Investigation of Handoff Algorithms for GSM Mobile Cellular Networks

One of the main feature of wireless cellular network is to achieve continuous (uninterrupted) services using handoff when mobile subscribers cross the boundaries of cells in the coverage area. Handoff calls are usually given higher priority than new calls initiated. Various algorithms are investigated using simulation and the results obtained show that the received signal strength with hysteresis and threshold in the serving cell (RSS-HTser.) and the received signal strength with hysteresis and threshold in the new cell (RSS-HTnew.) are the two methods which are closely representing the cellular system environment since they contain additional constrains in Handoff (HO) execution. Simulation are carried out by varying the governing parameters including the effects of fading on the received signal strength, averaging of signal strength, hysteresis and threshold, window size on average signal strength, and the standard deviation which represent worsening signal fading.


The Mobile Radio Channel:
The mobile channel places fundamental limitation on the performance of wireless communication systems. The radio channels are extremely random and do not offer easy analysis. Modeling the radio channel is the most difficult part of the radio system design and is characterized in a statistical fashion. The channel experiences two effects [1][3]:

Short-term Fading:
This fading is mainly caused by the multipath propagation of radio waves that are reflected or diffracted on obstacles such as buildings, vehicles etc. The transmitted signal reaches the receives with time delay signals and on paths with different lengths. Depending on the phase position of the signals of the individual paths, this results in interference of the received signal at the receiver.
Fades are more or less at fixed locations in a given environment. The distances of fading minima are frequency-dependent at about half a wavelength, thus in GSM 900 at approximately 15 an and GSM 1800 at approximately 8 cm.

Long-term Fading:
This fading is caused by shadowing, such as by buildings, therefore it has a greater distance of occurring in build-up urban areas. In GSM this type of fading occurs at about 12-60 m.

Reasons For Handoff (HO):
There are many reasons for HO execution. Usually every MS tries to utilize the radio channel with the best carrier to interference ratio (CIR) by monitoring the signal strength using the MS and the BS of the radio channel. Handoff execution is performed using the BS or the MS by the followings: Radio Link-type HO: This handoff occurs due to the mobility of the MS in the coverage area and depends on: Number of MSs in the cell.

Network Management HO:
The network executes handoff in case of imbalance in traffic intensity between neighboring cells and try to achieve the best balance of channels and other resources required between these cells.
Service-Related HO: It occurs due to degradation in the quality of service (QoS).

Problems Encountered With Handoff:
Handoff does not always occurs in the correct and successful way due to the limitations facing this process.

Ping-Pong Handoff:
It occurs when an MS reaches the boundary of the serving cell and entering the neighboring cell and then returns back to the serving cell [4] [5]. Hysteressis margin (the difference in signal strength level in the neighboring cell and the serving cell at which handoff is initiated), and the average window length are the two basic factors used to avoid ping-pong HO to happen. The effect of ping-pong can be reduced by increasing the hysteresis level to overcome signal fading in shadow region, or using appropriate average window length of the signal to overcome the time the MS spend in a shadow region. Certain limitations must be taken into account: The value of high hysteresis will limit the ping-pong happening also the number of handoff executions, but it will increase delay in HO at cells boundaries which is not accepted practically. The increase in average window length of signal strength causes handoff to be slow and not executed in the appropriate time resulting in lost call. It is important to known the averaging window (either rectangular or exponential of variable weights). The ping-pong cannot be avoided completely even when using appropriate variables (average signal strength and hysteresis), but can be reduced to acceptable level.

Number of Handoff:
Usually microcell radius is about 1 km radius or less is appropriate in urban areas in order to increase system capacity [1] [2]. In such a case the mobile station may cross cell boundaries (depending on MS speed) and therefore many handoff's are needed. Each handoff requires the provision of channels for the MS from the BS the mobile is entering its boundary. This will add an extra burden on the network [5][6] and cause call dropping or handoff failure, also increased number of handoff's requires modified handoff algorithms for a required quality of service (GoS).

Corner Effect on Handoff:
Mobile stations moving in microcells experience handoff in line of sight (LOS-HO) and non line of sight handoff (NLOS-HO). In the latter case the handoff will be difficult. The problem of MS's moving in microcells is when there is a sudden change at street corners or streets junction. A sudden large drop in signal level (20 to 30 dB) occurs. The corner effect is

Types of Handoff [8] [6]:
Network controlled handoff (NCHO): In this type of handoff, the network will decide the handoff by measuring the signal strength at the MS and is performed by the mobile switching center (MSC). This type is used in the 1 st generation advance mobile phone service (AMPS). Mobile Assisted handoff (MAHO): In this method, the MS perform the signal strength measurements and the mobile switching center (MSC) or the base station controller (BSC) control the handoff. Mobile Controlled Handoff (MCHO): The MS completely control and execute the handoff. This method is suitable for microcell system. The method is of the highest degree of decentralization and the benefits of decentralized handoff is the fast decision making. It is used in Digital European Cordless Telephone (DECT).

Detection of the Necessity of Handoff:
The need of handoff is specified by the measurement of signal strength or the measurement of carrier to interference ratio (CIR) which is considered as an important value in a cell and at a certain location. Low value of CIR will force to change the use of the present channel between a mobile station and a base station [1]. The mobile radio channel is a fading channel and this makes the initiation of handoff decision difficult. This effect may cause many unnecessary handoff's (e.g. ping-pong), therefore the need of effective, adaptive and fast method for handoff to deal with fast and temporal (i.e. change of radio channel environment) [9]. To deal with such channel environments, the average signal strength is adopted in order to reduce the effect of short term signal fading [3].

Received Signal Strength (RSS):
Handoff occurs when the signal strength of a neighboring base station received by the MS exceeds that from the serving base station [1].

Received Signal Strength with Threshold (RSS-T):
Handoff occurs when the signal strength of the serving base station is at a level below threshold and the signal strength of the neighboring base station is higher than the signal strength of the serving base station [10].
If the threshold is relatively large as at point T1 in figure 1, the case will be exactly similar to the RSS in section (6.1), and handoff will occur at point A in figure 1. If the threshold is less than that for example at T2 in figure 1, the handoff is delayed until the curve of signal strength intersects the threshold level at point B. If handoff is executed at T3 the delay will be more so the MS will travel large distance in the neighboring cell and MS connection is till with the serving base station, which results in degradation in the quality of connection with BS1 and results in call failure.

Received Signal Strength with Hysteresis (RSS-H):
Handoff occurs when the signal strength of the neighboring base station is higher than that of the serving base station by a hysteresis value (h) as in figure 1 at point C. This method reduces multiple HO (ping-pong HO), and handoff occurs even when the signal strength of the serving base station is strong enough to serve the MS in that cell, so RSS-H results in unnecessary handoff. To avoid unnecessary handoff the received signal strength with hysteresis and threshold of the serving base station is used.

Received Signal Strength with Hysteresis and Threshold of Serving BS (RSS-HT ser ):
In this case handoff to a new cell occurs only when the level of the received signal by the MS from the serving base station (BS 1 ) decreases to a level lower than the threshold and the received signal from the new base station BS 2 is higher that from BS 1 by a certain hysteresis as in figure 1 at point D.

Received Signal Strength with Hysteresis and Threshold of the New BS (RSS-HT new ):
An unintentional handoff to the wrong cell may sometimes occurs. To reduce such wrong handoff, handoff is delayed until the received signal strength received by the MS from a neighboring station is of enough value, to achieve the threshold of the intended new base station with the RSS-H algorithm, and this enhances system performance according to the following: When choosing a correct threshold for the new BS, this will reduce the number of unnecessary handoffs to the new cell when the cell signal strength is inadequate. When threshold value is high and appropriate, the number of unintended handoff, (wrong cell) is lowered.

Handoff Performance Parameters:
Handoff is classified into: Hard Handoff (HHO) is employed in GSM [1]. Soft Handoff (SHO) is used in Code Division Multiple Access (CDMA). In HHO, communication between the serving BS and the MS is disconnected by the serving BS cell before communication between the MS and the neighboring cell BS starts (i.e. communication of the MS is with one BS at a time).

BS 1
To evaluates the efficiency of handoff operation, it is necessary to define the performance of the measuring parameters of the HHO method [11] which are: The number of unnecessary handoff's which occur when the previous connection is of satisfactory performance (no need of handoff). The number of unsuccessful handoff that occur in the cell of which the MS receives inadequate signal strength.

Simulation of Handoff Algorithms:
A two cell mode have been considered in [12] [13].
Considering a three cells model [14] shown in figure 2.
The model network has three base stations BS 1 , BS 2 and BS 3 , the distance between any two cell centers is D meter with the mobile moving at constant speed along the straight line path.
The signal strength received by the MS from the three base stations can be written as:  D meters D: is the distance between any two cells centers in meters. d: is the distance moved by the MS from BS 1 towards BS 2 in meters. k 1 : is the signal strength of BS 1 and is taken as 1W (0 dB). k 2 : represent the path loss and is equivalent to ten times the path loss exponent (n) which depends on the propagation environment and take a value (3 -4) in urban area. u(d), v(d) and w(d) represent shadow fading, which follows Log-normal distribution is represented by zero mean, stationary Gaussian random process and of a standard deviation ( ) in dB.
The values of the parameters used in the simulation are assumed as follows: k 1 = 0 dB, k 2 = 30, D = 2000 m, correlation distance = 20, averaging constant = 30 and a standard deviation representing shadow fading.
The terms u(d), v(d), and w(d) can be generated using white Gaussian noise generator, then passing it to first degree filter [11].
The final equations representing shadow fading terms: Where g 1 (d), g 2 (d) and g 3 (d) are the signals resulting from the noise generators used for the serving, new wrong cells (BS 1 , BS 2 and BS 3 ) respectively. Figure 3 represents the received signal strength from BS 1 and BS 2 given by equations 1 and 2. Figure 4 represents the received signal strength received from BS 3 . Which is given by equation 3.
When dealing with random numbers in system's simulation (e.g. generation of random numbers of log-normal distribution as path loss, or exponential distribution to represent duration of a cell … etc. ) requires executions of simulation for many times (1000 to 2000) which means increasing the number of iterations and taking the average results to reach the stable state.  Assuming that there is no direct radio waves between the BS and the MS, the probability distribution of the envelop of the composite signals is a Rayleigh distribution, and its probability density function is given by: Where r is the envelop of the fading signal and is the standard deviation. By increasing the value of indicates the worsening environment (increased fading) as shown in figure 5. Figure 5 represents the average signal strength from BS 1 , BS 2 , while figure 6 is the average signal strength received for BS 3 .

Simulation Models for Handoff Process:
The methods mentioned in section 6 which are: RSS, RSS-T, RSS-H, RSS-HT ser , RSS-HT new of which the last two [RSS-HT ser and RSS-HT new ] are considered in the simulation due to resembling the mobile cellular environment and contain extra constrains in HO.

RSS-HT ser Model:
This model is explained in section 6.4 and the two cells model (figure 2 without BS 3 ) is used in the simulation. The signal strength received by the MS for a certain location when moving from BS 1 cell towards BS 2 cell is calculated using equations 1 and 2.
The signals are filtered to calculate the average signal strength in order to reduce the effect of shadow fading on the signal strength. Then comparison is drawn and if the handoff constrains are met the handoff is executed. The operation is repeated as the MS moves to the next location (1 meter distance) from BS 1 to BS 2 .
Finally the number of handoff occurrence are calculated from the simulation and then re-executed for large number of times and calculating the number of handoff to exclude the effect of the random variables on the results.

RSS-HT new Model:
This method assures the reduction of unintentional handoff to the wrong cell. Using the model of three cells instead of two cells model used in RSS-HT new as in figure 2. In this model ping-pong handoff is taken into consideration when HO occurs between BS 1 to BS 2 and BS 3 to BS 1 .

Comparisons of the RSS-HT ser and RSS-HT new Models:
The comparative performance of the RSS-HT ser and RSS-HT new models in the handoff process some results are presented [11]. Figure 8 represents the variations in average number handoff's versus hysteresis level for the two models. The average number of handoff in the RSS-HT new is less than that in the RSS-HT ser . for the same threshold level of -85 dB.
Figures 9 to 12 are for constant threshold level of -85 dB for comparison purpose. Figure 9 shows the average crossover point versus hysteresis level in the RSS-HT model is higher (longer time delay of HO) than that of RSS-HT ser . Figure 10 represents the relation between crossover point with hysteresis level for RSS-HT ser and RSS-HT new , it shows that the values of crossover periods of RSS-HT new is higher than RSS-HT ser but it is acceptable due to large decrease of the average number of handoff. Fig. 11 shows comparison between average number of handoff when using exponential and rectangular average signal window, while Fig. 12 shows comparison between call dropping probability when using exponential and rectangular average signal window (dav explained in section 9) and cell dropping probability when using exponential and rectangular window size, which clearly indicate the improved results of the average signal strength using exponential window. In addition, the rectangular window size deals with the signal strength when taking its average with equal weights, while the exponential window deals with the signal strength with different weights. It is clear that the received signal strength versus in level by large amount due to shadow fading and that is why exponential window is used.

Conclusions:
From the results obtained the following conclusions can be drawn.
-From the methods of detection of necessary handoff, the use of threshold level for the new cell (RSS-HT new gives less number of handoff for the same hysteresis level and