Wireless communication, UNIT 3, EC students, 8th sem

Wireless Communication: Unit 3 - Wireless Network Architecture and Operation

Prof. Suresha V, Dept. Of E&C E. K V G C E, Sullia, D.K-574 327 Page 1

UNIT-3

Wireless Network Architecture and Operation

Learning Objectives: Upon completion of this unit, the student should be able to

Understand the cellular concept and explain the advantages of frequency reuse.

Know a typical cellular cluster and explain the meaning of frequency reuse number.

Discuss how the capacity of a cellular system may be expanded.

Explain the difference between cell splitting and sectoring.

Discuss the use of backhaul networks for cellular systems.

Explain the concept of mobility management and discuss the operations it supports.

Discuss the concepts of power management and network security.

3.1 The Cellular Concept

(a). Introduction:

AT & T and Bell Labs offered first mobile telephone service.

Use high power BS Txs with elevated antenna provide a larger coverage area.

Typically 250 watt FM transmitter used paged a mobile when there was an incoming

call for the mobile.

The limitation of this system is limited users, no frequency reuse, cell congestion,

high power requirement.

Improvement: The main objective of cellular concept is to allocate more users in a

limited allocated spectrum. The basic system characteristics are

o Area divided into Cells, each served by base station with lower power transmitter

covers a few hundred meters in some cities..

o Each cell gets portion of total number of channels, neighboring cells assigned

different groups of channels in order to reduce the interference.

o Multiple lower-power base stations that service mobile users within their

coverage area and handoff users to neighboring base stations as users move.

(b). The cellular Advantage.

A large subscriber capacity and Efficient use of spectrum resources

Nationwide coverage & Adaptability to traffic density

Telephone service to both vehicle and portable user terminals including closed user

groups with voice dispatch operations with Toll quality

Affordability, which could eventually make it a mass-market service

Power requirement for mobile is less due smaller cell and low power transmitter

Longer battery life and smaller mobile station form factors.



(c). Cellular Limitation: Initial implement cost is large due to

Deployment of large no. of low power stations

Acquisition of lands for cell sites

The associated hardware like RBS TxrRXr , controller, Antennas and towers

NOTE: But the cellular concept allows a large enough increase in capacity to make

these operations economically feasible.

(D). Implementation of basic cellular architecture:

Instead of one base station covering an entire city, the city was broken up into cells,

or smaller coverage areas. Each of these smaller coverage areas had its own lower-power

base station. The radio channels must be allocated to these smaller cells in such way as to

minimize interference but at the same time provide the necessary system performance to

handle the traffic load within the cells.

(i). Frequency Reuse: ** (Jan-2015-10M)

It is a design process of selecting and allocating channel groups for all of the cellular base

stations within a system called frequency reuse. The key characteristic of a cellular network

is the ability to reuse frequencies to increase both coverage and capacity. Extensive

frequency reuse allows for many users to be supported at the same time. The frequency

reuse concept as illustrated in figure 3.1. Total spectrum allocated to the service provider is

broken up into smaller bands. A cell is assigned one of these bands. This means all

communications (transmissions to and from users) in this cell occur over these frequencies

only. Neighboring cells are assigned a different frequency band. This ensures that nearby

transmissions do not interfere with each other. The same frequency band is reused in

another cell that is far away. This large distance limits the interference caused by this co-

frequency cell.

Cluster 1 Cluster 2

Cluster 3



(ii). Cluster

It is a group of cell (see fig 3.1) that makes use of all the available radio spectrum.

Cluster has N cells with unique and disjoint channel.

Since adjacent cannot use the same frequency channels, the total frequency

allocation is divided up over the cluster and then repeated for other clusters in the

system.

The number of cells in a cluster is known as the cluster size or the frequency reuse

factor (1/N)

(iii). Illustration of cellular system capacity: An Example (June-2011-5M)

Consider service provider wants to provide cellular communications to a particular

geographic area. The provider is licensed = 5MHz. Each system subscriber bandwidth

(channel B.W) = 10 KHz.

If the service provider was to provide coverage from only one transmitter site, the

total theoretical number of possible simultaneous users = Total B.W/ Channel B.W

= 5 MHz/ 10kHz / user = 500 users.

If, however, the service provider implements a cellular system with 35 transmitter

sites, located to minimize interference and provide total coverage of area, determine

the new system capacity? Solution:

- Assume the cluster size N = 7

- The allocated B.W/cell= System B.W/ Number of cells in a cluster =5*106/7=714kHz

- Bandwidth per cell = 714 kHz.

- No. of cluster 35/7= 5.

- Each cell has a capacity =714kHz/10kHz/user = 71 users

- Total system capacity =35 cells*71 users/cell = 2485 users.

- This is a system capacity increase of =5 times.

Conclusion:

Smaller cells higher number of clusters higher Channel reuse higher

Capacity and + Lower power requirements for mobiles

o Undesirable factors: Additional base stations, More frequent handoffs and

Greater chance of ‘hot spots’

(iv). Cellular Hierarchy: It is created in the cellular system based on the cell size, as shown

in the below table. All type uses different radio link propagation & different technical cell

design concept.

Sl. No. Cell Type Cell diameter Operating Environment

1 Femtocells < 10 mts Personal Area network(PAN)

2 Picocells < 100 mt Indoor environment

3 Microcells 100-1000mts Outdoor to indoor and pedestrian

4 Macrocells >1000mts to Few Kms Vehicular and high antenna environment

5 Megacells Global coverage Vehicular and high antenna environment



3.2. CELL FUNDAMENTALS

(i).Introduction: A Cell is a small geographic area within which each cellular base station is

allocated a group of radio channels to be used.

The Footprint: The actual radio coverage of a cell and is determined from field

measurements or propagation prediction models.

o Although Real footprint is formless in nature, a regular cell shape is needed for

systematic system design.

o Why circle cannot be used to represent the coverage area of a base station? because

adjacent circles cannot be overlaid upon a map without leaving gaps or creating

overlapping regions.

o Thus, when considering geometric shapes which cover an entire region without overlap

and with equal area, there are three sensible choices: a square; an equilateral triangle;

and a hexagon.

(ii). Why Hexagonal model?

Figure 3.2 Use of hexagons to represent cellular coverage

A cell must be designed to serve the weakest mobiles within the footprint, and these are

typically located at the edge of the cell. For a given distance between the center of a

polygon and its farthest perimeter points, the hexagon has the largest area of the three.

By using the hexagon geometry, the fewest number of cells can cover a geographic region

The hexagon closely approximates a circular radiation pattern which would occur for an

Omni-directional base station antenna and free space propagation.



(iii). Reuse Number:

To gain the maximum reuse of the frequencies for a cellular system, cells are arranged in

clusters.

To determine the minimum size cluster that can be used it is necessary to calculate the

interference levels generated by the co-channel cells.

The reuse distance has been determined that relates cluster size N, cell radius R and the

reuse distance D.

The frequency reuse distance can be calculated by:

D = R (3N) 1/2

- Where R=cell radius and N=reuse pattern.

- Values of N can only take on numbers calculated from the following expression:

N = i2 + ij + j2, where i and j are integers.

Conclusion: If N decreased, then reuse distance D decrease, but Capacity C, Network

cost, Complexity, Handoff and Interference increase.

(iv).Relationship between cluster size (N) and reuse distance (D).

Example 1: For mobile system cluster size of 7, determine the frequency reuse distance if

the cell radius is five kilometers. Repeat the calculation for the cluster size of 4.

Solution:

Typical AMPS system uses a Cluster size of N=7, The reuse distance for cell is 5km.



Use the expression N= i2 + ij + j2, one can show that possible value for N is 7. As shown in

fig 4-4, the hexagons (cells) are arranged with one hexagon in the centre of a cluster

and six other hexagons surrounding the middle hexagon. Adjacent clusters repeat the

previous pattern. The re-use distance is found from the following equation:

(1). for N=7, wkt D = R (3N) 1/2 =5(3*7)1/2 = 5(21)1/2 = 5(4.5823) = 22.913km.

(2). For N=4, D = 5(3*4)1/2 = 5(12)1/2 = 5(3.464) = 17.32km. Hence a smaller cluster

size results in a smaller re-use distance.

(v). Cellular Interference Issues (S/I)

More complex calculation can yield the S/I ratio for a particular cluster size, N.

S/I ratio gives an indication of the quality of the received signal

Using simple mathematical model for S/I ratio calculations involving unidirectional cells

yield the results tabulated in the table below for several common values of N :

Table below shows Signal to interference (S/I) ratio for various cluster sizes.

Cluster Size, N S/I ratio

3 11.3 dB

4 13.8 dB

7 18.7 dB

12 23.3 dB

Smaller cluster sizes will yield a larger possible subscriber

From above table, the trade-off is a lowered S/I ratio and the corresponding decrease in

the radio link quality.

The AMPS system did not yield usable voice quality radio links unless an S/I ratio

exceeding 18dB was available.

This value of S/I was only possible for a cluster of size 7 and up. Therefore, the typical

AMPS system was deployed with a cluster size of N=7.

3.3 CAPACITY EXPANSION TECHNIQUES (Jan-15, July-14, Dec-12, May-10-10M)

Cellular capacity is a number of users in a cell.

The approaches to capacity expansion are either architecturally or technologically

enabled. They are

1. Cell splitting

2. Cell sectoring

3. Overlaid Cells

4. Channel allocation

5. Other capacity expansion schemes

o Lee’s Microcell Technology

o Smart Antenna Technology

o Migration to Digital Technology



1. Cell splitting

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

station and a corresponding reduction in antenna height and transmitter power)

Cell splitting preserves the geometry of the architecture and therefore simply scales the

geometry of the architecture

The increased number of cells would increase the number of clusters which in turn

would increase the number of channels reused, and capacity

Fig: 3.4 Increase capacities by cell splitting

Figure 3.4 shown assume that Cell A has become saturated and is unable to support its

traffic load. Using cell splitting, six new smaller cells with approximately one-quarter the

area of the larger cells are inserted into the system around A in such a way as to be

halfway between two co channel cells. These smaller cells will use the same channels as

the corresponding pair of larger co channel cells.

In order that the overall system frequency reuse plan be preserved, the transmit power

of these cells must be reduced by a factor of approximately 16 or 12dB.

As the splitting process moves toward completion the number of channels in the small

cells will increase until eventually all the channels in the area are used by the lower-

power group of cells and the original Cells A has had its power reduced and also joins the

new smaller cluster.

As traffic increases in other areas of the system this process may be repeated over again,

eventually the entire system will be resealed with smaller cells in the high-traffic areas

and larger cells on the outskirts of the system or in areas of low traffic or low population

density.

Conclusion:

Cell splitting effectively increases system capacity by reducing the cell size and therefore

reducing the frequency reuse distance thus permitting the use of more channels.



Advantages:

o Increases the system capacity.

o Reduces the cell size, frequency reuse distance.

o Increases the number of channels.

Disadvantages:

o Co channel interference increases

o Difficult to acquire appropriately located cell sites

o Prolonged conversion process, different cell size exists in the same area.

o No. of base station increases

o Trunking efficiency decreases and Handoff process increases

2. Sectoring

It increase capacity is to keep the cell radius unchanged and seek methods to decrease

the D /R ratio.

Uses directional antennas by replacing a single Omni-directional antenna at the base

station. It split the cell in to 3 new cells of 120o apart as shown in fig 3.5.

Figure 3.5: Increasing capacity by cell sectoring

It provides interference reduction, hence S/I ratio increases.

It does not require new cell sites and additional antennas and triangular mounting only.

Demerits: Increased network system architecture complexity

IIIustration of interference reduction due to cell sectoring as explained with fig 3.6 As shown in Figure 3.6 the sectoring of a cell results in a reduction in the amount of

interference that the sector experiences from its co channel neighbors in adjacent



clusters and conversely the amount of interference that the sector supplies to its co

channel neighbors.

Fig 3.6: Interference reduction due to cell sectoring

Before sectoring, for a cluster size of 7, a cell receives and gives interference to six other

nearest co channel cells in other clusters. Now, as shown by Figure for Cell AO, the

number of interfering cells has been reduced to two (AI and A2).

This results in a higher S/I ratio for that sector and its companion sectors in other

clusters. Table below tabulates these new values for a three-sector scheme for some

common values of cluster size.

Cluster Size N S/I Ratio in dB

3 16.08

4 18.58

7 23.44

12 28.12

Comparison Between Cell splitting and Cell Sectoring Sl

no. Cell Splitting Cell Sectoring

1 Cell splitting achieves capacity improvement by essentially rescaling the system.

It is an antenna technique to increase the system capacity

2 By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of channels per unit area.

It increase capacity is to keep the cell radius R unchanged and seek methods to decrease the D /R ratio

3 Here the number of handoffs decreases Here the number of handoffs increases



4 It increase in trunking efficiency It decrease in trunking efficiency

3. Overlaid Cells

It was first introduced in the section on cell splitting.

This method can be used to expand the capacity of cellular systems in two ways.

1. Split-band analog systems.

2. Reduced cluster size systems

1. Overlaid cells in a split-band system: A wideband analog system could be upgraded to

increase its capacity by overlaying and analog system with narrower bandwidth

requirements over it. Which is shown in figure 3.7

In such a split-band overlay system, channels are divided between a larger macrocell (using

AMPS or TACS) and the overlaid microcell (using NAMPS or NTACS) that is contained in its

entirety within the macrocell.

Figure 3.7 Overlaid cells in a split-band system

The channels assigned to the macrocell are used to service users in the area between the

microcells, and the channels assigned to the microcells service the microcells. With correct

system design of two areas just mentioned will be equal in size. The net effect of this design

is an increase in the total number of system channels since now the entire system

bandwidth is allocated to both the original wideband system and newer, more efficient

narrowband system. This type of system migration requires the use of dual-mode mobile

stations.

2. Overlaid cells in a reduced cluster size system: It may be applied to GSM or NA-TDMA

systems. As an example of this method, consider a system with a cluster size of N=4. On the

top of this system, a cluster of overlaid cells is applied with a cluster size of 3 as shown in

figure 3.8. If the channels for the overlaid cell cluster are taken from the underlaid cluster,

the system capacity increases since the area needed for the overlaid system for the overlay



system, the greater the increase in system capacity. This type of expansion allows operators

to migrate their systems using the same base station and mobile station equipment.

Figure 3.8 Overlaid cells in a reduced cluster size system

1. Channel allocation

Need for channel allocation to handle random traffic with different scenarios of

activities that might cause the amount of traffic to change. They are

Traffic in each cell is dynamic: For example

o During the events like rock concerts and sporting events, the amount of traffic

offered to cellular systems can change drastically for short periods on the scale of

hours.

o Other events like golf tournaments or state fairs could change traffic intensity for

a week or longer.

o The business district of a metropolitan area may experience changing levels of

traffic over the course of workday.

o The first scenario is so extraordinary that it is very difficult to design anything in

to the system to handle the extremely large increase in traffic offered to the

system. In many such cases, cellular providers will bring in portable cellular sites

(sometimes known as “cells on wheels” or COWs) to handle the increased

demand.

o A national cellular service provider may have dozens of COWs that are deployed

all over the country at any given time.

o COWs are also deployed during natural disasters to restore disrupted

communications.



o On the other hand, the traffic scenario within the business district can be dealt

with to some degree through channel allocation techniques.

Three main methods to achieve efficient channel allocation

1. Fixed channel scheme: The procedures of this scheme are

o Examines system wide traffic patterns over time.

o Allocating additional channels where needed to “fine-tuning” of the system. This means

that instead of equally dividing up the channels over the cells, some cells will receive

larger channel allocations than others.

o Use very complex algorithms to determine the final allocation of channels, and these

allocations are periodically updated as a traffic usage database grows and new patterns

of use emerge and periodically update this process.

2. Channel borrowing scheme: This scheme performs

o A high-traffic cell can borrow channels from low-traffic cells and keep them as needed or

until the offered traffic returns to normal.

o While borrowing channel, it should not effect on performance of the borrowed cell.

o After the traffic over the borrowed channel is complete, the channel is returned to use in

its original cell.

3. Dynamic channel allocation (DCA): It performs

o All the available channel are placed in channel pool

o Each channel assigned new call based on Signal to interference statistics

o Each Channel can used by each cell until SIR is met.

o This is an extremely complex system that uses many network resources to accomplish

its operation.

6. Other capacity expansion schemes

1. Lee’s Microcell technology

2. Smart Antenna Technology

3. Migration to newer technology

1. Lee’s Microcell technology

o In sectoring increases handoffs increasing loads on switching elements

o Here use zones instead of sectors

o It uses 3 antennas that provide coverage by “looking” into the microcell (see fig3.9).

o All 3 antennas are connected to the same BS by high-speed microwave or fiber links



o The antenna with the best reception of the mobile is used for both the uplink and

downlink.

Fig 3.9: Lee’s microcell concept for capacity expansion

o As the mobile travels with in the microcell the same channel can be used and there is

no need for handoff operations, hence it reduce number of hand offs

o As the mobile moves into another zone the base station simply switches the channel

to a different zone.

2. Smart Antenna Technology

o Not yet implemented. It is proposed for 3G techniques

o A BS could direct a narrow beam of radio wave at a particular mobile station

o Reuse the same channel over another narrow beam (smart antenna use phased array

technology, i.e, Adaptive steered antenna are ued).

o This technology is referred to as SDMA (Space Division Multiple Access)

o Many systems have space diversity to select the best signal out of two or more signals.

3. Migration to newer technology

o 1G need S/I=18dB for example AMPS and 2G need S/I=12dB example GSM,NA-

TDMA

o Newer technology is based on digital modulation technique ,e.g. 3G services

o 2G systems use TDMA and CDMA to achieve greater capacity

o Comparison of TDMA and CDMA

TDMA CDMA

1. Multiple time slots per multiple users.

1. Multiple users use the same channel simultaneously.



2. Immune to noise and interference

2.Interference handling capacity is inherent

3.Provide service with low S/I 3. Good S/I ratio

3.4 Cellular Backhaul Networks

o Backhaul – “Getting data to the network backbone or transmitting from a remote site

or network to a central or main site.

o The backhaul of 1G cellular system

The MSC and BS were typically connected together using T1/E1/J1 lines.

The connection between MSC & BS carried PCM-encoded voice signals at 64kbps.

T1/J1 can handle 24 voice band calls and an E1 can handle 30 voice calls.

o For 2G cellular system

Voice signals are transcoded at rate 8kbps or 16 kbps at BSC and sent over

T1/E1/J1 facilities at either allowing as many as 192 voice channels.

Fiber optic cables between MSC and PSTN, traffic was typically aggregated and, if

wanted, usually sent over larger T3 facilities that could provide for high traffic.

o For 2.5G, 2.5+G

o Cellular operator use own private wideband networks to backhaul both voice and

data between MSC and BS and from BSs to the BSCs and finally to the MSCs in an

effort to reduce costs.

o CDMA Cellular system data network connections

CDMA systems maintained the connection between the MSC and the BSC for

voice traffic.

Inter working function and packet data service node (IWF/PDSN) network

element that connects directly to the external packet network and BSC. As

shown in the fig 3.10



Fig 3.10: CDMA Cellular system data network connections

o The IWF/PSDN node is responsible for proper protocol conversion and mapping

between the wireless network and the external packet network.

o GSM cellular system data network connection

GSM introduced packet-switched data services GPRS.

Here addition to the traditional GSM network components, a GPRS public land mobile

network (PLMN) has been added that interface to PDN as shown in fig 3.11.

Fig 3.11: GSM Cellular system data network connections

Through this GPRS PLMN, the GSM subscriber is to access Web sites through public

server or corporate intranets through private enterprise server.

Voice services are supplied through the traditional GSM PLMN.

3.5 Mobility management ***



It is important characteristics of wireless communication system are the ability to

provide mobility to the user.

It explains how the network knows where the subscriber is ( Location Management) and

how it keeps track of and in contact with the mobile station as the user moves from one

cell to another (Hand Off Management)

Mobility management = Location management + Hand off management

1. Location management*** (Dec-12-10M, June-2010-10M) It is the process of keeping track of the present or last known location of the MS and

delivery of both voice and data to it is move around.

Its main objectives are

1. Provide continuous radio link

2. Direct the packet in a network

3. Determine MS status in network

4. Check availability of the MS

Basic functions performed by Location management:****

1. Location updating

2. Paging messages

3. Transmission of location information between network elements

1. Location Updating

o It is performed by MS

o After initial power up registration, the MS and BS will exchange their identification

information.MS attached to a BS and is located initially and

o Periodically checked for changes

o MS sends update message every time it changes point of access(AP) in a network and

exchange information for handoff.

o If a connection fails, systems page group of surrounding stations to track a MS

o It tuned on with new registration. Balance required between number of update

messages and number of cells to be paged

o Greater degree of certainty in locating the MS & Call blocking due to frequent paging.

o Two types of updating schemes

1. Static: It uses geographic layout determines updating requirements. Most

of cellular service provider uses this method as shown in figure 3.12

2. Dynamic :it takes user’s mobility determines updating requirements



Fig 3.12: Cellular location updating.

o In this approach, a group of cells is assigned a location area identification value (LAI).

o Each BS in the LA broadcasts its ID number in a periodic fashion over a control channel.

o The MSs that are attached to the base station within the LA are required to listen to the

control channel for the LA ID.

o If the LAID changes, the MS will have to send a location update message to the new BS.

o The BS will forward the updated information to the VLR database location in the fixed

position of the wireless network. Now, if their message for an MS, a paging message will

be sent to all the cells in the LA where the MS is listed as being present.

o The MS, unless it has moved in to another LA, will respond to the paging message.

o Drawback: PING-PONG Effect: This effect can occur if the mobile is moving in a path that

takes it back and forth between the borders of 2 LA s.

2. Paging messages

o Incoming call or message to MS initiates paging of the mobile

o Different paging schemes are

a) Broadcasting message: send a message either to cell or to a group of cells that is

meant to bring a response from single particular mobile. This response will start the

process by which communication between the PSTN or the PDN will be established

with the mobile. The paging of the mobile is more efficient if the exact cell the mobile

registered in is known. However, as pointed out, this information is not always

available, hence use next strategies.

b) Blanket paging: This type paging will be broadcast to all cells in a particular

location area. If successful the mobile will respond after the first paging cycle and

delay kept to a minimum. Otherwise, a scheme of sequential paging is used.

c) Sequential paging: Paged to the cell where it was last registered. Handoff initiated



when power from current RBS drops. Reduce ping-pong effect. Handover to and fro

between a cell pair frequently. Solution is to define threshold. Fine tuning algorithm

to improve system performance. Provide required QOS continuity during handoff

3. Transmission of the location information between network elements

For location updating to work correctly in a wireless network, there must exist

several data bases where MS information can be stored and accessed by the network

as needed.

When MS is turned on by a subscriber’s home network area, the home VLR will

temporarily store the uses information.

Within a particular network there are usually several MSCs used to support

network’s operation. Depending up on the particular network topology, each MSC

may contain HLR and VLR data base function are alternately single HLRs (configured

as a MSC/HLR/VLR) might service a group of MSC/VLRs (See figure 3.13).

For a small system another possibility is that a gateway MSC (GMSC) might house the

HLR function for a group of integrated MSC/VLRs. A gate way MSC is an MSC that

interface the mobile network with other network such as PSTN.

Fig 3.13: A typical Cellular system.

o Let us examine several possible scenarios that could occur during the operation of a

wireless network.

o The first possibility the user turned on a mobile within his or her home area.

The mobile registers with the VLR for the home area.

The co-located or system HLR confirms that the subscribers has network privileges.

Communication between remote HLR and MSC/VLR occurs using particular

signalling protocol over an SS7 network.



o The second case would occur when the user is away from his or her home location. Now

mobile registers with VLR of another MSC or a ‘’foreign’’ network. .

In these situations, the MSC/VLR must send a message to the subscribers HLR to

verify authentication information about the mobile.

The HLR will respond to the request by transmitting the information back to the

requesting MSC/VLR over SS7 signalling network.

A few comments about the communications between MSC/VLRs and VLRs are

appropriate here.

For a GSM cellular system and most other modern, the SS7 system is used to

communicate these massages using MTP and SCCP signalling.

2. Handoff management

o Handoff: If a subscriber moves from one cell to another, the cellular system must have

the ability to reconfigure the connection to the mobile from the current base station to

the new BS in the new cell. This connection hand over process is called hand-off.

(i). Handoff control: The algorithm used to determine when to make a handoff can be

located in a network element or in a mobile terminal. Two major types:

1. Network controlled handoff or NCHO (Jan-2015-06M, June-2013-10M)

2. Mobile-controlled handoff or MCHO

(ii). Parameters measured/used in the handoff algorithms: RSS (received signal

strength), System path loss, S/I ratios, BER (bit error ratio). Traditional handoff algorithms

would initiate handoffs when the power received from the current RBS dropped below that

received by another nearby RBS as shown in figure 3.14.

Figure 3.14 Typical handoff algorithms using RSS measurements



(iii). Handoff management:

o Cellular service provider engineers are continually fine tuning system handoff

algorithms to improve system performance.

o Measures of system performance might include

Call blocking and call dropping probability.

Required time to complete a handoff,

System handoff rate.

(iv). Typical cellular handoff operation ( Jun-2011-10M)

o Handoff management requires the transmission of messages between various network

elements to facilitate the handoff process. As depicted in figure 3.15

Steps involved in the operation:

1. Signal power levels being received by the current and handoff candidate radio BSs and

the MSs are first relayed to the RBS and then to the BSC.

2. When these levels meet the criteria for a handoff, the process is initiated. a handoff

message is sent to the mobile from the current RBS that request the MS register with a

new RBS that is also identified in the message.

Figure 3.15 Typical cellular handoff operations

1. When the mobile performs this task, the MSC\VLR is updated to reflect the new mobile

point of attachment and any other changed system parameters.



2. If the MSC\VLR most recently registered with is not the same as the last, then the new

VLR must send an interrogation message to the home HLR to obtain the subscriber

profile and authentication information.

3. The HLR responds over the SS7 network with the authentication information.

4. If the mobile is authenticated, then the RBS sends a message to the mobile assigning a

new pair of traffic channels to the MS and the RBS for the continuation of a voice

conversation.

5. The HLR database is updated so that it knows where the mobile is and the new VLR

database adds the new mobile to its list of subscriber terminals that are being serviced

by the particular MSC/VLR.

6. As a last act, the HLR sends a message to the old MSC/VLR to purge the mobile from its

list of actively attached subscriber terminals.

7. Additionally, any data packets that were intended for delivery to the MS from the old

MSC/VLR that may have been placed in a temporary network storage area should be

either deleted or redirected to the new MS access point.

3. 6. Power and Radio resources management

1. Power management (July-2014-10M, June-2013-10M)

o Transmission powers represent a key degree of freedom in the design of wireless

networks.

o Power management includes Interference management, Energy management and

Connectivity management.

o We know that cellular systems the use of many closely spaced low-power RBSs allows

for frequency reuse and hence increased system capacity. At the same time, Interference

also increases.

o Interference can reduce by use of power control algorithms for the adjustment of the MS

output power and RBS output power allow for nearly constant received signal strength

at both the MS and RBS receivers.

o This use of power control provides several system advantages: the amount of co channel

interference (CCI) is reduced.

o The last advantage has additional ramifications in the reduction of battery requirements,

which translates to longer time between charging and lighter and smaller mobile

terminals.

o Types of power control



1. Open-loop power control

Depends solely on mobile unit

No feedback from BS

Not as accurate as closed-loop, but can react quicker to fluctuations in SS.

2. Closed-loop power control

Adjusts signal strength in reverse channel based on metric of performance

BS makes power adjustment decision and communicates to mobile on control

channel

Power control algorithm: Design objectives are

Achieve SIR tolerance with good quality communications

Must constantly adjust to change in RSS caused by fading or mobility of MS

Usual Power control algorithm has 2 phases

Phase I: MS registers with BSS, Determine minimum output power, Avoid

possibility of a call drop

Phase II: Additional measurements to reduce power, Output power of RBS is

adjusted, Use complex algorithms achieve maximum SIR for all radio links.

Power saving schemes***

1. Discontinuous transmission (DTX)

Transmit during speech only, Extra over head need

Compensate low-power background during silence. It is adopted by MS, TRC, BSC

2. Sleep mode

During no activity

RF circuitry is powered off

Periodical awakening

3. Energy efficient designs

Semiconductor technologies

Power efficient modulation schemes

Software/hardware design

DSP technology

2. Radio Resources Management (RRM)

It is the system level control of co-channel interference and other radio transmission

characteristics in wireless communication systems

Two types of RRM

1. Static RRM: Involves manual as well as computer aided fixed cell planning or radio



Network planning.

2. Dynamic RRM: Adaptively adjust the radio network parameters to the traffic load, user

Positions, Quality of service requirements, etc.

o Provide functional improvements for RF operation

o Implement system power control to reduce interference

o Maximize capacity from above concept

o Best available radio channel selection

o Use wireless radio resource management scheme to enable handoff operations

4.7 Wireless network security(May-2010-4M)

Security Issues In Cellular Networks

1. Authentication, Integrity, Confidentiality, Access Control, Operating Systems

2. Web Services, Location Detection, Viruses And Malware, Downloaded Contents

3. Device Security

Network security: Techniques

◦ Encryption :Scrambling using key, Use secret key algorithms.

Prepared By: Prof.Suresha V, Professor, E&C Dept. KVGCE, Sullia.

Email:[email protected]. Cell No: 94485 24399.date:31-03-2015

Date post:	21-Jul-2015
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Wireless communication, UNIT 3, EC students, 8th sem

Education