivery

Service Fulfillment, including the Network Inventory,
Activation and Provisioning

Service Assurance

Customer Care

Billing

GSM


TDMA with FDD





200Khz channels with 200KHz guard bands
GSM 900 has 124 carriers
GMSK modulation, 270kbps per carrier
Up to 8 users, 24.8kbps per user
FEC reduces to 13kbps per user for voice


RF carrier divided into 8 slots, numbered 0..7

Timeslots carrying data

At most 8 traffic channels

Control messages

At least 1 control channels

More control (logical) channels

Packed into RF carrier

Traffic channels
(TCH)
BCH
Signaling channel
CCCH
DCCH
TCH/F: Full-rate Traffic Channel
TCH/H: Half-rate Traffic Channel
FCCH: Frequency correction
SCH: Synchronization
BCCH: Broadcast control
Two-way
Base-tomobile
PCH: Paging
AGCH: Access grant
RACH: Random access
SDCCH: Stand-alone dedicated control
SACCH: Slow associated control
FACCH: Fast associated control
Two-way


Type Approval Code (TAC): 6 decimal places, centrally assigned.

Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer.

Serial Number (SNR): 6 decimal places, assigned by the manufacturer.

Spare (SP): 1 decimal place.


Mobile Country Code (MCC): 3 decimal places, internationally standardized.

Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.

Mobile Subscriber Identification Number
(MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.


Country Code (CC) : Up to 3 decimal places.

National Destination Code (NDC): Typically 2-3 decimal places.

Subscriber Number (SN): Maximum 10 decimal places.

59




A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell
One extreme is to page every cell in the network for each call
- a waste of radio bandwidth
Other extreme is to have a mobile send location updates at the cell level. Paging cut to 1 cell, but large number of location updating messages.
Hence, in GSM, cells are grouped into Location Areas – updates sent only when LA is changed; paging message sent to all cells in last known LA




SMS allowed



Two way communications of the text messages
Maximum character length of 160 characters

This can change though depending on the operator or the character set used
Character sets supported are


ASCII + additional European characters
Unicode
First Text

Was sent in December 1992, to a Vodafone device

Sent by Neil Papworth, saying “Merry Christmas”
Standard

Defined by ETSI and is known as “GSM 03.40”


SMS Continued


The success is SMS was never planned for!
It was only ever intended as the Pager replacement, with limited use
 This will explain some of the design decisions made


GSM

At a defined time interval in GSM all devices will listen to a transmission.

This is when a Digital Control Channel (DCCH) packet of information is being sent across the network.





These DCCH packets are used to transfer essential information into the devices.
Information like a call is in coming
Paging signals from the Base stations, to work out if a handover is needed
One of these packet formats is called SMS point to point messaging, Paging, access control channel (SPACH)

This message type can be used to carry a text message.
Advantage of this method is a text message can still be delivered during a phone conversation.


SMS Packet format

All data is transferred in a single DCCH SPACH packet
SCA Service Centre
Address
PDU Type Protocol Data Unit
Type
VP Validity Period
MR
DA
Message Reference
Destination Address
UDL User Data Length
PID
DCS
UD
Protocol Identifier
Data Coding Scheme
User Data


GSM data

CSD: circuit switched data

Max 14kbps

Similar to voice call

Inefficient usage of spectrum

GPRS packet-based service

Upgrade of infrastructure

GGSN is a gateway to outside world

SGSN is a gateway within the network


Class A
Class A terminals have 2 transceivers which allow them to send / receive data and voice at the same time. This class of device takes full advantage of GPRS and GSM.
You can be taking a call and receiving data all at the same time.

Class B
Class B devices can send / receive data or voice but not both at the same time. Generally if you are using GPRS and you receive a voice call you will get an option to answer the call or carry on.

Class C
This device only allows one means of connectivity. An example would be a
GPRS data card in a laptop.




Packet switched
Upgrades the modulation scheme

From GMSK to 8-PSK


Maximum speed ~59 Kb/sec per time slot, ~473.6 Kb/sec for all 8 time slots
Variable data rate – depending on the channel conditions
Defines several different classes of service and mobile terminals
EDGE enabled data mobile
Page 67



Theoretical rates are constrained by mobile power and processing capabilities
Most mobiles support less than the maximum allowed by standard
Practically achievable data rates
Page 68

Migration:
1.
High speed circuits switched data
(HSCSD)
2.
Packet switched data
(GPRS,EDGE)
3.
Integrated packet services – possibly under different access scheme (UMTS)
Data Rate
GSM 2+
9.6 Kb/sec
EDGE
384 Kb/sec
GPRS
114 Kb/sec
HSCSD
64 Kb/sec
HSCSD - High Speed Circuit Switched Data
GPRS - General Packet Radio System
EDGE
UMTS
- Enhanced Data GSM Environment
- Universal Mobile Telephone Service
UMTS
2Mb/sec
1999
1Q
2000
2Q
2000
Timeline
3Q
2001
Page 69
4Q
2002

70
High Speed Downlink Packet Access

Standardized in 3GPP Release 5


Improves System Capacity and User Data Rates in the Downlink
Direction to 10Mbps
Adaptive Modulation and Coding (AMC)

Replaces Fast Power Control :
User farer from Base Station utilizes a coding and modulation that requires lower Bit Energy to Interference Ratio, leading to a lower throughput

Replaces Variable Spreading Factor :
Use of more robust coding and fast Hybrid Automatic Repeat Request
(HARQ, retransmit occurs only between MS and BS)