This paper presents prior determination of soft handover probability considering new direction of motion of mobile station (MS) coinciding with gravitation point of cells. Our simulation results for 3-cell scenario and considered new direction of MS motion can be potentially used as advance input to soft handover algorithms to minimize number of handovers.
The hard handover algorithms used in earlier GSM networks [
Different handover schemes [8,9] have already been proposed considering different directions of MS motion and network scenarios. However, determination of soft handover probability considering MS motion coinciding gravitation point of cells is comparatively little investigated. The soft handover probability and MS parameter settings have been mostly studied through simulation studies [10-15].
This paper describes determination of soft handover probability depending upon the direction of motion of MS in CDMA/WCDMA 3G networks considering constant speed linear motion of the MS. Our analysis is based on determining soft handover probability as function of soft handover threshold (Tadd ~ Tdrop) at a fixed value of drop timer (Tdt), where Tadd and Tdrop are add threshold and drop threshold respectively. The signal strength exceeding Tadd and its value less than Tdrop are used by MS to enter and delete a base station (BS) in the active set respectively. The results presented provide the prior estimation of soft handover probability which can be potentially used in adaptive soft handover algorithm development to achieve desired QoS.
Section II describes analysis of soft handover algorithms for WCDMA networks. Section III describes mathematical model to calculate the soft handover probability based on the distance between BS and MS and the direction of motion of MS from a reference point. Section IV presents simulation of two and three cell scenarios along with results indicating the effect of direction of MS motion on soft handover probability. Section V contains conclusions.
Soft handover are the CDMA specific handover types implemented in the UMTS networks and form important characteristic of the WCDMA access [
The soft handover decisions are taken based on radio resource control (RRC) report which is obtained by accurate measurement of of the common pilot channel (CPICH). This is determined from the measured value of the received signal code power (RSCP) and the received signal strength available from received signal strength indicator (RSSI). RSCP gives the power carried by the decoded pilot channel and RSSI provides the total wideband received power within the channel bandwidth. The is defined as:
Typically a 200 ms window is chosen for filtering the handover measurement data to eliminate effect of fast fading for avoiding unnecessary handovers. A trade-off is made between number of allowed handovers and its delay because longer averaging duration increases handover delay. Further, minimizing the effect of fast fading through averaging becomes difficult.
The WCDMA handover algorithm differs slightly from IS 95A algorithm used for North American cellular system also based on CDMA. A good insight into the WCDMA handover algorithm can be obtained considering the network scenario example of the original 3GPP specifications [
The signal strength between the BS and MS depends on three parameters i.e. path loss, shadow fading and the multipath component. The path loss between the BS and MS is given by Okumura-Hata model [
where, is carrier frequency (900 MHz), hb is height of the base station antenna, hm is height of the mobile antenna, d is the distance between BS and MS. The shadow fading occurs due to variations in the environmental conditions and the given by the Gaussian probability density function with zero mean and constant variance.
where d0 is decay factor, v is velocity of the MS, and is the variance. The mutlipath fading is due to the multiple paths that exist between BS and MS because of reflections from tall buildings and trees which can be eliminated by averaging function given below.
where dav is averaging distance. Therefore the signal strength between the BS and Ms can be calculated considering different number of BSs and using Equations (2)-(4).
Optimization of soft handover is essential to achieve higher QoS by minimizing call handling overheads of the network which greatly depends upon soft handover probability of a user. The user’s soft handover probability in a network scenario is mainly governed by soft handover threshold and the distance of moving MS from BSs in the active set at any given time; and thus the speed of MS. Therefore this section presents analysis of the effect of these parameters through MATLAB simulations of the geometrical network model and computing signal strength values using Equations (2)-(4). Although our analysis method for determining the effect of soft handover probability on the handover is applicable to any 3G network scenario but this paper presents the results considering 200 ms time window, 150 meters height of BS and 10 meters height of MS for two and three cell scenarios of
Considering users moving at constant speeds, the ratio of time they are in soft handover to the total time they are moving in the system gives a good measure of soft handover probability.
The soft handover threshold is used by WCDMA soft handover algorithm and it is commonly referred as soft handover margin indicating the maximum difference between the two pilot channel signal strengths that can be accommodated in the active set of a MS. Therefore an increase in this value for a given call shall add more BSs into the active set of a MS. Therefore minimizing soft handover probability requires optimization of soft handover margin which necessitates analysing the effect of threshold and the distance between a moving MS and different BSs in the active set.
The soft hand over probability can be calculated either by computing the ratio of the users in soft handover to the total number of users in a cell or finding the ratios between the time a user is in soft handover to the total time for which the user communicates. Considering that the user is moving with constant speed the soft handover probability can be expressed by Equation (5) below.
where and are the soft handover time and distance travelled by the user during soft handover and and are the total call time and distance covered by the user during call at speed ν.
Considering scenario in
where W is the soft handover window and is the distance between the two base stations considering cell size with radius.
where and are the received power at MS and BS transmitted power respectively and represents the channel path loss.
The variation in BS1 transmitted power with distance when it is placed at can be expressed by Equation (9).
where Po is the signal power at and path loss as function of i.e. is given by the ITU vehicular path loss model [
at near the cell edge of
BS1 and BS2 can be obtained by differentiating Equation (9) and using linear approximation as in Equation (10).
where A is a constant. Therefore soft handover probability can be expressed in terms of soft handover threshold by substituting the value of from Equation (10) into Equation (6) as given by Equation (11).
The signal strength between the BS and MS considering BS1, BS2, and BS3 can be calculated for fixed values of add threshold (Tadd), drop threshold (Tdrop), handover margin, and drop timer (Tdt) by eliminating multipath fading through averaging and taking into account the path loss and shadow fading effects. Now as MS moves further away from BS1 toward BS3 it perceives an increase in signal strength from BS3 and decreased signal strength from BS1. At a certain distance x at time t when the received signal power by MS due to BS3 reaches Tadd, the mobile requests a connection with BS3. If this request is accepted then the mobile goes into a soft handover mode and communicates with both base stations BS1 and BS3. If the MS moves further away from BS1, it stops the connection to BS1 when received signal power from BS1 drops below Tdrop. This soft handover probability can be computed for the network scenario in
Our present study is based on MATLAB simulation of the network scenarios shown in Figures 1(a)-(c) and assuming constant speed of MS as illustrated in
Variation of signal strength with distance for all the three scenarios is shown in Figures 4-5 using Eqations (7)-(9).The soft handover probabilities have been determined Equation (11) and its variation as function of soft handover threshold (Tadd ~ Tdrop) is shown in Figures 6-8. Variation in soft handover probability as function of soft handover threshold for scenario in
ing the two cell centres and the variation in the signal strength received by MS is shown in
Comparing the simulation results in Figures 6-8 it is clear that scenario in
ber of handovers, handover initiation points, and the limit of the active set size for different values of (Tadd ~ Tdrop) for different network scenarios.
This paper presents determination of soft handover probabilities as function of soft handover threshold (Tadd ~ Tdrop) for different network scenarios and considering constant speed motion of MS. The simulation results can be potentially useful to determine the range of soft handover threshold for achieving a guaranteed QoS for a given network scenario.
The usefulness of this paper can be extended to the practical cases where variable speed for motion is considered. A slight modification by introducing an algorithm for the estimation of average speed from samples, during period of observation. A more realistic scenario can be generated by considering obstacles in the channel and their contribution to fading and multipath reflections.