CELLULAR COMMUNICATIONS Cellular Basics

Spectrum Reuse  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

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

Cells

 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

Cells

Cell Footprint R 2 S = 33 2 R

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

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

Co-ordinates for hexagonal cellular geometry  With these co- ordinates, 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 co- ordinates.

Cosine Rule

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

Cluster Radius Radius of a cluster c u R = R 3

Cluster Size C: number of channels needed for (i,j) grid is proportional to surface area of cluster Surface area of one hexagonal cell is R 2 S = 33 2 R Surface area of a (hexagonal) cluster of C cells is u R R 2 u S = C S = 33 2 R 3 { } Combining these two expressions gives u R = R3C

Possible Cluster Sizes We have seen u R = R3C and also u 2 2 R = i + j +ij 3 R Thus: C= i + j +ij 2 2 with integeriand j.

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

Reuse distance 2 – reuse pattern One frequency can be (re)used in all cells of the same color

Reuse distance 3 – reuse pattern

Design Objectives for Cluster Size High spectrum efficiency many users per cell Small cluster size gives much bandwidth per cell High performance Little interference Large cluster sizes

The effect of decreasing cell size 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

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

Umbrella Cells

Fixed and Dynamic assignment  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

24 Increasing Capacity  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

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

Handoff/Handover  Maintain call while moving

Basic Network Architecture

Basic Architecture  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

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

Setting up calls/registration  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

Registration  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

Receiving a calls  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

Paging channel  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

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

Handover(EU)/Handoff(US)  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 Handoff Region BS i Signal strength due to BS j E X1X1 Signal strength due to BS i BS j X3X3 X4X4 X2X2 X5X5 X th MS P min P i (x) P j (x) 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.

Types of Handoffs  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.

Handoff Decisions  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

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

GSM

Groupe Speciale Mobile/Global System for Mobile

GSM Air Interface  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

Physical Channel  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

Single Burst/Slot

Frame Structure

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

Broadcast Control Channels

Common Control Channels

Dedicated Control Channels

Channel Coding

International Mobile Station Equipment Identity (IMEI)  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.

International Mobile Subscriber Identity ( IMSI)  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.

Mobile Subscriber ISDN Number ( MSISDN):  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 What is a location area (LA)?  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  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  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

SMS  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  SMS Packet format  All data is transferred in a single DCCH SPACH packet SCAService Centre Address MRMessage ReferencePIDProtocol Identifier PDU TypeProtocol Data Unit Type DADestination AddressDCSData Coding Scheme VPValidity PeriodUDLUser Data LengthUDUser Data

GPRS: General Packet Radio Service  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

GPRS architecture

GPRS handset classes  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.

Page 67  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 Enhanced Data GSM Environment (EDGE) EDGE enabled data mobile

Page 68 Practically achievable data rates  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 69 Migration: 1.High speed circuits switched data (HSCSD) 2.Packet switched data (GPRS,EDGE) 3.Integrated packet services – possibly under different access scheme (UMTS) GSM Migration Towards 3G

70 HSDPA 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)