Utilizing Mobile Station Capacity and Reduction in Code Blocking

Vipin Balyan1, Davinder S Saini2, Pranjal Aggarwal3, Gunjan Gupta4, Inderjeet Sharma5

Department of Electronics and Communication Engineering Jaypee Institute of Information Technology, Noida, Uttar Pradesh-20103071,3,5

Jaypee University of Information Technology, Solan, Himachal Pradesh – 1732152

vipin.balyan@jiit.ac.in1, davinder.saini@juit.ac.in2, pranjal.agg@gmail.com3, gunjan.gupta@yahoo.com4, inderjeetjiit@gmail.com5

Abstract—In this paper, a code assignment scheme is proposed which uses non utilized capacity of mobile station (MS) code tree. A call can be handled using direct hop or multi hop when BS don’t have enough capacity to handle the call. The utilization of MS capacity to handle will reduce code blocking probability of a call arrives at BS and BS don’t have enough capacity to handle the call. The different modes are compared for different distribution scenarios. Simulation and results are given to verify the proposed scheme. Keywords—Ad-Hoc; OVSF; assignment; code blocking; call establishment delay.

I. INTRODUCTION 4th Generation mobile communication will use

mobile stations (MSs) equipped with more battery power and processors capable of routing traffic through them for small distance. Ad Hoc networks are wireless networks without fixed infrastructure [1]. In this paper, an OVSF (Orthogonal Variable Spreading factor) [2] code tree assignment scheme for Ad Hoc-CDMA network is proposed in which a call can be handled with or without utilizing base station (BS) capacity. In this network, each mobile node in the network functions as a router that discovers and maintains routes (number of hops) for other nodes. The use of MS capacity to handle traffic at small distance will lead reduced code blocking probability with an additional disadvantage of high power consumption of MS and lower Quality of Service (QoS). The movement of nodes is random and will lead to change in network topology frequently and unpredictably. The code blocking probability is defined as a condition that a new call request is blocked even though the system has enough capacity to handle the call.

A. Problem Statement The code blocking can be due to internal and

external fragmentation. Internal fragmentation [3] occurs when a higher rate code is assigned to lower rate call request, its mainly due to non-quantized ( kR ) calls assigned to quantized ( 12 ,1l R l L− ≤ ≤ ) codes. Also, 2' l ogl k≥ and ' 1 1(2 2 )l l R− −− is wastage capacity. External fragmentation [3] is due to assignment of calls in scattered way. For example, we must assign 16R to a call request of 12R. The capacity 4R (16R-12R) which is 33% of the required bandwidth is wasted and which increases with increase in difference of requested call rate and assigned code. One of the options to reduce this wastage is to use multiple codes which may increase the complexity, cost of the base station (BS) and mobile station (MS).

In literature, a number of code assignment

TABLE I. DEFINITION OF VARIOUS ABBREVATIONS

UMSC

Capacity of mobile station already used.

ThMSC

Capacity threshold of the mobile station which can be used for providing capacity to other mobile stations to handle new call.

UN Number of mobile stations actually used to handle a call using multihop communication.

ThMHN

Number of mobile stations that can be used to handle a call using multihop communication.

TCBSC

Total capacity of base station.

UBSC

Capacity of base station already in use.

ThMHD

Distance threshold above which a call cannot be handled using multihop communication.

acD Distance of mobile station on which call is active from device or base station depending on the mode used.

ThDCD

Distance threshold above which a call cannot be handled using direct communication.

R Bit Rate (7.5 Kbps for WCDMA Networks)

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schemes are already proposed which aims at reducing code blocking probability. The single code assignment scheme uses only one code from the OVSF code tree to handle new call. The single code usage requires single rake combiner in the BS and UE. The multi-code assignment scheme uses multiple codes to handle quantized or non-quantized data rates. This requires multiple rake combiners equal to the number of codes required to handle new call which leads to increased complexity. Single code schemes: crowded first assignment (CFA) [3], leftmost code assignment (LCA) [3], fixed set partitioning (FSP) [6] and recursive fewer codes blocked (RFCB) scheme [7] are few popular single code assignment schemes. In CFA, the code assignment is carried out to serve higher rate calls better in future. It has two categories namely crowded first code (based upon number of busy children) and crowded first capacity (based upon children used capacity). In LCA, code assignment is carried out from left side of the OVSF code tree. In FSP, the code tree is divided into a number of sub trees according to the number of input traffic classes and their distribution. The RFCB scheme works on the top of CFA and the optimum code is the code which

makes least number of higher rates codes blocked. It resolves tie by recursively searching for best candidate. The adaptive code assignment (ADA) [8] scheme divide the tree into small portions according to arrival distribution reducing the number of codes searched for new calls. The dynamic code assignment (DCA) scheme in [9] handles new call using code reassignments. This is the best single code scheme to reduce code blocking but the cost and complexity in reassignments is too high which limits its usage for low to medium traffic conditions. A top down schemes scheme which searches an optimum code from root code and reduces code blocking significantly with lesser call establishment delay [10].

The rest of the paper is organised as follows. The proposed scheme is give in section II. Result and simulations are given in section III. Finally, paper is concluded in section IV.

II. PROPOSED MODES OF OPERATION

A. Call handle process: Mobile Stations acting as Adhoc networks

1. Mobile station oriented mode (MOM)

a) Single hop communication: For a requested call of rate 12 ,1l R l L− ≤ ≤ , if the intermediate node

satisfies 12Th U lMS MSC C R−− ≥ the path will be

provided by the node. If single hop communication is possible through n Mobile Stations(MS), then the MS with min( U

MSC ) will be used as it will lead to less utilization of power from MS.

b) Multihop communication: If

12Th U lMS MSC C R−− < the base station(BS) will

check the number of mobile stations UN out of

available mobile stations ThMHN that can be used to

handle the call such that, for ThU MHN N< ,

11 ( ) 2UN Th U l

MS MSi C C R−= − ≥ .

If a tie occurs for two paths to a MS, then tie is resolved by assigning call to the available shortest path. If still tie persists assign MS in the path with min( U

MSC ).

2. Base station oriented mode (BOM):If 1

1 ( ) 2UN Th U lMS MSi C C R−

= − < or 0UN = then BS will check for the capacity available with it such

that 12TC U lBS BSC C R−− ≥ , if the condition satisfies

then call is assigned otherwise call is blocked.

A1 A2 A3

A4 A5 A6 A7 A8

A9 A10 A11

B A

(a)

(b) Fig. 1. Mobile network in (a) Direct Hop and (b) Multi Hop pattern. Tree shown is max capacity available to a MS node.

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B. Switching of Modes / Handoff in Adhoc networks:

1. BOM to MOM If MS(s) enters coverage area of BS in which calls are in BS mode, then calls can switch from BOM to MOM. • If there are intermediate nodes present in

this coverage area satisfying 12Th U l

MS MSC C R−− ≥ then to save the BS capacity, the active call is switched to MOM and free BS capacity can be used for future calls (direct communication).

• If ThMHN becomes finite i.e. there are MS

present in this coverage area which can provide path to another MS through them to save BS capacity for future calls (if

11 ( ) 2UN Th U l

MS MSi C C R−= − ≥ ), then also

there is a handoff from base station to MS (using Multihop communication).

2. MOM to BOM

If a MS on which a call is active using capacity of another MS is moving

• If the active call was using the capacity of intermediate node (direct communication) but due to its movement distance from another MS increases then to

save the active call from being dropped due to Th

ac DCD D> , call is either switched to another MS or BS mode.

• If active call is in multi hop communication and Th

ac MHD D> then active call changes its BS due to which call has to be switched to MS of first BS to the MS in second BS.

• If the MSs present under this second base station cannot provide the required capacity to the active call using direct communication( 12Th U l

MS MSC C R−− ≥ ) or using multihop communication(

11 ( ) 2UN Th U l

MS MSi C C R−= − < ), then the

capacity of the second BS is checked. If 12TC U l

BS BSC C R−− ≥ satisfies, then handoff is made between device under first BS to the new second BS.

• If two calls generated together which can be handled by a MS, call with higher capacity

0 1 2 3 40

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Average Traffic Load (a)

Blo

ckin

g P

roba

bilit

y

Distribution[20:20:20:20:20]

BOM

DOM

MOC

0 1 2 3 40

0.05

0.1

0.15

0.2

Average Traffic Load (a)

Blo

ckin

g P

roba

bilit

y

Distribution[40:30:10:10:10]

BOM

DOMMOC

0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

Average Traffic Load (a)

Blo

ckin

g P

roba

bilit

y

Distribution[10:10:10:30:40]

BOM

DOMMOC

0 1 2 3 40

0.1

0.2

0.3

0.4

Average Traffic Load (a)

Blo

ckin

g P

roba

bilit

y

Distribution[12.5:25:25:25:12.5]

BOM

DOMMOC

(a)

(b)

(c)

Fig. 2. Blocking Probability of various modes for distributions: (a) [20, 20, 20, 20, 20], (a) [20, 20,20,20,20], (b) [40, 30,10,10,10], (c) [10, 10,10,30,40], (d) [12.5, 25,25,25,12.5]

(d)

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is handled and lower is rejected or handled using BS mode. It will lead to lower code blocking probability. III. SIMULATIONS AND RESULTS

Following simulation parameters are considered for comparison of various modes of call assignment. • There are 5 classes of users with rates R, 2R, 4R,

8R, 16R. • Arrival rate is Poisson distributed with mean

value varying from 0-4 calls/minute. • Call duration is exponentially distributed with

mean value of 3 minutes. • The maximum capacity of the tree is 128R (R is

7.5kbps). • Simulation is done for 5000 users and result is

average of 10 simulations. Let i, i∈ [1,5] is the arrival rate and μi, i∈[1,5] is service rate for ith class users. Define iii μλρ /= as traffic load of the ith class users. Also, for 5 class system the Average arrival rate is = =

51i iλλ .

Average traffic load )/(51 ii i= = μλρ .

Call duration of all the calls equal i.e. iμμ /1/1 = . Define [p1,p2,p3,p4,p5] as probability distribution matrix where pi, ]5,1[∈i , is the capacity portion required by the ith class calls on average. Four distribution scenarios are considered • [20,20,20,20,20]: Capacity of each rate is

uniform. • [40,30,10,10,10]: Capacity of low rate calls

dominating. • [10,10,10,30,40]: Capacity of high rate calls

dominating. • [12.5,25,25,25,12.5]: Capacity of medium rate

calls dominating. The results of base station oriented mode (BOM),

including Direct Hop Communication (DOM) and Multi Hop communication (MOC) are compared with each other in Figure 2. The code blocking probability of a new call requested of MOC scheme is lowest as compare to BOM and DOM for uniform and low rate dominating scenario. However, for high rate and medium rate dominating distribution DOM perform better than MOC as MS code tree capacity is limited to 32R. The Multi Hop communication suffers from higher call establishment delay as it searches more number of Mobile stations most of the times.

IV. CONCLUSION

Real time calls are delay sensitive and quality of service is an important parameter for them. A fixed amount of delay is however tolerable then handling

calls using single hop and multi hop communication lead to lower code blocking. MOC and DOM reduces code blocking significantly. In future work, can be done to address QoS requirements when a call is handled by multiple MSs with reliability.

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[2] E. M. Royer and C.-K. Toh, “A review of current routing protocols for ad hoc mobile wireless networks, “ IEEE Personal Communication, pp. 46-55, 1999.

[3] Y. C. Tseng, C. M. Chao and S. L. Wu, Code placement and replacement strategies for wideband CDMA OVSF code tree management, in: IEEE GLOBECOM’01, 2001, pp 562-566.

[4] D. S. Saini and S. V. Bhoosan, Code Tree Extension and Performance Improvement in OVSF-CDMA Systems, in: IEEE ICSCN 2007, 2007, pp 316-319.

[5] R. H. Hwang, B. J. Chang, M. X. Chen and K. C. Tsai, An efficient adaptive grouping for single code assignment in WCDMA mobile networks, Springer Wireless Personal Communication, 39(1) (2006), 41- 61.

[6] J. S. Park and D.C. Lee, Enhanced fixed and dynamic code assignment policies for OVSF-CDMA systems, in: ICWN 2003, 2003, pp 620-625.

[7] A. N. Rouskas and D. N. Skoutas, Management of channelization codes at the forward link of WCDMA, IEEE Communication Letter 9 (2005), 679-681.

[8] D. S. Saini and S. V. Bhoosan, Adaptive assignment scheme for OVSF codes in WCDMA, in : IEEE ICWMC, 2006, pp 65.

[9] T. Minn and K. Y. Siu, Dynamic assignment of orthogonal variable-spreading-factor codes in WCDMA, IEEE Journal on Selected Areas in Communications 18(8)(2000), 1429-1440.

[10] D. S. Saini and V. Balyan, “Top Down Code Search to Locate An Optimum Code and Reduction in Code Blocking for CDMA Networks”, Wireless Personal Communication Springer, published online, DOI 10.1007/s11277-013-1372-9.

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