Investigation of Macrocell / Microcell Channels Selection In Multitier Cellular Networks

A proposed method of Macrocell / Microcell channel selection in multitier cellular system using sojourn time of microcell overlapped region. To keep the handoff rate to acceptable level, low mobility users (with speed V < Vth) should undergo handoffs at microcell boundaries and high mobility users (with speed V > Vth) should undergo handoffs at macrocell boundaries. Investigation of variation of the number of channels in the microcells and the macrocell with the blocking probability. Also the variation of microcell radius on the blocking probability for different mobile stations speed.


Introduction:
In multimedia services in order to cope with increasing traffic (increase capacity) is to use microcell of radius few hundred meters. On the other had the number of handoffs increase extensively. In this multitier cell structure consisting of microcell overlay with macrocell is efficient solution. In this scenario, high speed mobile stations (MSs) are serviced in the macrocell and low speed MSs are served in the microcell to reduce the number of handoffs. The speed of the MS has to be estimated precisely in the overlap region [1][2].

Theoretical Analysis:
The proposed scheme consists of four microcells based on sojourn time of the microcells overlapped region such as handoff region. The sojourn time measuring region is the same as in conventional handoff method. When a call originates it is first assigned to microcell after a call origination, the MS starts measuring the overlap region sojourn time.
When cell originates in the overlap area, the MS measures the sojourn time until reaching the boundary of the overlap area.
The MS receives four paging signals from four adjacent microcells Base Stations (BSs) the first strongest paging signal received by the MS is the source microcell, the microcell sending the second strongest paging signal is the target microcell [1].
The cell selection is determined when the MS reaches the boundary of the microcell source. After cell selection, the MS handoffs to the target microcell or its l ocati ng m acrocel l [2] [3]. If the overl apped regi on soj ourn ti m e i s l onger than the threshold time, the MS is estimated to be a low-speed MS and handoff to microcell. If the MS is estimated to be high speed MS and handoff to macrocell. The advantages of this scheme are: · Good performance can be obtained in case of MS moving without charging direction, as in case of MS varying direction.
· In case of fast moving MS changes its direction the sojourn time of the MS will be long and the MS speed is regarded as low speed. In the measuring (overlap) region, the smaller the sojourn time, the lower the probability of changing direction in that region.
· In this scheme, the users (traffic) allocation to macrocell/microcell is efficient.
· Since the capacity of macrocell is much smaller than the embedded microcells of the macrocell in the multitier structure.
· The MS cells selection for handoff is that only fast moving MS's are allocated to macrocell which requires handoff and this can minimize blocking probability of macrocell.
· The limitation is that when the MS speed varies enormously, the sojourn time measuring region is small. Figure 1 shows the overlapped region model .
Let q and f be random variables and uniformly distributed between Let the base station of the source cell be located in (0,0) and the distance between A and B be Z.
The coordinate of A and B as well as the value of Z are given by: Where R is the Microcell radius.
The mean of Z can be found as: Using MATLAB 7.4 the value of E(Z) can be found as E(Z) = mean (Z).
If V is the Mobile Velocity Then: Assume the ORST has a Gaussian distribution, then the pdf of h T is given by is the mean value of ORST. h T s is standard deviation of ORST.
Let P em and P em be the probability of erroneous assignment of a call to macro cell and to micro cell then P em , P em and threshold time o t can be obtained as:

Simulation of the Model:
This simulation is performed using MATLAB 7.4 .
· V threshold (V th ) is 12 m/s.
· Numbers of channels is 10 in the microcell and 10 in the macrocell.

Results:
The number of channels is varied in the macrocell and in   72

Conclusions:
It can be noticed from the results obtained that equations 2 and 4 are easier evaluated using MATLAB 7.4 and are checked with the calculated values [1] and [5].
The results obtained for different MSs speed and different channels allocation for macrocell and microcells. This indicates that when number of MS increases the load on the microcells increase. While the load decreases on the macrocell. When increasing the number of channels in the microcell the blocking probability decreases exponentially.
With increasing speed, the number of channels in the microcell decreases for the same value of blocking probability.
Increasing the speed of the MS in the macrocell the number of channels increases for the same value of blocking probability. Increasing the microcells radius for constant macrocell radius, the blocking probability (load) on the macrocell decreases while it increases for microcell, the load increases with increasing MS speed.