Call Admission Control In 3g Wireless Computer Science Essay

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Unlike FDMA or TDMA systems, CDMA is interference limited and has a soft capacity that changes depending on the interference felt at the base station at a given time. Admitting a new call and user movement increases the interference level in the system. Therefore a robust Call Admission Mechanism is needed. The 3G cellular mobile systems which are based on WCDMA technology are expected to be interference limited. Soft capacity is one of the main characteristics of 3G (i.e, UMTS) and it requires new radio resource management strategies to serve diverse quality of service requirements. A simple CAC scheme can distinguish the calls between the two traffic types by setting different thresholds for call admission. This simple CAC scheme is evaluated in respect of blocking probabilities of both new and handoff calls. To this end, we formulate an aggregate Markov chain, which describes both the new-call and handoff-call arrival processes, under the assumption that they are both Poisson processes .In this report, a WCDMA prioritized uplink call admission control (CAC) algorithm for UMTS, which combines QoS negotiation and service differentiation by priority, is studied. This CAC scheme gives preferential treatment to high priority calls, such as soft handoff calls, by reserving some bandwidth margin (soft guard channel) to reduce handoff failures. In addition, queuing is also used to enhance the handoff success probability. The algorithm uses the effective load as an admission criterion and applies different thresholds for new and handoff calls. Finally, the study considers two types of services: voice and data calls. Results indicate that this algorithm reduces the drop handoff calls and increases the total system capacity; hence the GoS and the system performance can significantly be improved especially in case of high mobility environments.

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Key Words:Call Admission Control (CAC), Quality-of-Service (QoS), Universal Mobile Telecommunication System (UMTS),

INTRODUCTION

The third generation (3G) technology is a developing technology for the future wireless cellular/mobile communication. Nevertheless, along with the development of the third generation (3G) mobile technologies, the development of the consumer electronics will also grow. The third generation (3G) services add a valuable mobile dimension to services that have already become an integral part of modern life, such as the internet and intranet access, and video-conferencing. With the support of higher data transmission rate for mobile users, third generation (3G) networks are expected to support different broadband multimedia services, and hence, leading to the increasing provision of the products for consumer electronics, like video mobile phones and third generation (3G) broadband cards. Apart from the appearance of the products, a key factor that most end users concern is the diverse quality-of-service (QoS) requirement of the system. In other words, the network performance will affect the market of the products in consumer electronics, indirectly. [1]

With the development growth of the internet and dramatic increase in wireless access, there is a tremendous demand on multimedia delivery over wireless internet. The third generation (3G) wireless cellular networks, foreseen to be the enabling technology for multimedia services with up to 384 Kbps outdoor and 2 Mbps indoor bandwidth, makes it feasible for visual communication over the wireless link. Error rate is usually very high in wireless channel, which is caused by multi-path fading, inter-symbol interference, and noise disturbances. The channel error rate varies with the changing external environment, resulting in devastating effect on multimedia transmission/applications. [1]

The third generation (3G) wireless cellular mobile systems which are based on WCDMA technology are expected to be interference limited. Soft capacity is one of the main characteristics of third generation (3G) (i.e. UMTS) and it requires new radio resource management strategies to serve diverse quality-of-service (QoS) requirements. [4]

In wireless communication systems, Quality of Service (QoS) is one of the most important issues from both the users and operators point of view. All the parameters related to QoS are not same important for all users and applications. The satisfaction level of different users also does not depend on same QoS parameters.The quality-of-service (QoS) provisioning means that the multimedia traffic should get predictable service from the available resources in the communication system. Typical resources are CPU time (for communication softwareto execute) & the network bandwidth. The communication software must also guarantee an acceptable end-to-end delay & maximum delay jitter. i.e. Maximum allowed variance in the arrival of data at the destination. In most cases, quality-of-service (QoS) requirements are specified by the 3-tuple: (bandwidth, delay & reliability). [13]

CAC SCHEME

2.1 CAC Scheme

In the CAC algorithm new call arrival rates are estimated continuously and if they are higher than a predetermined level some calls are blocked irrespective of whether a channel is available or not. The objective of this scheme is to maintain the new call arrival rate lesser than a predetermined level. In this scheme a comparison is made with the existing two schemes namely pre-request scheme and the guard channel scheme and various advantages and disadvantages are given for the two schemes and then a CAC algorithm is developed which provides a better QoS than the existing two schemesMaintaining the Integrity of the Specifications

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Figure 1:-Flow chart for CAC

Call Admission Algorithm (CAC)

In the CAC algorithm the acceptable load is calculated based on simulation results and this value is used for comparison purpose. The estimated load is also calculated and it is checked with the acceptable load. If the estimated load is lesser than or equal to the acceptable load, then attempts are made to allocate channels for all the incoming calls. If the estimated load is greater than the acceptable load then only a fraction of the incoming calls will be allocated channels and the remaining fraction of the calls will be discarded even if there are available channels

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Figure 2:-Flow Chart for Call Admission control Scheme

implement the admission control for WCDMA systems, first an estimate of the total interference should be computed to be employed in the decision process of acceptance or rejection of new connections. In this section the uplink capacity and load estimation of a homogenous, uniformly loaded network will be presented. The analysis carried out will focus on the UTRA-FDD mode. Furthermore, the analysis assumes perfect power control operation. Hence, a mobile station (MS) and its home base station (BS) use only the minimum needed power in order to achieve the required performance. The CDMA capacity has been subject to extensive research work , hence only a short description is given here.

The value of the bit-energy-to-noise-density ratio Eb corresponds to the signal quality, since it determines the bit error rate, BER. Let be the target bit-energy-to-noise-density ratio required to achieve a particular BER, or equivalently a particular frame error rate ( Eb / No). That means the maximum bit (BER) or blocks (BLER) error rates, can be mapped into an equivalent Eb / No constraint denoted by .If we assume perfect power control, then Eb / No . The resulting BER can then be approximated using:

(6)

In the uplink, the criteria for the received power for ith MS can be written as:

;i=1,2,…N (7)

: is the spreading factor for processing gains of MS i

:Bit Rate of MS i

The chip rate of WCDMA(3.84 Mcps)

The require for the mobile I and for a certain service quality

Received power of desired signal

=The power of unwanted signal

Minimum required power (sensitivity)

*=* (8)

called load factor increment for the new user i

(9)

The total load factor of such an interference system is the sum of the load factor increments brought by N active mobile users. Therefore

(10)

Assuming perfect power control on the reverse link, and that every user has the same service rate (constant R, v, and ), equation (1) Can be rewritten as

(11)

one can calculate the maximum number of simultaneously active users which can be permitted as:

(12)

When the system is 100% loaded, it has reached pole capacity or the maximum theoretical capacity of WCDMA system. Letting the 1 in above equation yields:

(13)

This section provides a brief description of the system under study. The throughput-based CAC algorithm computes the increase in the load caused by the uplink admission of a new user in the cell and accepts the new connection only if the following inequality is satisfied,

Where is the current uplink load of the cell and is the uplink load threshold. We consider a single cell with perfect power control in a homogenous and uniformly loaded network. Since the network is assumed to be homogeneous, the performance of the system can be deduced from the performance of a single cell analyzed in isolation. Only uplink direction is considered and it is assumed that whenever the uplink channel is assigned the downlink is established. Two types of services are considered, voice and data. Each type has two classes, newly originating calls and soft handoff calls. The soft handoff calls have higher priority than new calls. The system contains a separate queue for each handoff calls type and a predetermined load threshold. The system model is depicted in Fig3.2.

Figure.3: Detailed Cell Model

The arrival process of new and handoff calls is Poisson with rates,

for voice handoff call , data handoff calls, voice new calls and data new calls, respectively. The channel holding time for each type of calls is exponentially distributed with mean while the queuing time of each handoff calls class is exponentially distributed with mean j. This algorithm has the following steps:

When a call arrives, load factor threshold for new and handoff calls and QoS requirements ( in term of BER) are determined firstly using (6). Then the load increase of the arrived call and the current cell load factor before accepting the arrived call are calculated, new using(8). After calculating the current load of the target cell , it is compared with the load factor threshold of the arrived call of type If the current cell load factor plus the load increase is less than or equal the required load factor threshold for the arrived call, then the arrived call can be admitted to enter the target cell. Otherwise, the arrived call is queued or rejected based on queue availability. Queued soft handoff calls can be accepted if sufficient bandwidth gets available, or can be terminated due to timeout.

SIMULATION RESULTS

Simulation Parameters

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In this paper, two services are simulated, voice and data and video. Characteristics of these services are listed in Table3.3.1Simulation Parameters

Parameter

Value

Radio Access Mode

Radio Access Mode WCDMA (FDD)

Service classes

Voice ,Data,Video

Bit rates

Voice:12.2Kbps;Data: 144Kbps

Video:144 kbps

Required Eb/No

Voice :5.6dB; Data :3.2dB

activity

Voice :0.4; Data : 1,

Video:3.2

Fractional load

62.5% - 65%

Interference factor (f)

0.5

Chip-rate

3.84Mbps

Thermal noise

1.0 e-15 W

Channel holding time

180 sec

Arrival rate Poisson

( 0.2-2 calls/sec)

Channel holding time

Exponential (3 min)

Queue limit

0,5,5,5

Queuing time

Exponential (15 s)

System Performance

This proposed Algorithm is evaluated based on three Quality of Service (QoS) metrics: The blocking probability for newly originating calls, the forced termination probability and the total system carried traffic. The blocking probability is the probability that a new call is denied access to the system, while the forced termination probability is the probability that a call that has been admitted will be terminated prior to the call's completion. The Grade of service is considered here to evaluate the system performance and defined as:

Where is the handoff blocking probability, and is the new call blocking probability of calls belonging to traffic of type i. =10 . Which indicates priority level for handoff call to new call? Smaller QoS means better system performance. The system capacity is evaluated using the total carried traffic (i.e., rate of call departure), so as the total carried traffic increase the system capacity in term of supporting more calls increases. The total carried traffic is evaluated using:

(16)

New calls and handoff calls are treated differently. Handoff calls are given higher priority to new calls, and load factor threshold for handoff calls and new calls are different. Handoff calls share residual capacity exclusively besides sharing available capacity with new calls. In simulation we consider the following three scenarios:

Scenario1: All call services classes (new calls and soft handoff calls) are treated equally where they have the same load threshold and no queuing is used.

Scenario2: Same as 1, and in addition to that, the handoff calls are allowed to be queued till the resource is available or the time out is reached.

Scenario3 (proposed algorithm): Same as 2, and in addition to that, the handoff calls have higher load threshold than new calls. This scenario is repeated using different channel holding times.

Average service time for all services is 180 seconds. Arriving rates of all services are changed. Scenario3 is repeated using different service times (120s and 90sec). It is clear that our proposed algorithm has better system capacity and this improvements increase as channel holding time decreases. In general as shown in these figures, the system has a better performance under this proposed algorithm.

CONCLUSION

Call admission control is a very important measure in CDMA system to guarantee the quality of the communicating links. In future wireless networks multimedia traffic will have different QoS requirements. In this paper, the uplink capacity and load estimation formulas is formulated. Then, a prioritized throughput based uplink call admission control algorithm for a WCDMA cellular system with perfect power control is presented. To give priority to soft handoff calls, we introduce queuing techniques and the idea of 'soft guard channels', which is represented by reserving a small fraction of the cell load for the higher priority calls.

One of the key quality-of-service (QoS) measures in wireless cellular networks is the handoff voice call dropping probability as dropping a call-in-progress is generally not considered as acceptable or user-friendly. Call admission control (CAC) scheme is very important measure in wireless cellular networks to guarantee the quality-of-service (QoS) provisioning of the communicating links. Handoff prioritization is the common characteristic of these schemes. In future wireless cellular networks multimedia traffic will have different quality-of-service (QoS) requirements.