Abstract

To provide quality of services to the end users during vertical handoff period,

heterogeneous wireless networks have to be aware of quality of services (QoS) within

each access network. The traditional vertical handoffs algorithms are based on

received signal strength (RSS) are not of QoS concerned and hence cannot fulfill the

requirements of the users. Here, I propose a new vertical handoff algorithm which

uses received signal to inference plus noise ratio (SINR) from various access

networks as the handoff criteria. In this algorithm, the SINR from one network is

converted to the equivalent SINR of the target, so that the handoff algorithm can have

the knowledge of achievable bandwidths from both access networks to make handoff

decisions with QoS consideration. Moreover, power of the mobile station is controlled

to maintain the SINR so that number handoff can be minimized due to ping pong

effect. It has been observed that SINR based vertical handoff algorithm can

consistently offer the end users with maximum available bandwidth during vertical

handoff contrary to the RSS based vertical handoff algorithms. Also, it is observed that

the performance of RSS based handoff is different in different network conditions as

against the SINR based algorithm. System level simulations also reveal the

improvement of overall system throughputs using SINR based vertical handoff,

compared to the RSS based vertical handoff.

1

Introduction

The popularity of wireless communication is increasing quite rapidly through out the

world after the introduction of cellular and broadband [2] technologies. The real

potential of broadband wireless networks lies with mobility. A hot debate is centered

on building metropolitan area networks using WiMAX (Worldwide Interoperability for

Microwave Access)[1] technology based on the IEEE 802.16 standards. The demand

of broadband and cellular technology is increasing due to its superior quality of

services (QoS), greater coverage area as well as low cost effectiveness. The success

of Wi-Fi network with IEEE 802.11x technology makes it possible to access

broadband anywhere with low cost. The introduction of broadband wireless WiMaX

solution based on IEEE 802.16 technology makes it possible a standard based low

cost solution for the last mile. In particular, with its coverage of 30 miles and non line

of sight technology based on OFDM, it will be able to construct a metropolitan network

where broadband access from anywhere within the area is possible. With the inclusion

of mobility, WiMaX could become the ultimate solution that provides a low latency,

high bandwidth, and wide area connectivity to mobile users which is long sought after

by the industry. A metropolitan network will cover an area of up to 30 miles. Current

study shows that the effective range for broadband coverage under IEEE802.16a is 4

to 5 miles. The eventual network might be composed of many base stations

connected together to provide broadband connectivity to hundreds of stationary and

mobile users. The intended applications of such a network are real-time media

streaming and VOIP. The network must guarantee that the continuous services will

not be disrupted while a mobile user switched its connectivity from one station to

another due to signal fading or change of provider. The effectiveness of mobility

depends on whether a moving node can maintain continuous connectivity with the

base station without packet loss or delay during handoff. One characteristic is the

handoff distance which specifies the minimum coverage between adjacent base

stations for a moving node at maximum specified speed. Due to the proliferation of

existing wireless technologies, a metropolitan network will consist of various wireless

accessing technologies with different link speed and mobility support. In case WiMAX

2

becomes the major broadband service provider to the metropolitan area and GPRS

(General packet radio service)[10] the major cellular service provider, users must be

able to easily roaming among different technologies without interruption. The success

will depend on the integration of mechanisms to deal with handoffs. Within a

metropolitan network, a mobile user could switch between different access

technologies due to coverage and provider changes, like GPRS.

WiMaX and GPRS are viewed as the future complementary access technologies.

From one side, UMTS (Universal Mobile Telecommunication System) core network

GPRS that uses GSM[10] (Global System for Mobile Communication) technology is

capable of providing data transmission with medium speed over wide area, supporting

high numbers of mobile users. On the other side, IEEE 802.16 broadband network,

WiMaX can offer high data rates relatively in large geographical areas as well as high

data rate as compared to the cellular GPRS and are expected to be widely deployed

in the future network generation. The main problem of next generation network is to

seamlessly transfer the connection of a mobile host exiting the coverage of the GPRS

to another access network with larger coverage area like WiMax. In other words,

interoperability is needed to support the mobile users between GPRS, with

mobile internet access, keeping the connection on line when moving to WiMax

access network, thus providing always on connectivity and vice versa. But the main

issue will be to provide fast vertical handover between these heterogeneous access

networks of larger coverage area, considering the quality of service (QoS), continuous

service as well as cost effectiveness. Therefore, better algorithm is necessary in the

handover procedure instead of the received signal strength (RSS) based algorithm. It

has been experimented that SINR is better than the RSS based since, it considers the

noise and interference factors in the background of the networks. Vertical handoff is

work of my thesis using SINR based approach. Moreover step is taken to control the

transmission power of the mobile to maintain the SINR for reducing the number of

handoff and saving the battery power depending on the noise and interference

present.

3

Chapter 1

1. Aim of the work

Vertical handoff process is always associated with two networks of different

technologies. Maintaining continuous and quality of service to a mobile user is more

challenging task in a vertical handoff process. The aim of the work is to make vertical

handoff between WiMaX and GPRS without compromising the continuity and quality

of service to the users. Normally, received signal strength (RSS) from a mobile user is

considered as the means of handoff decision. But, it has a lot of pitfalls. Therefore,

instead of RSS, another approach known as Signal to noise plus interference ratio

(SINR) has been used as the means of handoff decision between WiMaX and GPRS

to reduce the inefficiency associated with RSS based approach. Moreover,

transmission power control mechanism has been introduced for the mobile to maintain

its SINR for effective throughput of the system. The calculation of SINR in mobile

station needs the amount of noise and interference associated with the background.

1.1 Organization of the dissertation

The rest of the thesis is organized in the following way:

Chapter 2 includes related works

Chapter 3 includes background works of a handoff process

Chapter 4 includes a brief description about GPRS and WiMaX Technology

Chapter 5 includes the proposed algorithm

Chapter 6 includes the simulation result

4

Chapter 2

1. Related work

This work consists of the summary of different papers of different authors, related

to the work carried out by me. The main aim of this chapter is to bring in to focus

the related topics of my work. Following are the related work taken into

consideration for carrying out work.

a) Power Control by Interference Prediction for Broadband Wireless Packet

Networks [3]: A Kalman-filter method for power control is proposed for

broadband, packet-switched TDMA wireless networks. By exploiting the temporal

correlation of co-channel interference, a Kalman filter is used to predict future

interference power. Based on the predicted interference and estimated path gain

between the transmitter and receiver, transmission power is determined to achieve

a desired signal-to-interference-plus-noise ratio (SINR). A condition to ensure

power stability in the packet-switched environment is established and proven for a

special case of the Kalman-filter method. The condition generalizes the existing

one for a fixed path-gain matrix, as for circuit-switched networks. Specifically,

power control has been shown to be a useful technique to improve performance

and capacity of time-division-multiple-access (TDMA) wireless networks. In

addition to performance improvement, power control is actually essential in solving

the near-far problem in code-division-multiple-access (CDMA) networks. In this

paper, we focus on broadband packet-switched TDMA networks with data rates up

to several megabits per second. The advantage of the Kalman filter is that it is

simple, due to its recursive structure and robust over a wide range of parameters,

and it possibly provides an optimal estimate in the sense of minimum mean square

error. Kalman filters have been applied successfully to many systems [BH97]. As

for wireless networks, [DJM96] proposes using a Kalman filter for call admission in

CDMA networks. But, here in this report it is shown that Kalman filtering is also

useful for power control in TDMA networks.

5

b) Combined SINR Based Vertical Handoff Algorithm for Next Generation

Heterogeneous Wireless Networks [4]: Next generation heterogeneous wireless

networks offer the end users with assurance of QoS inside each access network

as well as during vertical handoff between them. For guaranteed QoS, the vertical

handoff algorithm must be QoS aware, which cannot be achieved with the use of

traditional RSS as the vertical handoff criteria. In this paper, the author of this

paper proposes a vertical handoff algorithm which uses received SINR from

various access networks as the handoff criteria. This algorithm considers the

combined effects of SINR from different access networks with SINR value from

one network being converted to equivalent SINR value to the target network, so

the handoff algorithm can have the knowledge of achievable bandwidths from both

access networks to make handoff decisions with QoS consideration. His analytical

results confirm that the new SINR based vertical handoff algorithm can

consistently offer the end user with maximum available bandwidth during vertical

handoff contrary to the (received signal strength) RSS based vertical handoff,

whose performance differs under different network conditions. System level

simulations also reveal the improvement of overall system throughputs using SINR

based vertical handoff, comparing with the RSS based vertical handoff. Having the

relationship between the maximum achievable data rate and the receiving SINR

(γAP) from both WLAN and WCDMA (γBS) makes the SINR based vertical handoff

method applicable, in which the receiving SINR from WCDMA γBS is being

converted to the equivalent γAP required to achieve the same data rate in WLAN,

and compared with the actual receiving SINR from WLAN. With the combined

effects of both SINR being considered, handoff is triggered while the user is getting

higher equivalent SINR from another access network. It means that given the

receiver end SINR measurements of both WLAN and WCDMA channel, the

handoff mechanism now has the knowledge of the estimated maximum possible

receiving data rates a user can get from either WLAN or WCDMA at the same time

within the handover zone, where both WLAN and WCDMA signal are available.

This gives the vertical handoff mechanism the ability to make handoff decision with

multimedia QoS consideration, such as offer the user maximum downlink

6

throughput from the integrated network, or guarantee the minimum user required

data rate during vertical handoff.

c) SINR Estimation for Power Control in Systems with Transmission

Beamforming [5]: The author of this paper takes a unified approach to the

downlink transmission power control, while a transmission beamforming is applied

in the base station. The proposed scheme is based on the estimation of signal-to-

interference-and-noise-ratios (SINRs) by antenna array measurement and using

this estimate in transmission power control. Hence power control algorithms

needing SINR-levels can be applied instead of the simple relay power control. The

SINR estimation technique does not require any additional measurements

compared to a separate adaptive beamforming and power control, since the

required measurements are needed for the adaptive beamforming update. The

estimation is based on the knowledge of the level of caused interference to the

multiple access links in the same cell, and the utilization of relay power control

commands in SINR estimation. Adaptive beamforming is an antenna array

technique used to reduce the interference experienced by the receivers. Antenna

array is a group of antennas in the transmitter or in the receiver of a radio link This

method considers systems with transmission beamforming in base stations. The

interference reduction of transmission beamforming is based on spatial filtering, in

which transmitted waveforms are either amplified or cancelled depending on the

directions of departure to the antenna array.

d) On the Use of SINR for Interference-aware Routing in Wireless Multi-hop

Networks [6]: This work considers the problem of mitigating interference and

improving network capacity in wireless multi-hop networks. An ongoing aim of this

research is to design a routing metric which is cognizant of interference. To

address this issue, and based on the measurement of the received signal

strengths, we propose a 2-Hop interference Estimation Algorithm .With the use of

the received signal level, a node can calculate the signal to interference plus noise

ratio (SINR) of the links to its neighbors. The calculated SINR is used to infer the

7

packet error rate (PER) between a node and each of its first tier interfering nodes

set. Then, the residual capacity at a given node is estimated using the calculated

PERs. Based on the capacity estimation analysis, a new routing metric, EBC

(Estimated Balanced Capacity), is proposed. EBC uses a cost function at the aim

of load-balancing between the different flows within the network. Extensive

simulations show that EBC improves tremendously the network capacity and also

enhances the VoIP calls quality.

e) Vertical handover criteria and algorithm in IEEE 802.11 and 802.16 hybrid

network [7]: Hybrid networks based, for instance, on systems such as WiMAX

and WiFi can combine their respective advantages on coverage and data rates,

offering a high Quality of Service (QoS) to mobile users. In such environment,

WiFi/WiMAX dual mode terminals should seamlessly switch from one network to

another, in order to obtain improved performance or at least to maintain a

continuous wireless connection. This paper proposes a new user centric algorithm

for vertical handover, which combines a trigger to continuously maintain the

connection and another one to maximize the user throughput (taking into account

the link quality and the current cell load). This aims of this paper are defining an

efficient user-driven vertical and over mechanism which does not require any

change on network and protocol architecture, and that can furthermore e easily

applied in current WiFi/WiMAX hybrid systems. To is purpose, the author has

introduced the estimation of two common network performance parameters, data

rate and network load, based on a measurement of Signal to Interference-plus-

Noise Ratio (SINR) level and channel occupancy respectively. Then they propose

a novel algorithm which embeds two independent triggers: the first one aims at

maintaining the wireless connection, the second one at maximizing the network

performance.

f) A Performance Evaluation of Vertical Handoff Scheme between Mobile

WiMax and Cellular Networks [8]: This paper proposes a cost-effective scheme

for fast handoff between mobile WiMax and cdma2000 networks. The smoothly

8

integration scheme proposed in this paper adopts the advantages of both loosely

integration and tightly integration schemes: cdma2000 and Mobile WiMax

networks provide their own services independently and, on vertical handoff

between them, support seamless services by fast handoff. Since we present

protocol stacks as well as operation flows considering cdma2000 and Mobile-

WiMax standard specifications, the proposed scheme can be implemented with

minimal modification of existing Mobile WiMax and cdma2000 networks. As result

of simulation, the performance of the proposed scheme is proved compared with

others.

9

Chapter 3

3. Background of the work

The background of the work covers those which involves in carrying out the handoff

between two networks. Before performing handoff it is important to know these

activities to carry out the handoff operation smoothly.

3.1 Handoff in Wireless Mobile Networks

Mobility is the most important feature of a wireless cellular communication system.

Usually, continuous service is achieved by supporting handoff (or handover) from one

cell to another. Handoff is the process of changing the channel (frequency, time slot,

spreading code, or combination of them) associated with the current connection while

a call is in progress. It is often initiated either by crossing a cell boundary or by

deterioration in quality of the signal in the current channel. Handoff is divided into two

broad categories hard and soft handoffs. They are also characterized by “break before

make” and “make before break.” In hard handoffs, current resources are released

before new resources are used; in soft handoffs, both existing and new resources are

used during the handoff process. Poorly designed handoff schemes tend to generate

very heavy signaling traffic and, thereby, a dramatic decrease in quality of service

(QoS). The reason why handoffs are critical in cellular communication systems is that

neighboring cells are always using a disjoint subset of frequency bands, so

negotiations must take place between the mobile station (MS), the current serving

base station (BS), and the next potential BS. Other related issues, such as decision

making and priority strategies during overloading, might influence the overall

performance.

3.2 Types of Handoffs

Handoffs are broadly classified into two categories—hard and soft handoffs. Usually,

the hard handoff can be further divided into two different types intra and intercell

handoffs known as horizontal as well as vertical handoff respectively. The main topic

of our discussion is the vertical handoff between WiMaX and GPRS systems.

10

3.2.1 Horizontal Handoff

In this handoff process, the handoff of a mobile terminal takes place between base

stations supporting the same network technology. For example, the changeover of

signal transmission due to the mobility of the mobile terminal from an IEEE 802.11b

base station to a neighboring IEEE 802.11b base station is considered as a horizontal

handoff process. Signal strength and channel availability are needed to consider in

horizontal handoffs.

11

Figure 2: Soft Handoff

Figure 1: Hard Handoff

Figure 3: Horizontal Handoff

GPRS GPRS

3.2.2 Vertical handoff:

The vertical handover was introduced with the development of different wireless

technologies and the coexistence of their networks including GSM, GPRS, and UMTS

as cellular networks and WiFi, WiMAX as broadband access networks. This handoff

process of a mobile terminal takes place among access points supporting different

network technologies. For example, the changeover of signal transmission from an

IEEE 802.16 WiMax base station to a cellular GPRS network is considered as a

vertical handoff process. Due to the different technologies of the networks, more than

one interface is required during the handoff process.

3.3 Handoff Initiation [9]

A hard handoff occurs when the old connection is broken before a new connection is

activated. It is assumed that the signal is averaged over time, so that rapid fluctuations

due to the multipath nature of the radio environment can be eliminated. Figure 5

shows a MS moving from one BS (BS1) to another (BS2). The mean signal strength of

BS1 decreases as the MS moves away from it. Similarly, the mean signal strength of

BS2 increases as the MS approaches it. This figure is used to explain various

approaches described in the following subsection.

3.3.1 Relative Signal Strength

This method selects the strongest received BS at all times. The decision is based on a

mean measurement of the received signal. In Figure 5 the handoff would occur at

position A. This method is observed to provoke too many unnecessary handoffs, even

when the signal of the current BS is still at an acceptable level.

12

Figure 4: Vertical Handoff

WiMaX GPRS

This method allows a MS to hand off only if the current signal is sufficiently weak (less

than threshold) and the other is the stronger of the two. The effect of the threshold

depends on its relative value as compared to the signal strengths of the two BSs at

the point at which they are equal. If the threshold is higher than this value, say T1 in

Figure 5 this scheme performs exactly like the relative signal strength scheme, so the

handoff occurs at position A. If the threshold is lower than this value, say T2 in Figure

5 the MS would delay handoff until the current signal level crosses the threshold at

position B. In the case of T3, the delay may be so long that the MS drifts too far into

the new cell. This reduces the quality of the communication link from BS1 and may

result in a dropped call. In addition, this results in additional interference to co-channel

13

h

BS1 Signal BS2 Signal

BS1 BS2 A B C D

T1T2

Figure: 5 Movement of a mobile station in the handoff zone

users. Thus, this scheme may create overlapping cell coverage areas. A threshold is

not used alone in actual practice because its effectiveness depends on prior

knowledge of the crossover signal strength between the current and candidate BSs.

3.3.2 Relative Signal Strength with Hysteresis

This scheme allows a user to hand off only if the new BS is sufficiently stronger (by a

hysteresis margin, h in Figure 1) than the current one. In this case, the handoff would

occur at point C. This technique prevents the so-called ping-pong effect, the repeated

handoff between two BSs caused by rapid fluctuations in the received signal strengths

from both BSs. The first handoff, however, may be unnecessary if the serving BS is

sufficiently strong.

3.3.3 Relative Signal Strength with Hysteresis and Threshold

This scheme hands a MS over to a new BS only if the current signal level drops below

a threshold and the target BS is stronger than the current one by a given hysteresis

margin. In Figure 1, the handoff would occur at point D if the threshold is T3.

3.4 Handoff Decision

The decision-making process of handoff may be centralized or decentralized (i.e., the

handoff decision may be made at the MS or network). From the decision process point

of view, one can find at least three different kinds of handoff decisions.

3.4.1 Network-Controlled Handoff

In a network-controlled handoff protocol, the network makes a handoff decision based

on the measurements of the MSs at a number of BSs. Network-controlled handoff is

used in first-generation analog systems such as AMPS (advanced mobile phone

14

system), TACS (total access communication system), and NMT (advanced mobile

phone system).

3.4.2 Mobile-Assisted Handoff

In a mobile-assisted handoff process, the MS makes measurements and the network

makes the decision. In the circuit-switched GSM (global system mobile), the BS

controller (BSC) is in charge of the radio interface management.

3.4.3 Mobile-Controlled Handoff

In mobile-controlled handoff, each MS is completely in control of the handoff process.

MS measures the signal strengths from surrounding BSs and interference levels on all

channels. A handoff can be initiated if the signal strength of the serving BS is lower

than that of another BS by a certain threshold.

3.5 Desirable features of handoff

An efficient handoff algorithm can acquire many desirable features. Some of the major

desirable features of any handoff algorithm are described below.

3.5.1 Reliability

A handoff algorithm should be reliable. This means that the call should have good

quality after a handoff. Many factors help in determining the potential service quality of

15

Desirable handoff features

Reliability Seamless Interference

Performance Load balancing No of handoff

Maximize MaintainMinimize

Figure 6: Desirable handoff features

a candidate base station. Some of these factors include signal-to-interference ratio

(SIR), signal-to-noise ratio (SNR), received signal strength (RSS), and bit error rate

(BER).

3.5.2 Seamless

A handoff algorithm should be fast so that the mobile device does not experience

service degradation or interruption during the handoff process. Service degradation

may be due to a continuous reduction in signal strength or an increase in co-channel

interference (CCI).

3.5.3 Interference

A handoff algorithm should avoid high interference. The Co-channel and interchannel

interferences can degrade the transfer rate of a wireless network. Co-channel

interference is caused by devices transmitting on the same channel and on the other

hand, interchannel interference is caused by devices transmitting on adjacent

channels.

3.5.4 Load balancing

A handoff algorithm should balance traffic in all cells, whether of the same or different

network type. This helps to eliminate the borrowing of channels from the neighboring

cells to reduce the probability of new call blocking.

3.5.5 Minimizing the no of handoff

The number of handoffs should be minimized in a handoff scenario, because more the

number of handoff attempted, the greater the chances that a call will be denied access

to a channel, resulting in a higher handoff call dropping probability.

3.6.2 Vertical handoff decision

In vertical handoffs, whether a handoff should take place or not depends on many

network characteristics. Following characteristics are particularly important for this

type of decision in addition to the two in the horizontal decision.

16

3.6.2.1 Quality of Service

Handing over to a network with better conditions and higher performance would

usually provide improved service facility. Transmission rates, error rates, and other

characteristics have to be measured in order to decide which network can provide a

higher assurance of continuous connectivity.

3.6.2.2 Cost of Service

The cost of the different services to the user is a major issue, and could sometimes be

the decisive factor in the choice of a network. The cost of service of new network set

by the internet provider may be higher than the previous one.

3.6.2.3 Security

Risks are inherent in any wireless technology. Perhaps the most significant source of

risks in wireless networks is the technology’s underlying communications medium.

That is the airwave which is open to intruders.

3.6.2.4 Power Requirements

Wireless devices have limited battery power. When the level decreases, handing off to

a network with low power may require much time.

3.6.2.5 Proactive Handoff

In proactive handoff, the users are involved in the vertical handoff decision. By

permitting the user to choose a preferred network, the system is able to accommodate

the user’s special requirements.

3.6.2.6 Velocity

The velocity of the mobile device has a greater effect on vertical handoff decision than

in horizontal handoffs. Because of the heterogeneous networks, handing off to an

embedded network when traveling at high speeds is discouraging since a handoff

back to the original network would occur very shortly afterward.

17

3.6.2.7 Radio Link Transfer

Radio link transfer, the second part of the handoff process, is the task of establishing

links to a call at the new base station. The radio link is transferred from the old to the

new base station.

3.6.2.8 Channel Allocation

The final handoff stage is channel assignment which consists of the allocation of

channels at the new base station.

3.7 Mobility management

Mobility in handoff means movement of a user from one location to another from time

to time. Mobility of a user in a wireless communication system has a big impact on

maintaining the continuity of the service to the users. Mobility management has widely

been recognized as one of the most important and challenging problems for seamless

handoff of a mobile device across wireless networks. In this situation, such technology

needs to be used so that mobile users receive their services without the disruption of

communications. Two main aspects need to be considered in mobility management

are location management and handoff management.

3.7.1 Location management

It means locating mobile terminals in order to deliver data packets to them. Operations

of location management include:

3.7.1.1 Location registration

Also know as location update or tracking, i.e. the procedure that the mobile node

informs the network and other nodes of its new location by updating the corresponding

location information entries stored in some databases in the networks. Figure 7

shows the flow chart of a mobile station showing its process of association to a foreign

network to make continuation of packet receiving during and after the vertical handoff

process.

18

3.7.1.2 Location paging

Also know as locating or searching. In most cases location information stored in

databases is only the approximate position of a mobile device. Location paging is the

procedure that the network tries to find the mobile device’s exact locality when

calls/packets need to be delivered to the mobile device. Some key research issues for

location management include:

19

Gets subnet infoPacket dropping

IP Registration Request

Send Request

Authenticates

Assigns IP

Tunnel Establishment

Transmitting

Delivered

Tunnel Establishment

Figure 7: Registration flow chart of a mobile node in a foreign network

MN FA HA CN

MN Mobile Node HA Home Agent FA Foreign Agent CN Correspondent Node

3.7.1.3 Addressing

It means how to represent and assign address information to mobile nodes.

3.7.1.4 Database structure

It is for how to organize the storage and distribution of the location information of

mobile nodes. Database structure can be either centralized or distributed.

3.7.1.5 Location update time

It means when a mobile node should update its location information by renewing its

entries in corresponding databases.

3.7.1.6 Paging scheme

Paging means how to determine the exact location of a mobile node within a limited

time.

3.8 Handoff management

It means controlling the changes required during the handoff in order to maintain the

connection with the mobile node. Operations of handoff management include:

3.8.1 Handoff triggering

Initiating of handoff process is according to some conditions. Possible conditions may

include e.g. signal strength, workload, bandwidth, cost, network topology change etc.

3.8.2 Connection re-establishing

It means establishing new connection between the mobile node and the new access

point.

3.8.3 Packet routing

It means routing the data through the new connection path after establishment of the

connection to the target access point.

20

Chapter 4

4. GPRS System

GPRS (General Packet Radio Service) is rapidly becoming a global standard for

sending and receiving high-speed data across the GSM network. It is also known as

GSM-IP (Internet Protocol) because it connects users directly to Internet Service

Providers. It uses existing GSM network to transmit and receive TCP/IP based data to

and from GPRS mobile devices. GPRS now makes it possible to deploy several new

devices that have previously not been suitable over traditional GSM networks due to

the limitations in speed (9600bps), message length of the Short Message Service

(160 characters), dial up time and costs. These applications include Point Of Sale

Terminals, Vehicle tracking systems, and monitoring equipment. It's even possible to

remotely access and control in-house appliances and machines. Since, GPRS is a

Radio Service, like a radio, a GPRS enabled device is "always on", so as long one’s

equipment in switched on, he has an open channel for sending and receiving data.

Being used the packet-switched technology, GPRS users are always connected,

always on-line, and may be charged only for the amount of data that is transported.

Voice calls can be made simultaneously over GSM-IP while a data connection is

operating, depending on the phone Class and Type. Thus GPRS is efficient, fast and

cost effective as compared to GSM technology as explained follows.

Efficient - GPRS mobile devices only use the GSM network when data is

transferred. The GSM connection is not dedicated to each user; therefore it can

be shared with many users resulting in efficient use of the network.

Fast - GPRS gives speeds of upto 5 times faster than GSM. GPRS offers

maximum data rates of 56Kbps (down) and 14.4kbps (up); however, this is

shared bandwidth therefore actual data rates are potentially lower.

Payment based on data usage - Billing is not based on time, but on the

amount of data actually transferred.

21

4.1 Architecture of GPRS [11]

GPRS provides packet radio access for Global System for Mobile Communications

(GSM) and uses time-division multiple access (TDMA) for providing services to the

users. GPRS is a data network that overlays a second-generation GSM network. This

data overlay network provides packet data transport at rates from 9.6 to 171 kbps.

Additionally, multiple users can share the same air-interface resources

simultaneously. Following is the GPRS Architecture diagram:

22

HLR

GGSNAUC

EIR

GGSN

SGSN SGSN

Internet X.25 networkPSTN

BSC

MSC Internal Backbone Network

SignalingCircuit Switched GSM

Packet Switched Data Signaling

Figure 8: GPRS Network Architecture

Circuit Switched GSM

GPRS

4.1.1 GPRS Mobile Stations

New Mobile Station are required to use GPRS services because existing GSM phones

do not handle the enhanced air interface or packet data. A variety of MS can exist,

including a high-speed version of current phones to support high-speed data access,

a new PDA device with an embedded GSM phone, and PC cards for laptop

computers. These mobile stations are backward compatible for making voice calls

using GSM.

4.1.2 GPRS Base Station Subsystem

Each BSC requires the installation of one or more Packet Control Units (PCUs) and a

software upgrade. The PCU provides a physical and logical data interface to the base

station subsystem (BSS) for packet data traffic. The BTS can also require a software

upgrade but typically does not require hardware enhancements.

When either voice or data traffic is originated at the subscriber mobile, it is transported

over the air interface to the BTS, and from the BTS to the BSC in the same way as a

standard GSM call. However, at the output of the BSC, the traffic is separated; voice

is sent to the mobile switching center (MSC) per standard GSM, and data is sent to a

new device called the Serving GPRS support node (SGSN) via the PCU over a frame

relay interface. Following two new components, called GPRS support nodes (GSNs),

are added.

4.1.3 Gateway GPRS support node (GGSN)

The Gateway GPRS Support Node acts as an interface and a router to external

networks. The GGSN contains routing information for GPRS mobiles, which is used to

tunnel packets through the IP based internal backbone to the correct Serving GPRS

Support Node. The GGSN also collects charging information connected to the use of

the external data networks and can act as a packet filter for incoming traffic.

23

4.1.4 Serving GPRS support node (SGSN)

The Serving GPRS Support Node is responsible for authentication of GPRS mobiles,

registration of mobiles in the network, mobility management, and collecting

information for charging for the use of the air interface.

4.1.5 Internal Backbone

The internal backbone is an IP based network used to carry packets between different

GSNs. Tunneling is used between SGSNs and GGSNs, so that internal backbone

does not need any information about domains outside the GPRS network. Signaling

from a GSN to a MSC, HLR or EIR is done using SS7.

4.1.6 Routing Area

GPRS introduces the concept of a routing area. This is much the same as a Location

Area in GSM, except that it will generally contain fewer cells. Because routing areas

are smaller than Location Areas, less radio resources are used when a paging

message is broadcast.

4.2 WiMaX System

WiMaX stands for Worldwide Interoperability for Microwave Access is based on wireless

broadband technology. WiMAX technology based on the IEEE 802.16 specifications to enable

the delivery of last-mile wireless broadband access as an alternative to cable and DSL. WiMAX

has a rich set of features with a lot of flexibility in terms of deployment options and

potential service offerings. Some of the more salient features that deserve highlighting

are as follows:

4.2.2 OFDM-based physical layer

The WiMAX physical layer (PHY) is based on orthogonal frequency division

multiplexing, a scheme that offers good resistance to multipath, and allows WiMAX to

operate in (non line of sight) NLOS conditions. OFDM is now widely recognized as the

method of choice for mitigating multipath for broadband wireless.

24

4.2.3 Very high peak data rates

WiMAX is capable of supporting very high peak data rates. In fact, the peak PHY data

rate can be as high as 74Mbps when operating using a 20MHz wide spectrum. More

typically, using a 10MHz spectrum operating using TDD scheme with a 3:1 downlink-

to-uplink ratio, the peak PHY data rate is about 25Mbps and 6.7Mbps for the downlink

and the uplink respectively.

4.2.4 Scalable bandwidth and data rate support

WiMAX has a scalable physical-layer architecture that allows for the data rate to scale

easily with available channel bandwidth. This scalability is supported in the OFDMA

mode, where the FFT (fast fourier transform) size may be scaled based on the

available channel bandwidth.

4.2.5 Adaptive modulation and coding (AMC)

WiMAX supports a number of modulation and forward error correction (FEC) coding

schemes and allows the scheme to be changed on a per user and per frame basis,

based on channel conditions. AMC is an effective mechanism to maximize throughput

in a time-varying channel. The adaptation algorithm typically calls for the use of the

highest modulation and coding scheme that can be supported by the signal-to-noise

and interference ratio at the receiver such that each user is provided with the highest

possible data rate that can be supported in their respective links.

4.2.6 Link-layer retransmissions

For connections that require enhanced reliability, WiMAX supports automatic

retransmission requests (ARQ) at the link layer. ARQ-enabled connections require

each transmitted packet to be acknowledged by the receiver; unacknowledged

packets are assumed to be lost and are retransmitted.

4.2.7 Support for TDD and FDD

IEEE 802.16-2004 and IEEE 802.16e-2005 supports both time division duplexing and

frequency division duplexing, as well as a half-duplex FDD are cost effective.

25

4.2.8 Orthogonal frequency division multiple access (OFDMA)

Mobile WiMAX uses OFDM as a multiple-access technique, whereby different users

can be allocated different subsets of the OFDM tones. OFDMA facilitates the

exploitation of frequency diversity and multiuser diversity to significantly improve the

system capacity.

4.2.9 Flexible and dynamic per user resource allocation

Both uplink and downlink resource allocation are controlled by a scheduler in the base

station. Capacity is shared among multiple users on a demand basis, using a burst

TDM scheme. When using the OFDMA-PHY mode, multiplexing is additionally done in

the frequency dimension, by allocating different subsets of OFDM subcarriers to

different users. Resources may be allocated in the spatial domain as well when using

the optional advanced antenna systems (AAS). The standard allows for bandwidth

resources to be allocated in time, frequency, and space and has a flexible mechanism

to convey the resource allocation information on a frame-by-frame basis.

4.2.10 Support for advanced antenna techniques

The WiMAX solution has a number of hooks built into the physical-layer design, which

allows for the use of multiple-antenna techniques, such as beamforming, space-time

coding, and spatial multiplexing. These schemes can be used to improve the overall

system capacity and spectral efficiency by deploying multiple antennas at the

transmitter and/or the receiver.

4.2.11 Quality of service support

The WiMAX MAC layer has a connection-oriented architecture that is designed to

support a variety of applications, including voice and multimedia services. The system

offers support for constant bit rate, variable bit rate, real-time, and non-real-time traffic

flows, in addition to best-effort data traffic. WiMAX MAC is designed to support a large

number of users, with multiple connections per terminal, each with its own QoS

requirement.

26

4.2.12 Robust security

WiMAX supports strong encryption, using Advanced Encryption Standard (AES), and

has a robust privacy and key-management protocol. The system also offers a very

flexible authentication architecture based on Extensible Authentication Protocol (EAP),

which allows for a variety of user credentials, including username/password, digital

certificates, and smart cards.

4.2.13 Support for mobility

The mobile WiMAX variant of the system has mechanisms to support secure

seamless handovers for delay-tolerant full-mobility applications, such as VoIP. The

system also has built-in support for power-saving mechanisms that extend the battery

life of handheld subscriber devices. Physical-layer enhancements, such as more

frequent channel estimation, uplink subchannelization, and power control, are also

specified in support of mobile applications.

4.2.14 IP based architecture

The WiMAX Forum has defined a reference network architecture that is based on an

all-IP platform. All end-to-end services are delivered over an IP architecture relying on

IP-based protocols for end-to-end transport, QoS, session management, security, and

mobility. Reliance on IP allows WiMAX to ride the declining costcurves of IP

processing, facilitate easy convergence with other networks, and exploit the rich

ecosystem for application development that exists for IP.

4.3 Architecture of WiMaX System [12]

The network reference model describes the architecture of WiMaX developed by the

WiMAX Forum defines a number of functional entities and interfaces between those

entities is shown in the figure 3 given below. The design of WiMAX network is based

on the following major principles. They are spectrum, topology, inter-working, IP

connectivity and mobility management.

27

28

BSBS

MSMS MS

AccessNetwork

ASNGW

IPNetwork

AAA

BSS

Gateway

Connectivity Service Network (CSN)

ASP

IPNetwork

Internet

PSTN

3GPP

BS

Figure 9: IP Based WiMaX Network Architecture

The WiMAX Forum has defined an architecture that defines how a WiMAX network

connects with other networks, and a variety of other aspects of operating such a

network, including address allocation, authentication, etc. An overview of the

architecture is given in the illustration. This defines the following components:

ASN: the Access Service Network

BS: Base station, part of the ASN

ASN-GW: the ASN Gateway, part of the ASN

CSN: the Connectivity Service Network

AAA: AAA Server, part of the CSN

NAP: a Network Access Provider

NSP: a Network Service Provider

4.3.1 Base station (BS) [13]

The BS is responsible for providing the air interface to the MS. Additional functions

that may be part of the BS are micro mobility management functions, such as handoff

triggering and tunnel establishment, radio resource management, QoS policy

enforcement, traffic classification, DHCP (Dynamic Host Control Protocol) proxy, key

management, session management, and multicast group management.

4.3.2 Access service network gateway (ASN-GW)

The ASN gateway typically acts as a layer 2 traffic aggregation point within an ASN.

Additional functions that may be part of the ASN gateway include intra-ASN location

management and paging, radio resource management and admission control, caching

of subscriber profiles and encryption keys, AAA client functionality, establishment and

management of mobility tunnel with base stations, QoS and policy enforcement, and

foreign agent functionality for mobile IP, and routing to the selected connectivity

service network (CSN).

4.3.3 Connectivity service network (CSN)

29

The CSN provides connectivity to the Internet, ASP, other public networks, and

corporate networks. The CSN includes AAA servers that support authentication for the

devices, users, and specific services. The CSN is own by NSP also provides per user

policy management of QoS and security. The CSN is also responsible for IP address

management, support for roaming between different NSPs, location management

between ASNs, and mobility and roaming between ASNs. The CSN also provides per

user policy management of QoS and security. The CSN is also responsible for IP

address management, support for roaming between different NSPs, location

management between ASNs, and mobility and roaming between ASNs.

Chapter 5

30

5. Proposed System

It has been observed that the traditional received signal strength (RSS) based

algorithm suffers from major drawbacks which made us to choose another one which

provide better service quality to the users during handoff. This new approach not only

provide enhance service quality but also more efficient as compared to RSS based.

This new approach is based on signal to noise plus interference ratio (SINR).

This algorithm is proposed for vertical handoff between those different networks which

supports time division multiple access multiplexing (TDMA).

The proposed algorithm is SINR based and also controls the transmission power of

the transmitter. This algorithm will provide better quality of services as well better

system throughput as compared to RSS based algorithm because SINR is adaptive to

noisy and overload condition and controlling transmission power minimizes the energy

consumption and reduces the interference as well as increases the system capacity.

In this proposed algorithm, the power required for the transmitter is calculated based

on the required target SINR for the receiver. Since the algorithm is for TDMA systems,

prior to the power calculation, interference and noise is measured using Kalman filter

method for the slot n. In TDMA the interference and noise measures of the previous

slot can be used to determine the transmission power required for the next slot to

maintain the target SINR at the receiver. Therefore the measure of SINR of the slot n

is used to adjust the power of the transmitter for the slot n+1 for achieving the required

SINR at the receiver even when the user keeps moving. The SINR received by the

mobile station from other networks is being converted equivalent SINR value of the

current network required to achieve the same data rate in the current network. With

this combined effects, handoff is triggered when the receiver receives greater SINR

from another network. So the handoff algorithm can have the knowledge of achievable

bandwidths from both access networks to make handoff decisions with QoS

consideration.

5.1 Major advantages of SINR based algorithm over RSS based

31

Use of RSS based vertical handoff cannot provide the user with quality of

service (QoS) throughput, as the vertical handoff algorithm itself is not QoS

aware. But SINR can provide QoS since SINR takes into account the

interference and noise at the transmission end.

Analysis results show that SINR based vertical handoff provides higher

average throughput for end users as compared to the RSS based vertical

handoff with various thresholds settings, and also can adapt to different

network conditions, such as different noise level and load factor. Simulation

results further confirm that the SINR based vertical handoff improves the

overall system throughputs.

In real networks, interference power will depend on the user location as well as

the density of the users. Therefore, only the SINR based vertical handoff can

guarantee multimedia QoS specifying the achieved date rate for end user

inside vertical handover zone. This is also another important reason that our

SINR based vertical handoff can adapt to the network conditions and can

provide consistently maximum available throughputs to the end user, which

RSS based handoff cannot achieve.

SINR based vertical handoff algorithm can consistently offer the end user with

maximum available bandwidth during vertical handoff contrary to the RSS

based vertical handoff, whose performance differs under different network

conditions.

SINR based does handoff actually when it is necessary. But RSS based

sometimes does unnecessary handoffs under interference and noisy condition

even though the signal strength in current network is still greater than the

threshold.

SINR based handoff will be able reduce the ping pong effect as controlling the

power of transmission it will be able to provide the required SINR for the mobile

even if the mobile user is closed to the boundary of the neighboring network.

32

5.2 System assumptions for the algorithm

In the networks time is divided into slots. Let each data message be divided

into a number of packets, each of which can be sent in one time slot. Allows

multiple, contiguous time slots to be used by the same transmitter for sending a

message, thus producing temporal correlation for interference

The channel gain between a mobile station and its base station is measured as

follows:

Gain (or loss) = , is the received power and is the transmitted

power.

The medium-access control (MAC) protocol used allows at most one terminal in

each sector or cell to send data at a time. Therefore, no data contention occurs

within the same sector or cell. Also, a terminal can transmit in contiguous time

slots. Moreover, the base station knows which terminal is scheduled to

transmit at different times.

Base stations do not exchange control information among themselves on a per

packet basis in real time due to the large volume of data.

The interference power is equal to the difference between the total received

power and the power of the desired signal.

5.3 Power Control using Kalman-filter method

A Kalman-filter method for power control is proposed for broadband, packet-switched

TDMA wireless networks. By exploiting the temporal correlation of co-channel

interference, a Kalman filter is used to predict future interference power. Based on the

predicted interference and estimated path gain between the transmitter and receiver,

transmission power is determined to achieve a desired signal-to-interference-plus-

33

noise ratio (SINR). Performance results reveal that the Kalman-filter method for power

control provides a significant performance improvement.

Although the Kalman-filter method is applicable to both the uplink (from terminal to

base station) and the downlink (from base station to terminal) we will focus on the

downlink here.

5.4 Interference prediction by Kalman Filter method

We apply the Kalman-filter method to predict interference power for predicting SINR

by adjusting transmission power. Using this method, each terminal continuously

measures the interference power for its assigned radio channel (e.g., the same time

slot of the consecutive TDMA frames). Let be the actual interference-plus-noise

power in dBm received at a given base station in time slot n. In fact, is required to

be estimated by the Kalman filter. Assume that the noise power, which depends on

the channel bandwidth, is given and fixed. The total interference is simply the thermal

noise plus the measured interference. The system dynamics of the interference plus

noise power can be modeled as:

Where represents the fluctuation of interference power when terminals start new

transmissions and/or adjust their transmission power in the time slot.

Let be the measured interference power plus noise power in dBm for slot n then

By the Kalman filter theory, the time and measurement update equations for the

interference power are:

34

Where, are the a priori and a posteriori estimates of .

, are the a priori and posteriori estimate error variances respectively.

is the Kalman gain, and and are the variances for the process

noise and measurement noise respectively.

is estimated based on the interference measurements in the last W slots

as follows:

is used to capture the non stationary interference

is the average measured interference with noise over the last slots.

Also can be given by

Where, is a given constant between 0 and 1

5.5 Determination of Transmission power

Let is the target SINR, is the transmission power and the path gain from the

transmitting base station to the mobile station for slot n, respectively. and

35

represent the actual and predicated interference power in dBm and let and

denote the respective value in mW. Based on the base station transmits in slot n

with power

The goal of this of transmission power is to choose just enough power to achieve the

target SINR .

When p(n) is the power of the base station selected by (11) for slot n, the actual

receiving SINR (n) at the mobile station is

Where is the actual interference power in mW for the slot n.

Thus (12) implies that when predicted interference is accurate to actual

interference , the target SINR is achieved.

Even if is not exactly equal to , the method helps in reducing the spread of

as long as and are correlated.

Steps for the Kalman Filter

The Kalman Filter method for power control is summarized below:

36

1. For each slot n, each base station measures the interference power for the time

slot.

2. The interference measurements are used as input to the Kalman filter in equation

(3) to (10) to predict the interference power in slot n+1.

3. Based on the MAC protocol in use, the base station tracks the path gain and

selects the transmission power by (11) to meet a given target SINR for the terminal

that transmits in slot n+1.

4. The power level is used for transmission to the mobile station in slot n+1.

5.6 Calculation SINR at the mobile station from WiMaX and GPRS networks

5.6.1 Date rate using Shannon capacity formula

According to Shannon capacity formula, the maximum achievable data rate Rij

received by the user i from the base station j is given by:

is SINR received at user i when associated with GPRS or WiMaX . is the

gap between uncoded QAM and capacity, minus the coding gain.

Thus the if and are maximum achievable data rate from WiMaX as well

as from GPRS respectively, these can be represented in terms of the receiving

SINR from the two networks as:

Where, is SINR received from WiMaX on

High Speed Downlink Packet Access (HSDPA) Channel and is SINR

received from GPRS on HSDP. The relationship between and is

37

5.6.2 Calculation for SINR at the mobile station

In this discussion, we consider the downlink traffic, as they normally require higher

bandwidth than uplink.

The SINR received by user i from WiMaX base station can be represented

as:

is the transmitting power of

is the channel gain between user i and

is the background noise power at user receiver end.

The SINR received by user i from GPRS base station can be

represented as:

is the total transmitting power of

is the transmitting power of to user j

is the channel gain between user i and

5.6.3 Throughput calculation

38

In this analysis, consider a point to point model, in which a user is moving at speed

v from ( ) to ( ), as shown in the following figure. The vertical handoff

has shown to be taken place at point .

The total downlink throughputs ө can be represented as

Where is cell residence time, and and is maximum data rate received from WiMaX and GPRS.

• For the RSS based vertical handoff, the is dependent on the minimum

required receiving power from WiMaX base station

• In SINR based vertical handoff, is calculated based on the receiving SINR

from WiMaX and GPRS.

• So, It will be possible to compare the average throughputs for different vertical

handoff algorithm with different .

1X

wsR

hX

gsR

Figure 10: Point to point model

39

PACKETS

PACKETS

GPRS network WiMaX network

Corresponding network

Figure 11: Integration of GPRS and WiMax networks

Internet

CN

BS

MSC

Gateway

MS

BS

MSC

Gateway

N

BS

Gateway

References

1. “A Mobile-IP Based Mobility System for Wireless Metropolitan Area Networks”

--- by Chung-Kuo Chang.

2. “www.wiremaxforum.org” --- by WiMaX forum

3. “Power Control by Interference Prediction for Broadband Wireless Packet

Networks” ---by Kin K. Leung.

40

4. “Combined SINR Based Vertical Handoff Algorithm for Next Generation

Heterogeneous Wireless Networks” by --- Kemeng Yang, Iqbal Gondal, Bin Qiu and

Laurence S. Dooley, 2007.

5. “SINR Estimation for Power Control in Systems with Transmission Beamforming”

---by Vesa Hasu, Student Member, IEEE, and Heikki Koivo, Senior Member, IEEE,

2005.

6. “On the Use of SINR for Interference-aware Routing in Wireless Multi-hop

Networks” ---by Riadh M. Kortebi, Yvon Gourhant, Nazim Agoulmine.

7. “Vertical handover criteria and algorithm in IEEE 802.11 and 802.16 hybrid

networks” ---by Z. Daia, R. Fracchiaa, J. Gosteaub, P. Pellatia,G.Vivier.

8. “A Performance Evaluation of Vertical Handoff Scheme between Mobile WiMax and

Cellular Networks” ---by Seongsoo Park, JaeHwang Yu, ,JongTae Ihm

9. “Handoff in Wireless Mobile Networks” ---by Qing-An Zeng, Dharma P. Agrawal

10. “http://www.tutorialspoint.com/gprs/gprs_architecture.htm”

11. “http://www.en.wikipedia.org/wiki/WiMAX#Architecture”

12. ” http://www.tutorialspoint.com/wimax/wimax_network_model.htm”

41