cellular communication system

CHAPTER 4

CELLULAR COMMUNICATION SYSTEM

CHAPTER OUTLINE

CELLULAR COMMUNICATION COMPONENT

ANTENNA IN CELLULAR COMMUNICATION

CELLULAR COMMUNICATION CONCEPT

HAND OVER PROCESS IN CELLULAR COMMUNICATION SYSTEM

RADIO CHANNEL & MODULATION TECHNIQUE IN CELLULAR COMMUNICATION

PART 1

MAIN COMPONENT IN CELLULAR COMMUNICATION SYSTEM

AT THE END OF THIS TOPIC STUDENT SHOULD BE ABLE TO:

4.1 Understand the main component in cellular communication system.

4.1.1 Describe the Mobile Switching Centre (MSC).

4.1.2 Describe the elements connected to the MSC.

a. Home Location Register.

b. Visitor Location Register.

c. Equipment Identity Register (EIR).

d. Authentication Centre (AuC)

e. Gateway Mobile Switching Centre (GMSC).

f. SMS Gateway (SMS-G).


4.1.3 Describe the Base Station Subsystem (BSS).

4.1.4 Describe the types of Base Transceiver.

a. Base Transceiver Station (BTS).

b. Base Station Controller (BSC).

4.1.5 Describe the Mobile Unit that functions as a transceiver.

4.1.6 Explain the function of sim card used in mobile units.

4.1.7 Relate between MSC, BSS, BTS, BSC and mobile unit in the form of diagrams.

Wireless Communications

• Multimedia wireless Communications at any Time and Anywhere

• Brief history• Ancient Systems: Smoke Signals, Carrier Pigeons

• Radio invented in the 1880s by Marconi

• Many sophisticated military radio systems were developed during and after WW2

• Cellular has enjoyed exponential growth since 1988, with more than 2 billion users worldwide today

• Ignited the recent wireless revolution, 1980-2003

• Growth rate tapering off

• Is there a future for wireless?

Current Wireless Systems

• Cellular systems

• Wireless LANs

• Satellite Systems

• Paging Systems

• Bluetooth

• Ultrawideband Radios

• Zigbee Radios

What is Cellular Communication System

• Cellular communication is designed to

provide communications between two moving units, or between one mobile unit and one stationary phone or land unit (PSTN).

• A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the channel from base station to base station as the caller moves out of range (handover/handoff).

To make this tracking possible…..

• Each cellular service area is divided into small regions called cells.

• Each cell contains an antenna and is controlled by powered network station, called the base station (BS).

• Each base station is controlled by a switching office, called a mobile switching center (MSC).

• The MSC coordinates communication between all the base stations and the telephone central office (exchange). It is a computerized center that is responsible for connecting calls, recording call information, and billing.

COMPONENTS IN CELLULAR COMMUNICATION SYSTEM

3 main components: • Mobile Station (MS) – UE,

SIM

• Base Station Subsystem (BSS) – BTS, RBS, BSC

• Network and Switching Subsystem (NSS) – MSC, VLR, HLR,

Network & Switching Subsystem (NSS)

• Mobile Switching Center (MSC)

• Home Location Register (HLR)

• Visitor Location Register (VLR)

• Equipment Identify Register (EIR)

• Authentication Centre (AuC)

• Gateway Mobile Switching Center (GMSC)

• SMS Gateway (SMS-G)

Mobile Switching Center (MSC)

• The MSC is the heart of the GSM network.

• From technical perspective MSC is just an ordinary Integrated Services Digital Network (ISDN) exchange

• One MSC can handles multiple BSCs and also interfaces with other MSC's (Using E-Interface).

• It also handles inter-BSC handoffs as well as coordinates with other MSC's for inter-MSC handoffs.


• MSC performs the telephony switching functions of the system.

• Controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN).


• Difference between a MSC and an exchange in a fixed network, MSC has to take into account the impact of the allocation of radio resources and the mobile nature of the subscribers and has to perform in addition, at least the following procedures:

• required for location registration

• procedures required for handover


• MSC can be connected to only one VLR or more VLR. Therefore, all mobile stations that move around under base stations connected to the MSC are always managed by the same VLR.

• MSC would communicate typically with one EIR. While it is possible for an MSC to communicate to multiple EIRs, this is highly unlikely since the EIR provides a centralized and geographic independent function.

Ericsson Mobile Switching Center Server (MSC-S)

The Mobile Switching Center Server (MSC-S) provides control of high-capacity switching in mobile circuit core networks

The Elements Connected to The MSCThe MSC connects to the following elements:

a. The Home Location Register (HLR)

b. The Visitor Location Register (VLR)

c. Equipment Identify Register (EIR)

d. Authentication Centre (AuC)

e. Gateway Mobile Switching Center (GMSC)

f. SMS Gateway (SMS-G)

Home location register (HLR)

• HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network.

• The HLRs store details of every SIM card issued by the mobile phone operator.

• Each SIM has a unique identifier called an

IMSI which is the primary key to each HLR record.

Visitor Location Register (VLR)

• When the mobile user visits a PCS network other than the home system, a temporary record for the mobile user is created in the visitor location register (VLR) of the visited system.

• The VLR temporarily stores subscription information for the visiting subscribers so that the corresponding MSC can provide service.

• In other words, the VLR is the "other" location register used to retrieve information for handling calls to or from a visiting mobile user.

Home Location Register (HLR)

• Examples of other data stored in the :• GSM services that the subscriber has

requested or been given.

• GPRS settings to allow the subscriber to access packet services.

• Current location of subscriber (VLR and serving GPRS support node/SGSN).

• Call divert settings applicable for each associated MSISDN.

Responsibilities of the HLR include:

• management of service profiles

• mapping of subscriber identities (MISDN, IMSI)

• supplementary service control and profile updates

• execution of supplementary service logic e.g. incoming calls barred.

• passing subscription records to VLR

• directly receives and processes MAP transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around.


• VLR is a database as same as HLR that contains all subscriber information data for call handling and mobility management

• VLR provide dynamic data management (HLR static data management)

• The VLR keeps track of all subscribers roaming in the VLR service area.

• In GSM system the VLR is integrated with the MSC


• VLR contains:

• Selective information function from the HLR

• IMSI (the subscriber's identity number).

• Authentication data.

• MSISDN (the subscriber's phone number).

• GSM services that the subscriber is allowed to access.

• access point (GPRS) subscribed.

• The HLR address of the subscriber.

Function of the VLR include:

• Executing supplementary service programs (outgoing calls barred)

• Initiating authentication and ciphering

• Initiating paging

• Mapping of various identities (MSISDN, IMSI, TMSI, MSRN)

• Passing location information to HLR

Function of the VLR include:

• To inform the HLR when subscriber has arrived in the area covered by the VLR.

• To track where the subscriber when idle mode.

• To allow or disallow which services the subscriber may use.

• To allocate roaming numbers during the processing of incoming calls.

• To purge the subscriber record if becomes inactive whilst in the area and deletes the subscriber's data after some period and informs the HLR

• To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR.

Equipment Identity Register (EIR)

• The EIR is a database that keeps tracks of handsets on the network using the IMEI.

• The EIR was introduced to identify, track and bar such equipment from being used in the network

• There is only one EIR per network.

• Composed of three lists.

• The White List

• The Gray List

• The Black List

Equipment Identity Register (EIR)

Authentication Centre (AuC)

• AUC is always integrated with HLR for the purpose of the authentication.

• The Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. The Ki is stored in the AUC and used to provide the triplets, same Ki is also stored in the SIM.

• AUC stores the following information for each subscriber

• The IMSI number,

• The individual authentication key Ki

• A version of A3 and A8 algorithm.

Authentication Centre (AuC)

In AUC following steps are used to produce one triplet:

1. A non- predictable random number, RAND, is produced

2. RAND & Ki are used to calculate the Signed Response (SRES) and the Ciphering Key (Kc)

3. RAND, SRES and Kc are delivered together to HLR as one triplet.

HLR delivers these triplets to MSC/VLR on request in such a way that VLR always has at least one triplet.

Gateway Mobile Switching Center

(GMSC)• There is another important type of MSC, called a

Gateway Mobile Switching Center (GMSC).

• The GMSC functions as a gateway between two networks.

• If a mobile subscriber wants to place a call to a regular land line, then the call would have to go through a GMSC in order to switch to the Public Switched Telephone Network (PSTN).

Gateway Mobile Switching Center (GMSC)

SMS Gateway (SMS-G)

• The SMS GMSC (SMS gateway MSC) is

a gateway MSC that can also receive

short messages.

• The gateway MSC is a mobile network’s

point of contact with other networks.

Base Station Subsystem (BSS)

• BSS is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the NSS.

• The BSS performs all the radio-related functions.• The BSS is comprised of the following functional units:

• Base Station Controller (BSC) • Base Transceiver Station (BTS)

• BSS communicate to Mobile Station (MS) using Air

Interface


• Function of BSS

• transcoding of speech channels,

• allocation of radio channels to mobile phones,

• paging,

• transmission and reception over the air interface

• and many other tasks related to the radio network.

• BTS + BSC = BSS.

• BTS provides physical connection between MS to network using Air Interface (Um)

• Main function of BTS is for maintaining the Um interface and minimizing the transmission problem (Um very sensitive for disturbance)

• Using Abis interface for connection between BTS and BSC

Base Transceiver Station (BTS)


• A BTS usually placed on center of the cell

• Its transmitting power defines size of the cell

• Each BTS has between 1 to 16 transceiver depending on density of the user in the cell

• Each BTS serve in a single cell


The site controller stations that function depends on the directions from the MSC.

Using voice channels - VC (or trafic channel TC) and control channels – CC as radio channels communications in each cell

Supervise the call, monitoring the quality of the speech and also the measurement of the strength of the voice signal.

Send and receive voice signals and data signals to/from users.

Interface between users equipment UE and switching systems MCS


• The BTS is the Mobile Station's access point to the network.

• It is responsible for carrying out radio communications between the network and the Mobile Station’s.

• It handles speech encoding, encryption, multiplexing (TDMA), and modulation/demodulation of the radio signals.

• It is also capable of frequency hopping (changing carrier frequency while communicating)

• One BTS usually covers a single 120 degree sector of an area.

• Usually a tower with 3 BTSs will accommodate all 360 degrees around the tower.

Base Station Controller (BSC)

• The BSC controls multiple BTS.

• It handles allocation of

- radio channels,

- frequency administration,

- power and signal measurements from the MS,

- Handovers from one BTS to another (if both BTSs are controlled by the same

BSC).


• A BSC my be collocated with a BTS or it may be geographically separate.

• It may even be collocated with the Mobile Switching Center (MSC).

GSM BTS Type

Mobile Station (MS)

• MS is the physical equipment used by a GSM subscriber

• It comprises two parts:

• Subscriber Identity Module (SIM)

• The Mobile Equipment (ME)

Mobile Equipment (ME)

• ME provides the radio and processing needed to access the GSM network, plus a man machine interface MMI to enable the user to access services.

Mobile Equipment (ME)

ME Function

• Radio transceiver and signal processing

• Radio related operations: power control; timing advance; discontinuous transmission (DTX); slow frequency hopping (SFH).

• Call handling

• man-machine interface, display, keypad, speech transducers.

• interfaces to external equipment e.g. laptops / palmtops

Subscriber Identity Module (SIM)• Provides personal mobility - user can have

access to subscribed services irrespective of a specific terminal.

• Contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information.

• The SIM card may be protected against unauthorized use by a password or personal identity number.

SIM

Function of SIM card

• SIM is a smart card which plugs into the mobile equipment and contains information about the mobile subscriber.

• Carries all the subscriber specific information used by an MS.

• Major functions are to identify the current user of an MS and to take part in security and confidentiality procedures.

• Stores recent location data and may also store personal information for the user such as abbreviated dialing codes (telephone directory).

Specific functions include:

• Permanent storage of a subscriber’s International Mobile Subscriber Identity (IMSI) and Authentication key (Ki)

• Semi permanent storage of system information e.g. current Location Area Identity (LAI), encryption key Kc and lists of preferred / forbidden GSM networks

• Semi permanent storage of user data, ‘telephone directory’, short messages

• Participation in mobility procedures e.g. user authentication, generation of ciphering key, instigation of location updates.

• Protected by PIN

The SIM contains several pieces of information:

International Module Subscriber Identity (IMSI) – This number identifies the mobile subscriber. It is only transmitted over the air during initialisation.

Temporary Mobile Subscriber Identity (TMSI) – This number identifies the subscriber, it is periodically changed by the system management to protect the subscriber from being identified by someone attempting to monitor the radio interface.

The SIM contains several pieces of information:

Location Area Identity (LAI) – Identifies the current location of the subscriber .

Subscriber Authentication Key (Ki) – This is used to authenticate the SIM card.

Mobile Station International Services Digital Network (MSISDN) – This is the telephone number of the mobile. It is comprised of a country code, a national a subscriber number.

GSM System Architecture

PART 2

ANTENNA USE IN CELLULAR COMMUNICATION SYSTEM


4.2 Know the types of antenna used in cellular communication systems.

4.2.1 Identify the types of antenna.

a. Omnidirentional antenna.

b. Sectorized antenna.

4.2.2 State the characteristics of the antenna in

Types of Antenna Used in Cellular Communication System

a. Omnidirectional Antenna

b. Sectorized Antenna

Omni Directional

• The omnidirectional antenna radiates or receives equally well in all directions.

• It is also called the "non-directional" antenna because it does not favor any particular direction.

• This antenna is built using a metallic bar

Omni Directional

• The highest radiated power is in the direction perpendicular to the antenna and reradiated power drops as the angle increases above or below the horizon.

• Such an antenna is suitable for cell phone towers because most of the radiated power travels parallel to the horizon.

• In an omnidirectional cell, the BTS (centre-excited) is equipped with an antenna system that transmits and receives equally well in all directions. A theoretical circular shape of coverage will be achieved with this antenna

Omni Directional

Omni Directional

Sectorized Antenna

• By focusing the beam in a more focused area, offers greater range and throughput with less energy.

• Many operators will use sector antennas to cover a 360-degree service area rather than use an Omni directional antenna due to the superior performance of sector antennas over an Omni directional antenna.

• In sectorized cell, it can be either edge excited or centre excited. In a centre excited cell, it can be a 3x120o and 6x60osectors while in the edge excited, the BTS is located at the edge of the cell and provides coverage to multiple(3) cells.

Sectorized Antenna

Sectorized Antenna

Sectorized Antenna

• Advantages:• More power utilisation

• More user can access the channel

• Disadvantages• Need more antenna to cover 1 cell

• High cost

PART 3

CONCEPTS OF CELLULAR COMMUNICATION SYSTEM

4.3 Understand the concepts of cellular communication system.

• 4.3.1 Define Cell and Cluster.

• 4.3.2 Describe techniques to improve coverage and capacity in cellular systems.

a. Cell splitting

b. Sectoring

c. Microcell zone concept

• 4.3.3 Describe the term frequency reuse in cellular communication systems.

4.3 Understand the concepts of cellular communication system.

• 4.3.4 Describe the relationship between frequency reuse and cell splitting as techniques to maximize the traffic capacity of cellular systems.

• 4.3.5 Describe how to determine total number of channels in a given bandwidth.

• 4.3.6 Describe the Signal to Noise ratio (S/N ratio).

Cell and Cluster

CELL

• A cell is formally defined as an area where in the use of radio

communication resources by the MS is controlled by a BTS.

• The size and shape of the cell and the amount of resources

allocated to each cell dictate the performance of the system

to a large extent, given the number of users, average

frequency of calls being made, average duration of call time,

and so on.

Cell and Cluster

• Cell Area

Cell and Cluster

• In cellular communication, we used hexagon to represent the cell area

• Rural Area population Density is low; cell area is kept large

• Built up area population density is large cell area is small

Cell and Cluster

• The middle circles represent cell sites.

• This is where the base station radio equipment and their antennas are located.

• A cell site gives radio coverage to a cell.

• Most cells have been split into sectors or individual areas to make them more efficient and to let them to carry more calls.

• Antennas transmit inward to each cell

• It cover a portion or a sector of each cell, not the whole thing

Cell and Cluster

Cell Types

• The density of population in a country is so varied that different types of cells are used:

• Macrocells

• Microcells

• Selective cells

• Cluster:

• A set of hexagons (cells) can be packed in

clusters such that no two similar cell are

adjacent.

• Possible cluster sizes are 1,3,4,7,9,12, etc.

• Frequency can only be reused outside and

not within the same cluster.

Cell and Cluster

Cell and Cluster

4.3.2 Describe techniques to improve coverage and

capacity in cellular systems.

a. Cell splitting

b. Sectoring

c. Microcell zone concept

System Expansion Techniques

• As demand for wireless services increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users. More channels must therefore be made available per unit area.• This can be accomplished by dividing each initial

cell area into a number of smaller cells, a technique known as cell-splitting.

• It can also be accomplished by having more channels per cell, i.e. by having a smaller reuse factor. However, to have a smaller reuse factor, the co-channel interference must be reduced. This can be done by using antenna sectorization.

System Expansion Techniques --Sectorization

• Keep the cell radius but decrease the D/R ratio. In order to do this, we must reduce the relative interference without increasing the transmit power.

• Sectorization relies on antenna placement and directivity to reduce co-channel interference. Beams are kept within either a 60° or a 120° sector.


• If we partition a cell into three 120° sectors, the number of co-channel cells are reduced from 6 to 2 in the first tier.

• Using six sectors of 60°, we have only one co-channel cell in the first tier.

• Each sector is limited to only using 1/3 or 1/6 of the available channels. We therefore have a decrease in trunking efficiency and an increase in the number of required antennas.

• But how can the increase in system capacity be achieved?

System Expansion Techniques --Micro cells

• Micro cells can be introduced to alleviate capacity problems caused by “hotspots”.

• By clever channel assignment, the reuse factor is unchanged. As for cell splitting, there will occur interference problems when macro and micro cells must co-exist.

System Expansion Techniques --Micro cells

Cell Splitting

The process of subdividing a congested cell into smaller cells each with its own base station.

Lowering antenna height, antenna down tilting and reducing transmitter power.

Cell Splitting

• Increasing capacity by increasing the number of times that a channels reused

Cell splitting

• During splitting, the designer must minimize changes in the system

• The value of the reuse factor must be relatively prime with respect to the value of the type of split.

• Hence the voice channel frequency assignments at the old cell remain the same when new cell are added.

Cell splitting

• Cell splitting increases the number of BSs in order to increase capacity. There will be a corresponding reduction in antenna height and transmitter power.

• Cell splitting accommodates a modular growth capability. This in turn leads to capacity increase essentially via a system re-scaling of the cellular geometry without any changes in frequency planning.

• Small cells lead to more cells/area which in turn leads to increased traffic capacity.

Cell splitting

Cell splitting

• For new cells to be smaller in size, the transmit power must be reduced. If n=4, then with a reduction of cell radius by a factor of 2, the transmit power should be reduced by a factor of 24 (why?)

• In theory, cell splitting could be repeated indefinitely.

• In practice it is limited• By the cost of base stations

• Handover (fast and low speed traffic)

• Not all cells are split at the same time : practical problems of BS sites, such as co-channel interference exist

• Innovative channel assignment schemes must be developed to address this problem for practical systems.

Cell splitting

Sectoring

• Sectorization

• increased C/I (carrier-to-interference ratio, C/I) by eliminating some co-channel cell at the

• expense of reducing trunking efficiency

• cell size can be reduced

Sectoring

• Cell sectoring will improved the value of C/I by reducing the number of interferers.

• However, trunking effiency (the number of users which can be offered a particular GOS) will be reduced

• Trunking efficiency can also be measured in term of number of channel per sector (cell)

Microcell Zone Concept

• The increased number of handoffs required when sectoring is employed results in an increased

• load on the switching and control link elements of the mobile system.


• In this scheme, each of the three (or possibly more) zone sites (represented as Tx/Rx in Figure 4.1) are connected to a single base station and share the same radio equipment.

• The zones are connected by coaxial cable, fibre optic cable, or microwave link to the base station.

• Multiple zones and a single base station make up a cell. As a mobile travels within the cell, it is served by the zone with the strongest signal.

• This approach is superior to sectoring since antennas are placed at the outer edges of the cell, and any base station channel may be assigned to any zone by the base station.

Figure 4.1 The microcell concept


• As a mobile travels from one zone to another within the cell, it retains the same channel.

• Thus, unlike in sectoring, a handoff is not required at the MSC when the mobile travels between zones within the cell.

Figure 4.2 Illustration of how a distributed antenna system (DAS) may be used inside a building. Figure produced in Site Planner®. (Courtesy of Wireless Valley Communications Inc.)


• This technique is particularly useful along highways or along urban traffic corridors.

• The advantage of the zone cell technique is that while the cell maintains a particular coverage radius, the co-channel interference in the cellular system is reduced since a large central base station is replaced by several lower powered transmitters (zone transmitters) on the edges of the cell.

4.3.3 Describe the term frequency reuse in cellular communication systems.

Frequency Reuse

• Base stations in adjacent cells are assigned

channel groups which contain completely

different channels than neighbouring cells.

• The base station antennas are designed to

achieve the desired coverage within the

particular cell.

Frequency Reuse

• By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits.

Frequency Reuse

• The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.

Figure 4.2

Based on Figure 4.2:

• Illustration of the cellular frequency reuse concept.

• Cells with the same letter use the same set of frequencies.

• A cell cluster is outlined in bold and replicated over the coverage area.

• In this example, the cluster size, N, is equal to seven, and the frequency reuse factor is 1/7 since each cell contains one-seventh of the total number of available channels.

Frequency Reuse

• Figure 4.2 is conceptual and is a simplistic model of the radio coverage for each base station, but it has been universally adopted since the hexagon permits easy and manageable analysis of a cellular system.

• The actual radio coverage of a cell is known as the footprint and is determined from field measurements or propagation prediction models.

4.3.4 Describe the relationship between frequency reuse and cell splitting as techniques to maximize the traffic capacity of cellular systems.

The relationship between frequency reuse and cell splitting

• To understand the frequency reuse concept, consider a cellular system which has a total of S duplex channels available for use.

• If each cell is allocated a group of k channels (k < S), and if the S channels are divided among N cells into unique and disjoint channel groups which each have the same number of channels, the total number of available radio channels can be expressed as:

S = kN (4.1)


• The N cells which collectively use the complete set of available frequencies is called a cluster.

• If a cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of capacity and is given by

C = MkN = MS (4.2)


• As seen from Equation (4.2), the capacity

of a cellular system is directly proportional

to the number of times a cluster is

replicated in a fixed service area.

• The factor N is called the cluster size and

is typically equal to 4, 7, or 12.


• If the cluster size N is reduced while the cell

size is kept constant, more clusters are required

to cover a given area, and hence more capacity

(a larger value of C) is achieved.

• A large cluster size indicates that the ratio

between the cell radius and the distance

between co-channel cells is small.


• Conversely, a small cluster size indicates that

co-channel cells are located much closer

together.

• The value for N is a function of how much

interference a mobile or base station can

tolerate while maintaining a sufficient quality

of communications.


• From a design viewpoint, the smallest possible value of N is desirable in order to maximize capacity over a given coverage area (i.e., to maximize C in Equation (4.2)).

• The frequency reuse factor of a cellular system is given by 1/N, since each cell within a cluster is only assigned 1/N of the total available channels in the system.

4.3.5 Describe how to determine total number of channels in a given bandwidth.

• Example 4.1

If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses

a. four-cell reuse,

b. seven-cell reuse, and

c. 12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.

SolutionGiven:

Total bandwidth = 33 MHzChannel bandwidth = 25 kHz × 2 simplex channels = 50 kHz/duplex channelTotal available channels = 33,000/50 = 660 channels(a) For N = 4, total number of channels available per cell = 660/4 ≈ 165 channels.(b) For N = 7,total number of channels available per cell = 660/7 ≈ 95 channels.(c) For N = 12,total number of channels available per cell = 660/12 ≈ 55 channels.

A 1 MHz spectrum for control channels implies that there are 1000/50 = 20 control channels out of the 660 channels available. To evenly distribute the control and voice channels, simply allocate the same number of voice channels in each cell wherever possible. Here, the 660 channels must be evenly distributed to each cell within the cluster. In practice, only the 640 voice channels would be allocated, since the control channels are allocated separately as 1 per cell.

• (a) For N = 4, we can have five control channels and 160 voice channels per cell.

• In practice, however, each cell only needs a single control channel (the control channels have a greater reuse distance than the voice channels). Thus, one control channel and 160 voice channels would be assigned to each cell.

• (b) For N = 7, four cells with three control channels and 92 voice channels, two cells with three control channels and 90 voice channels, and one cell with two control channels and 92 voice channels could be allocated.

• In practice, however, each cell would have one control channel, four cells would have 91 voice channels, and three cells would have 92 voice channels.

• (c) For N = 12, we can have eight cells with two control channels and 53 voice channels, and four cells with one control channel and 54 voice channels each.

• In an actual system, each cell would have one control channel, eight cells would have 53 voice channels, and four cells would have 54 voice channels.

4.3.6 Describe the Signal to Noise ratio (S/N ratio).

Signal-to-Noise (S/N) Power Ratio

• The presence of noise degrades the performance of analog and digital communication.

• The extent to which noise affects the performance of communication systems is measured by the output signal to noise power ratio or SNR.

• Signal-to-noise ratio is defined as the power ratio between a signal (meaningful information) and the background noise (unwanted signal).

• Signal-to-noise ratio (SNR or S/N) is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise.

• SNRs are often expressed using the logarithmic decibel scale (dB).

http://en.wikipedia.org/wiki/Power_(physics)

http://en.wikipedia.org/wiki/Signal_(information_theory)

http://en.wikipedia.org/wiki/Noise_(electronic)

Noise Factor and Noise Figure

• Noise factor (F): The noise factor of a system is defined as;

• where SNRin and SNRout are the input and output

power signal-to-noise ratios, respectively.

• Noise figure (NF): a measure of degradation of the signal-to-

noise ratio (SNR), caused by components in a radio

frequency (RF) signal chain. It is the noise factor, given in

dB.

oo

ii

NS

NSF

Noise Calculation

• SNR is ratio of signal power, S to noise power, N.

• Noise Factor, F

• Noise Figure, NF

dBN

SSNR log10

oo

ii

NS

NSF

)(log10

log10

dBNS

NS

FNF

oo

ii

139

4.0 CELLULAR

COMMUNICATION SYSTEM

4.5

Understand the process of hand-over in cellular communication system.

140

4.5 Understand the process of hand-over in cellular communication system

4.4.1 Define the process of hand-over

4.4.2 Sketch the process of hand-over between cells.

4.4.3 Describe the relationship between Signal to Noise ratio

(S/N ratio) and the hand-over when the call is in progress.

4.4.4 Describe the types of hand-over.

a. Hard hand-over.

b. Soft hand-over.

4.4.5 Describe roaming and paging in cellular

communication system.

141

4.4.1 Define the process of hand-over

• When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station.

• This handoff operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station.

• Processing handoffs is an important task in any cellular radio system.

• Many handoff strategies prioritize handoff requests over call initiation requests when allocating unused channels in a cell site.

Handoff

• When a mobile user is engaged in conversation, the MS is connected to a BS via a radio link.

• If the mobile user moves to the coverage area of another BS, the radio link to the old BS is eventually disconnected, and a radio link to the new BS should be established to continue the conversation.

• This process is variously referred to as automatic link transfer, handover, or handoff.

Handover / Handoff

• Occurs as a mobile moves into a different cell during an existing call, or when going from one cellular system into another.

• It must be user transparent, successful and not too frequent.

• Not only involves identifying a new BS, but also requires that the voice and control signals be allocated to channels associated with the new BS.

Handover / Handoff• =handoff threshold -Minimum acceptablesignal to maintain the call

• too small:

• Insufficient time

to complete handoff

before call is lost

• More call losses

• too large:

• Too many handoffs

• Burden for MSC

145

Hard handoff between the MS and BSs.

146

Handoff

• Handoffs must be performed successfully and as infrequently as possible, and be imperceptible to the users.

• In order to meet these requirements, system designers must specify an optimum signal level at which to initiate a handoff.

147

Handoff acceptance signal level

• Once a particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station receiver (normally taken as between –90 dBm and –100 dBm), a slightly stronger signal level is used as a threshold at which a handoff is made.

• This margin, given by Δ = Pr handoff – Pr minimum usable, cannot be too large or too small. If Δ is too large, unnecessary handoffs which burden the MSC may occur, and if Δ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions.

148Illustration of a handoff scenario at cell

149

Understand the process of hand-over in cellular

communication system.

4.4.4 Describe the types of hand-over.

a. Hard hand-over.

b. Soft hand-over.

c. Softer hand-over

150

Hard handover• The definition of a hard handover or handoff is one where

an existing connection must be broken before the new one is established.

• One example of hard handover is when frequencies are changed.

• As the mobile will normally only be able to transmit on one frequency at a time, the connection must be broken before it can move to the new channel where the connection is re-established. This is often termed and inter-frequency hard handover. While this is the most common form of hard handoff, it is not the only one.

• It is also possible to have intra-frequency hard handovers where the frequency channel remains the same.

• Although there is generally a short break in transmission, this is normally short enough not to be noticed by the user.

151

Soft handover

• The new 3G technologies use CDMA where it is possible to have neighbouring cells on the same frequency and this opens the possibility of having a form of handover or handoff where it is not necessary to break the connection.

• This is called soft handover or soft handoff, and it is defined as a handover where a new connection is established before the old one is released.

• In UMTS most of the handovers that are performed are intra-frequency soft handovers.

152

Softer handover

• The third type of hand over is termed a softer handover, or handoff. In this instance a new signal is either added to or deleted from the active set of signals.

• It may also occur when a signal is replaced by a stronger signal from a different sector under the same base station.

• This type of handover or handoff is available within UMTS as well as CDMA2000.

153

4.4.5 Describe roaming and paging in cellular communication system.

154

Roaming • In short, roaming is a term used to describe

the ability of phones to connect to the

network of a different carrier, abroad or at

home in

• to offer users the same features they use

while on their “home” network – making and

receiving calls and text messages and surfing

the web.

155

Roaming

• Roaming is possible thanks to the

international agreements carriers have with

other carriers, in order to offer their wireless

services in other regions of a country or of

the world.

• There are various types of roaming agreements between carriers, with some of them being free, but most of them will bring extra charges to your monthly cell phone bill.

156

Roaming • Also worth remembering is that roaming

services have to be activated with some carriers in order for your phone to work abroad.

• So, if you plan to use the handset in other countries, you’ll have to enable the service with your mobile operator before departing.

157

Why is it so important for them to offer roaming features?

• First of all, it’s all about marketing.

• Each carrier, especially major ones, want their subscribers to know that they’ll be able to use the handset, which is purchased in most cases for a subsidized two-year contract, abroad and enjoy the same services.

158

Why is it so important for them to offer roaming features?

• And second of all, roaming agreements between carriers aren’t exactly controlled by a regulator (except in the EU), which means that mobile operators can jack up prices and raise their profit margins when it comes to charging for used voice minutes, SMS and MMS messages, and especially data used when roaming.

Roaming• When a mobile user moves from one PCS system

(e.g., the system in New York City) to another (e.g., the system in Los Angeles), the system should be informed of the current location of the user. Otherwise, it would be impossible to deliver the services to the mobile user.

• To support mobility management, protocols such as EIA/TIA Interim Standard 41 (IS‑41 or ANSI‑41) or Global System for Mobile Commu nications (GSM) Mobile Application Part (MAP) have been defined for PCS networks.

160

Roaming• A wireless roaming network has FIVE(5) components

that make it work:

1. A database for storing customer profile information such as features, dialing capabilities, and the home serving area identification. This is called the home location register (HLR).

2. A database of mobile numbers used by each switch on the network.

3. A signaling network for transmitting data messages between switches.

4. Routing specifications that direct the data messages to the appropriate destination.

5. Public long-distance connections for call delivery

161

Roaming

• A registration cycle keeps track of a phone as it travels around the network. It begins when a wireless user powers on their phone. The general steps for this process are:

• When the phone is powered on, it sends a data message to the cellsite. This data message contains the Mobile Identification Number (MIN or phone number) and the Electronic Serial Number (ESN). The cellsite forwards this information to the switch.

• The switch compares the MIN with a table of all MINs in the network. It will determine if the MIN belongs to a home customer, or to a visiting customer. In either case, the switch will request the subscriber's feature profile from the Home Location Register (HLR). The HLR for home customers may be integrated into the same switch or stored on a separate platform.

162

Roaming• If the HLR is a separate platform, or

if the customer is visiting from another system, the switch then sends a data message to the HLR across the signaling network.

• Routing specifications stored at Signaling Transfer Points (STPs) provide the necessary information to direct the message to the home location register.

163

Roaming • When the Home Location Register

(HLR) receives the message, it checks the MIN & the ESN.

• If the numbers are valid, the HLR records the location of the phone and returns a message containing the subscriber's feature list and calling restrictions to the visited switch.

164

Roaming

• Once the visited switch receives the return message, it creates a Visitor Location Register (VLR) to store information about the roamer, including the MIN, ESN, features, etc... This register will be used by the roamer as long as they are registered in the visited system.

Paging Systems

• Broad coverage for short messaging

• Message broadcast from all base stations

• Simple terminals

• Optimized for 1-way transmission

• Answer-back hard

• Overtaken by cellular

166

Paging

• Practically every cellular system has some kind of broadcast mechanism.

• This can be used directly for distributing information to multiple mobiles, commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one to one communication between the mobile transceiver and the base station.

• This is called paging. The three different paging procedures generally adopted are sequential, parallel and selective paging.

167

Paging

• The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE, cells are grouped into Tracking Areas).

http://en.wikipedia.org/wiki/GSM

http://en.wikipedia.org/wiki/UMTS

http://en.wikipedia.org/wiki/3GPP_Long_Term_Evolution

168

Paging

• Paging takes place by sending the broadcast message to all of those cells.

• Paging messages can be used for information transfer.

• This happens in pagers, in CDMA systems for sending SMS messages, and in the UMTS system where it allows for low downlink latency in packet-based connections.

http://en.wikipedia.org/wiki/Paging_(telecommunications)

http://en.wikipedia.org/wiki/CDMA

http://en.wikipedia.org/wiki/Short_message_service

http://en.wikipedia.org/wiki/UMTS

4.5 UNDERSTAND RADIO CHANNELS AND MODULATION

TECHNIQUES USED IN CELLULAR

COMMUNICATION SYSTEM.

169

Subtopic

4.5.1 Describe the types of radio channel. a. Control channel (CC).

i. Reverse Control Channel (RCC).

ii. Forward Control Channel (FCC).

b. Voice channel (VC).

i. Forward Voice Channel (FVC).

ii. Reverse Voice Channel (RVC).

170

Subtopic

4.5.2 Describe the modulation technique used in radio channel in 4.6.1.

4.5.3 Describe the cellular call procedures involved in making different types of calls.

a. Mobile to wireline

b. Mobile to mobile

c. Wireline to mobile

171

4.6 Understand radio channels and modulation techniques used in cellular communication system.

4.5.1 Describe the types of radio channel.

a. Control channel (CC). i. Reverse Control Channel (RCC).

ii. Forward Control Channel (FCC).

b. Voice channel (VC). i. Forward Voice Channel (FVC).


172

Operational Channels• In each cell, there are FOUR(4) types of

channels that take active part during a mobile call.

• These are:a. Control channel (CC).

i. Forward Control Channel (FCC).

ii. Reverse Control Channel (RCC).

b. Voice channel (VC).

i. Forward Voice Channel (FVC).


173

Forward Control Channel (FCC)

• Forward Control Channel (FCC): Control channels are generally used for controlling the activity of the call, i.e., they are used for setting up calls and to divert the call to unused voice channels.

• Hence these are also called setup channels.

• These channels transmit and receive call initiation and service request messages. The FCC is used for control signalling purpose from the BS to MS.

174

Reverse Control Channel (RCC)

• Reverse Control Channel (RCC): This is used for the call control purpose from the MS to the BS.

• Control channels are usually monitored by mobiles.

175

Forward Voice Channel (FVC)

• Forward Voice Channel (FVC): This

channel is used for the voice

transmission from the BS to the MS.

176

Reverse Voice Channel (RVC)

• Reverse Voice Channel (RVC): This is

used for the voice transmission from the

MS to the BS.

177

4.5.2 Describe the modulation technique used in radio channel in

4.6.1.

178

Modulation Basics

179

Modulation Basics

• The primary difference between analog and digital modulation is the source information.

• If the source is analog, the modulation is analog.

• If the source is digital, the modulation is digital.

• The carrier is always analog

• The modulated wave is always propagated through the air as a series of sine or cosine waves - not pulses!

180

Modulation and Demodulation

181

Modulation Basics

182

Types of Modulation

183

Amplitude Modulation (AM)

184

Frequency Modulation (FM)

185

Phase Modulation

186

Amplitude Shift Keying (ASK)

• Amplitude Shift Keying (ASK)

– The digital source bits are represented by different amplitude

levels

– Not commonly used alone in wireless

– The amplitude in a wireless environment is much more prone to

impairments than the frequency or phase of the signal

– ASK types of modulation are more commonly used in wire line

where amplitude doesn’t suffer so much degradation

187

Frequency Shift Keying (FSK)

• Frequency Shift Keying (FSK)– The frequency of the carrier is switched depending

on whether the source data is a “one” or “zero”

– GSM uses a type of FSK called Gaussian Minimum Shift Keying (GMSK)

188

Phase Shift Keying (PSK)

• Phase Shift Keying (PSK)

– The phase of the carrier is switched depending on

whether the source data is a “one” or “zero”

– Most current and future wireless systems use a

type of PSK

189

Quadrature Phase Shift Keying (QPSK)

• Quadrature Phase Shift Keying (QPSK)

– The modulated wave shifts between four phases,

90° apart to create a “00”, “01” “10” or “11”

– CDMA uses QPSK on Forward Link

190

Quadrature Amplitude Modulation (QAM)

• Quadrature Amplitude Modulation (QAM)

– ASK and PSK are combined

– More common on Microwave and wireline

technologies such as DSL

– 16-QAM shown in diagram. 256-QAM is also

possible

191

QPSK, BPSK and QAM

192

QPSK BPSK 16-QAM

193

4.5.3 DESCRIBE THE CELLULAR CALL PROCEDURES INVOLVED IN MAKING DIFFERENT TYPES OF CALLS.

MOBILE TO WIRELINE

MOBILE TO MOBILE

WIRELINE TO MOBILE

Introduction

• Phone calls over the cellular network requires the use of two full-duplex voice channels simultaneous user channels and control channels.

• RBS transmit and receive signals using channels called the forward control channel and forward voice channel.

• While the cellular unit is transmitting and receiving signals using the back channel and control channel speakers.

194

Ho w

to

cal l fr

o m

m ob ile

un it to

th e PS TN

(wi

rel

ine )

195

Cellular users dial the number he/she want, and click the 'SEND'. This will cause the number dialled and cellular dialler will be sent to the MTSO

If the number identification is valid , MTSO will connect the call to the PSTN network. Next to complete the guide called

Through RBS, MTSO will get the channel guide that is free and direct the cellular unit to tune it to the channel

After MTSO received confirmation that the mobile unit is tuned, mobile users will receive a progress tone from the MTSO.

MTSO will stop ringing when cellular users began to answer the call and conversation started.

How to call from mobile unit to the other mobile

units.

196

Mobile users who want to make a call, dialled the desired number and press

'SEND'.

MTSO will receive the caller's identification number and the number is then checked its authenticity and it determines whether the

mobile unit is in use(on-hook) or not (off-hook).

MTSO will send the command 'paging' to all users, then called RBS and will receive

instruction

Positive signals from users who are called will cause the MTSO try and get a free channel and

directing users ,manually tuned to their respective channels. Then the cellular units called

will ring.

MTSO will stop ringing when the called user answered and the conversation began to be

instituted by both users.

How to call from PSTN (Wireline)

to mobile units

197

MTSO received a call from the caller via the PSTN.

MTSO will check digit dialling and determine whether the cellular unit is

used or not.

If used (on-hook), the MTSO will switching 'paging' to mobile users and try to get the

free channels and direct him to tune to that channel

Mobile unit will send confirmation to the MTSO through the RBS channel and then send

a call progress tones which produce tones.

MTSO will stop ringing when it receives a positive signal that the mobile user has

answered the call and the conversation can begin.

cellular communication system

Education

mobile switching centre

msc coordinates communication

mobile stations

mobile units

mobile nature

subsystem nss msc

switching office

technical perspective