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THE EVALUATION OF SOFT HANDOFF PERFORMANCE

BETWEEN IS-95A AND IS-95B/CDMA2000

BONGKARN HOMNAN, VIKORN KUNSRIRUKSAKUL AND WATIT BENJAPOLAKUL

Communication System Laboratory, Department of Electrical Engineering

Chulalongkorn University, Bangkok, 10330, Thailand.

Phone: +66-2-218-6482, Fax: +66-2-251-8991, E-mail : [email protected]

ABSTRACTThis paper evaluates the performance of soft handoff

between IS-95A and IS-95B/cdma2000. Cdma2000 soft

handoff also supports in IS-95B. One different thing

between IS-95A and IS-95B/cdma2000 soft handoff is

the value of thresholds (add threshold, drop threshold,drop timer threshold) which is dynamic in IS-

95B/cdma2000. This paper compares the performance

between both algorithms by using the followingperformance Indicators: quality of traffic channel (EB/N0),

outage probability, new call blocking probability (PB),

handoff call blocking probability (PHO), expected number

of base stations in active set (NOBS), expected number of

changes in active set (NOupdate) and Trunking Resource

Efficiency (TRE) with Poisson distributed call arrival and

exponential distributed holding time process. By

comparison of all parameters between both algorithms, IS-

95B soft handoff tends to have a higher performance than

IS-95A soft handoff, especially, TRE, PB and PHO butNOupdateis worse than that of IS-95A soft handoff.

KEYWORDS: CDMA systems, wireless personalcommunication systems, soft handoff.

1. INTRODUCTIONNormally handoff procedure is used to maintain the

quality of service for the mobile users. Too many

changing of serving base stations for mobile stations

affects system loading and performance. Soft handoff

procedure is used in CDMA mobile communicationsystems. It differs from hard handoff used in TDMA or

FDMA mobile communication systems. The differencebetween them is that, in soft handoff, the user attempts to

have simultaneous traffic channels communicating with

more than one base station, so this scheme is a diversity of

handoff for better EB/N0but unfortunately mobile stations

use more resources than those of hard handoff resulting in

such as lower TRE. The other interested parameters are

NOBSand NOupdate analyzed by Zhang and Holtzman [1]

in soft handoff algorithm.

Soft handoffs in IS-95B [2,3] (narrowband CDMA

systems) and in cdma2000 (wideband CDMA systems),

which are both dynamic threshold soft handoffs, are

compared with the performance of the static threshold of

IS-95A soft handoff. Chheda [4] also compared the IS-

95B soft handoff algorithm to IS-95A soft handoff

procedure by varying the forward traffic loading in order

to investigate the average Walsh code per user, Handoff

Message factor, carried traffic gain/loss and probability of

blocking by assuming that the call-blocking threshold isset to 100% of total power, and can relate maximum users

and carried user to call-blocking probability via Erlang B

traffic. However in this paper we define call arrival

process with a Poisson traffic distribution and holding

time with an exponential distribution. Thus, a better view

of soft handoff performance comparison between IS-95A

and IS-95B/cdma2000 can be expected. By assigninginitial constant values of parameters of add threshold

(T_ADD), drop threshold (T_DROP) and drop timer

threshold (T_TDROP) to both algorithms and constant

value for SOFT_SLOPE, ADD_INTERCEPT, DROP_INTERCEPT

to IS-95B algorithm, the performance indicators affecting

the performance of soft handoff at any traffic loading

include [1,4,5] : EB/N0, outage probability, PB, PHO, NOBS,

NOupdateand TRE with specific holding time process.

The paper is divided into 5 sections. Section 2

describes examples of IS-95A and IS-95B/cdma2000 soft

handoffs, section 3 describes the system model andsection 4 describes computer simulation and results,

conclusion and future work are presented in the last

section.

2. EXAMPLES OF IS-95A AND IS-95B/

CDMA2000 SOFT HANDOFF

Fig. 1. IS-95A Soft Handoff Process.


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There are channel lists for mobile station whose

members include the active set (AS), the candidate set(CS), the neighbor set (NS) and the remaining set (RS).

An example of IS-95A soft handoff process as shown in

Fig. 1 can be explained as follows [6].

(1) When the NS s pilot strength mobile station receives,exceeds T_ADD, mobile station sends a Pilot

Strength Measurement Message (PSMM) and

transfers pilot to the CS.

(2) Base station sends an Extended Handoff Direction

Message (EHDM), a General Handoff Direction

Message (GHDM) or Universal Handoff Direction

Message (UHDM) to mobile station in order to do

next step.

(3) Mobile station transfers pilot to the AS and sends

Handoff Completion Message.

(4) When the AS s pilot strength drops below T_DROP,

mobile station sends a PSMM.

(5) Base station receives the PSMM.(6) Base station sends an EHDM, a GHDM or a UHDM.

(7) Mobile station moves pilot from the AS into the NSand send HCM (AS s pilot strength decreases below

T_DROP for T_TDROP seconds).

There are 3 new parameters : SOFT_SLOPE,

ADD_INTERCEPT and DROP_INTERCEPT used in IS-

95B soft handoff algorithm when pilots in the AS are

more than three [2,3].

Fig. 2. IS-95B/cdma2000 Soft Handoff Process.

From Fig. 2, an example of IS-95B/cdma2000 softhandoff process can be described as follows.

(1) When the NS s pilot P2 strength exceeds T_ADD.

Mobile station transfers this pilot to the CS.(2) When the CS s pilot P2 strength exceeds

[(SOFT_SLOPE/8)*10*10log10(PS1)+ADD_INTERCEPT/2],

mobile station sends a PSMM.

(3) Upon receiving an EHDM, a GHDM or a UHDM,

mobile station transfers the pilot P2 to the AS, and

sends a HCM.

(4) When pilot P1 drops below [(SOFT_SLOPE/8)*10*

10log10(PS2)+DROP_INTERCEPT/2], mobile station starts the

handoff drop timer.

(5) When handoff drop timer expires, mobile station sends

a PSMM.(6) Upon receiving an EHDM, a GHDM or a UHDM,

mobile station transfers the pilot P1 to the CS and

sends a HCM.

(7) When pilots P1 strength drops below T_DROP,mobile station starts the handoff drop timer.

(8) Finally, when handoff drop timer expires, mobile

station moves the pilots P1from the CS to the NS.

Note that the conditions of IS-95B/cdma2000 soft

handoff algorithm in this paper are partly different

from those used in [4] but conform to TIA/EIA/IS-95B

[2] and TIA/EIA/IS-2000-5 [3]. Moreover, in this

paper, soft handoff procedures are based on statistical

modeling with Poisson arrival process and exponential

holding time process while Chheda s proposed

simulations [4] are not. Thus, a better view of soft

handoff performance comparison between IS-95A andIS-95B/cdma2000 can be expected.

3. SYSTEM MODEL

3.1 ASSUMPTIONS1) The mobile stations in the system are perfectly

reverse power controlled.

2) Rayleigh fading is neglected in order to mostly

reduce its effect.

3.2 SERVI CE AREAThere are 19 hexagonal cells (1 center cell, 6 first tier

cells and 12 second tier cells). The radius of each cell is3000 m. of equal size with an omni-directional antenna.

3.3 RADIO PROPAGATION MODELMobile station at distance r from base station has an

attenuation !as in (1)

" # 10/10, $%$! rr & (1)where $is the dB attenuation due to shadowing, with zero

mean and standard deviation ' [7]. [8] suggests the

choice of %= 4 for power law and '= 8 dB for standard

deviation and 50% correlation of shadowing between

cells.

3.4 TRAFF IC MODELThe call arrival process is assumed to be a Poisson

process and arrives uniformly in coverage area. Holding

time is exponential distributed with a mean of 120 seconds

over the service area. Forward link power is controlled at

the required power levels within 1 dB standard deviation.

3.5 MOBIL I TY MODELA mobile station moves with uniformly distributed

direction (0-2() [9]. The initial velocity is a Gaussianvariable with a mean of 40 km/h and a standard deviation

of 10km/h [10]. Only the velocity within the range of

[0,60] km/h is selected We assume a mobile to change its


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velocity at random intervals, which are exponentially

distributed [9,10] with a mean of 30 seconds. The updatedand previous speeds are uniformly correlated in the range

of approximately 30%. The new direction correlates to

the previous one with 30% and the new angle is assumed

to be a uniformly distributed random variable.

4. SIMULATION AND RESULTS

4.1 PARAMETERS VALUES1) T_ADD (dB) -12, -13, -14 [4, 11]2) T_DROP (dB) -14, -15, -16 [4, 11]

3) T_TDROP (seconds) 5

4) T_COMP*0.5 (dB) 1 [4]

5) SOFT_SLOPE/8 2.25 [4]

6) ADD_INTERCEPT/2 (dB) 3 [4]

7) DROP_INTERCEPT/2 (dB) 3 [4]

8) Traffic channels/cell 50 [12]

9) Voice activity factor 0.4 [4]10) Orthogonality factor 0.8 [11]

11) Processing gain 128 [11]

12) Maximum BS power (watt) 5 [11]

13) Required Eb/N0(dB) 7 [13,14]

The pilot, paging and synchronization channel power

percentages are set up as follows:

14) Pilot (%) 15 [11]

15) Paging (%) 12 [11]

16) Synchronization channel (%) 1.5 [11].

The Eb/N0 in each cell is re-calculated every 0.1

second.

4.2 RESULTS

4.2.1 THRESHOLDS VARYING

(30 ERLANG/CELL )There are 3 cases by using T_ADD/T_DROP as

follows : -12/-14, -13/-15 and 14/-16 in Table 1, Table 2

and Table 3 respectively for simulations. All parameters

in Table 1, 2 and 3 are the averaged ones by using data

from center and first tier cells. It can be seen that almost

all investigated parameters of IS-95B show higher

performance than those of IS-95A (in this specific traffic).

NOBSis the expected number of base stations in active set[5] and is defined as a combined impact of 1-way to 6-

way handoff percentage. Because of lower NOBS in IS-

95B soft handoff when it is compared with IS-95A soft

handoff (Table 1: 11.33%, Table 2: 14.37%, Table 3:

7.89%), the values of PBand PHO of IS-95B soft handoff

is mostly lower (PB; Table 1: 89.96%, Table 2: 68.58%,

Table 3: 31.56%, PHO; Table 1: 92.55%, Table 2: 77.29%,

Table 3: -2.13%). In this paper, we assign the priority of

handoff call to be higher than that of new call. TRE is the

expected system efficiency, where efficiency is 1/ NOBS[5]. TRE is equal to 100% for hard handoff and TRE is

less than 100% for soft handoff [5]. TRE of IS-95B soft

handoff is higher than TRE of IS-95A about 11.15%,

14.39% and 7.61% in Table 1, Table 2 and Table 3,

respectively.

The performances of IS-95B that are worse than those

of IS-95A are QOS, outage probability and NOupdate. The

effective forward link Eb/N0 (QOS) received at mobilestation is the combination of all sites (NOBS) involved in a

call. Eb/N0 of IS-95B is lower (Table 1: 1.01%, Table 2:

1.01%, Table 3: 0.57%) but they are very little different.

Because of the effective Eb/N0of IS-95B is lower, outage

probability is also lower than that of IS-95A but the values

in Table 1-3 are very low. The expected number of

changes in active set NOupdateis used to measure network

loading [5]. When both algorithms are compared, IS-95B

soft handoff has a higher NOupdate(Table 1: 39.17%, Table

2: 12.88%, Table 3: 27.00%) because of the effects of the

process as shown in Fig. 2.

Table 1. Comparison of soft handoff performancebetween IS-95A and IS-95B/cdma2000 algorithm.

(Traf fi c = 30 erl ang/cell , T_ADD=-12 dB, T_DROP=-14 dB)

Performance

indicatorsIS-95A

Soft Handoff

IS95B/cdma2000

Soft Handoff

Eb/N0(dB) 6.90 6.83

Outage Prob. 0.0079 0.0121

PB 0.0687 0.0069

PHO 0.0483 0.0036

1-way Soft (%) 61.09 73.17

2-way Soft (%) 28.63 20.94

3-way Soft (%) 9.98 5.77

4-way Soft (%) 0.31 0.12

5-way Soft (%) 0 0

6-way Soft (%) 0 0

NOBS 1.50 1.33

TRE (%) 66.89 75.28

NOupdate (%) 4.08 5.70

Table 2. Comparison of soft handoff performance

between IS-95A and IS-95B/cdma2000 algorithm.(Traf fi c = 30 erl ang/cell , T_ADD=-13 dB, T_DROP=-15 dB)

Performance

indicators

IS-95A

Soft Handoff

IS95B/cdma2000

Soft Handoff

Eb/N0(dB) 6.95 6.88

Outage Prob. 0.0053 0.0091

PB 0.1426 0.0448

PHO 0.1114 0.0253

1-way Soft (%) 48.90 64.58

2-way Soft (%) 29.87 22.88

3-way Soft (%) 19.96 11.87

4-way Soft (%) 1.24 0.65

5-way Soft (%) 0.026 0.017

6-way Soft (%) 0 0


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NOBS 1.74 1.49

TRE (%) 57.60 67.28

NOupdate (%) 5.75 6.60

Table 3. Comparison of soft handoff performance

between IS-95A and IS-95B/cdma2000 algorithm.(Traf fi c = 30 erlang/cell , T_ADD=-14 dB, T_DROP=-16 dB)

Performance

indicatorsIS-95A

Soft Handoff

IS95B/cdma2000

Soft Handoff

Eb/N0(dB) 6.96 6.92

Outage Prob. 0.0037 0.0074

PB 0.2044 0.1399

PHO 0.1175 0.1200

1-way Soft (%) 45.52 50.42

2-way Soft (%) 23.43 26.38

3-way Soft (%) 27.34 21.06

4-way Soft (%) 3.55 2.09

5-way Soft (%) 0.15 0.05

6-way Soft (%) 3.1E-03 0

NOBS 1.90 1.75

TRE (%) 52.80 57.15

NOupdate (%) 6.22 8.52

4.2.2 TRAFF IC LOAD VARYING

(T_ADD= -13 dB, T_DROP= -15 dB)

6.8

6.9

7.0

7.1

10 20 30 40 50

Traffic load (erlang)

Eb/No

(dB)

IS-95A

IS-95B

Fig. 3. Eb/N0as a function of traffic load.

It can be shown for both algorithms that when traffic

load is increased, QOS is lower because of more

interference and less NOBS. Eb/N0values of IS-95B are

worse than those of IS-95A but not more than 1.2% which

is quite low as shown in Fig. 3.

0.000

0.005

0.010

0.015

10 20 30 40 50


Outagepr

obability IS-95A

IS-95B

Fig. 4. Outage Probability as a function of

traffic load.

From Fig. 2, IS-95B soft handoff algorithm results in

high outage probability than IS-95A soft handoff

algorithm. This results from that the values of NOBS

(1/TRE) and Eb/N0 are less than those of IS-95A. All of

the values in Fig. 4 are low and still accepted.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

10 20 30 40 50


Newcallblockingprobability

IS-95A

IS-95B

Fig. 5. New call blocking probability as a function of

traffic load.

IS-95B soft handoff algorithm as shown in Fig. 3,

provides a greater improvement than IS-95A soft handoff

algorithm under increasing offered traffic load.

0.00

0.05

0.10

0.150.20

0.25

0.30

0.35

0.40

10 20 30 40 50


Handoffcallblockingprobability

IS-95A

IS-95B

Fig. 6. Handoff call blocking probability as a function of

traffic load.


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40%

50%

60%

70%

80%

10 20 30 40 50


TR

E

IS-95A

IS-95B

Fig. 7. Trunking Resource Efficiency as a function of

traffic load.

IS-95B soft handoff call has better blocking

probability of handoff than that of IS-95A soft handoff

when the offered traffic is under 20 erlang as shown in

Fig 6.

The blocking probability of new call and handoff call

can be reduced by using lower T_TDROP in both

algorithms.

TRE for the two algorithms increases with an increase

in the offered traffic to the system. TRE of IS-95B soft

handoff algorithm is increased from that of IS-95A

algorithm as shown in Fig. 7.

5%

6%

7%

8%

9%

10 20 30 40 50


NOupdate

IS-95A

IS-95B

Fig. 8 NOupdate as a function of traffic load.

NOupdate of IS-95B soft handoff algorithm is slightly

higher than that of IS-95A soft handoff algorithm. As

offered traffic increases, NOupdatedecreases because when

interference increases, the AS s pilot strength decreases,as shown in Fig. 8.

5. CONCLUSION AND FUTURE WORKThe IS-95B soft handoff algorithm is flexible because

it uses dynamic thresholds and has more conditions than

IS-95A soft handoff algorithm. The performance of this

new algorithm is better especially TRE, PB and PHO

which are essential performance indicators for CDMAmobile communications system but NOupdateis worse than

that of IS-95A soft handoff. By evaluation of all

parameters between both algorithms, IS-95B soft handoff

tends to have a higher performance than IS-95A soft

handoff. The results show that it will be better for using

IS-95B/cdma2000 to improve soft handoff performance incurrent CDMA mobile communication networks and next

generations.

For future work, we will investigate on thecharacteristics of dynamic thresholds in IS-

95B/cdma2000 and parameters optimization in these

algorithms. In addition, we are also interested in the

methods to improve handoff performance.

ACKNOWLEDGEMENTThe authors wish to thank the Royal Golden Jubilee

foundation of Thailand Research Fund for research

support, Thailand IMT-2000 groups from many

Universities and Institute of Technologies and Mr.

Preecha Weraachakul, CDMA system manager of the

Communication Authority of Thailand (CAT) for their

valuable discussions and information.

REFERENCES[1] N. Zhang and J. M. Holtzman, Analysis of a CDMA

Soft-Handoff Algorithm, IEEE Trans. Veh. Technol.,47(2), 1998, 710-714.

[2] TR45 TIA/EIA/IS-95B, Mobile Station-Base Station

Compatibility Standard for Dual-Mode Spread Spectrum

Systems, Oct. 31, 1998.

[3] TIA/EIA/IS-2000-5, Upper Layer (Layer 3) Signaling

Standard for cdma2000 Spread Spectrum Systems, Aug.

1999.

[4] A. Chheda, A Performance Comparison of The

CDMA IS-95B and IS-95A Soft Handoff Algorithms,IEEE VTC99, 49th, 2, 1999, 1407-1412.

[5] D. Wong and T. J. Lim, Soft Handoff in CDMA

Mobile Systems, IEEE Personal Communications, 4(6),1997, 6-17.

[6] TIA/EIA/IS-95A, Mobile Station-Base Station

Compatibility Standard for Dual-Mode Spread Spectrum

Systems, July. 1993.

[7] W.C.Y Lee, Mobile Cellular Telecommunications

Systems. New York; McGraw-Hill, 1989.

[8] A. J. Viterbi, A. M. Viterbi, K. S. Gilhousen and E.

Zehavi, Soft Handoff Extendeds CDMA Cell Coverage

and Increase Reverse Link Capacity, IEEE Journal on

Selected Areas in Communications, 12(8), 1994, 1281-1288.

[9] H-G. Jeon, S-H Hwang and S-K Kwon, A Channel

Assignment Scheme for Reducing Call Blocking Rate in

A DS-CDMA Cellular System, IEEE Universal Personal

Comm. Record. 1997, 6th, 2, 637-641.

[10] B. Worley and F. Takawira, Handoff Scheme in

CDMA cellular systems,IEEE COMSIG 98, 1998, South

African, 255-260.

[11] J. Y. and W. C. Y. Lee, Design Aspects and System

Evaluations of IS-95 based CDMA Systems, IEEE

Universal Personal Comm. Record. 1997, 6th, 2, 381-385.

[12] D. M. Lee and D. C. Son, Performance Estimation of

Prioritized Queueing Handoff Scheme in CDMA Cellular

System,IEEE VTC98,48th, 2, 1998, 1049-1053.


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[13] W. C. Y. Lee, Overview of Cellular CDMA, IEEE

Trans. Veh. Technol., 40(2), 1991, 291-302.[14] K. S. Gilhousen, I. M. Jacobs, R. Padovani, A. J.

Viterbi, L. A. Weaver and C. E. Wheatley, On the

Capacity of a Cellular CDMA System, IEEE Trans. Veh.

Technol., 40(2), 1991, 303-311.

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