RNC classifies the cells before the UF computation for services and forms a group of can-didate cells. This classification can speed up the cell selection and provides a suitable and reasonable access decision. Three constraints are adopted to select cells into the candidate cell group, including signal strength constraint, cell admission constraint and terminal capa-bility constraint. Define n as the cell index, which is jointed into the candidate cell group N
Figure 3.1: The UFAS method procedure
when cell n passes the three constraints according to service characteristics.
3.2.1 Signal strength constraint
For an MN, if the strength of pilot or beacon signal from the BS is too small or the distance between the MN and the BS is too far, the cell associated with the BS is not useable. The MN measures the signal strength around in order to filter the cells with enough power out, that is, to eliminate the cell on the far side or the cell with deficient signal strength. If the received signal strength from cell n, denoted by P Wn, exceeds a given power threshold, the cell is classified as feasible. Since WLAN networks differ from WCDMA networks, the predefined signal strength threshold would be different.
3.2.2 Cell admission constraint
This constraint is used to check that the admittance does not sacrifice the quality of the existing connection. The admission control functionality is located in RNC. The cell loading of the WCDMA cell n, ρn, is derived in the Appendix A. Similarly, ρn for WLAN cell n is derived in the Appendix B. The constraint is designed to hold the cell loading to be under a predefined threshold load ρth to ensure the request can be accepted by the CAC scheme.
In WCDMA networks, the threshold load ρth is related to the radio network planning, cell capacity and coverage [20]. For instance, the operator sets the cell loading ρth to be 0.6 in average. When a new service comes and wants to enter the cell, the admission control calculates the effect of this service on the cell loading to ensure the loading of cell n ρn will not over ρth. The increasing load ∆L of the service is denoted by,
∆L = 1
1 + (E W
b/N0)·R·υ
, (3.1)
where W is the chip rate, (Eb/N0) is the required spectral efficiency of the service, υ is the assumed voice activity factor of the service, and R is required bit rate of the service.
Notably, the value of R for new service request is the bit rate of the new service, and for handoff request, it is the total bit rates of the handoff service.
In WLAN networks, the increasing load ∆L is the required data rate of service s. The current load is derived in the Appendix B. Notably, the threshold load ρth is 1.0 so that the current load is the upper bound for the next time since WLAN networks do not have capacity limitation.
By allowing the service into the cell, the RNC does remove the cell from the candidate cells if the load is larger than ρth,
NW CDM A = NW CDM A \ {n}, if ρn+ ∆L >= ρth. (3.2)
Usually, the call admission control strategy has two different thresholds according to two service request types, ongoing call (handoff request) and new call (new service request).
Notably, the ρth for the handoff request will plus a reservation margin since blocking of new call attempts is more tolerable than forced termination of ongoing calls. The margin is also set by radio network planning [20].
3.2.3 Terminal capability constraint
The final constraint reflects the radio module capability and also has different determi-nation on two service request types. By this constraint, the candidate cell group N for the service on an MN is determined. Due to the terminal capability, the candidate cell group is determined based on the condition of radio module occupancy. The determination of N for these cases is described as follows.
The handoff service can hand off any cell of the current network. The cell that uses the free radio module is also considered since it can support the handoff service. Contrarily, for the new service, the selection of candidate cells depends on the free radio modules. When MN opens his first connection, the new service can select any cell from WCDMA and WLAN
networks. If the MN has already used services and then invokes an new one, the cell that currently in use can not be changed or the existed services may be blocked. Hence, the cell that currently in use is a choice and other choice depends on the free radio module.
Moreover, if both radio modules are occupied, then the new service needs to join into the existed connection to avoid other existed services’ blocking.
3.2.4 Illustration of Cell classification
In the following, we listed three cases to depict the work of cell classification. The first two cases are about the handoff request because the handoff services may be linked with WLAN or WCDMA networks originally. And this will affect the consideration of available RF modules. The first case is shown as,
For handoff request which uses WCDMA RF module, For cell n is a WCDMA cell,
N = N ∪{n} , if P Wn> P W (W CDM A), and ρn+ ∆L < ρth. (3.3)
For cell n is a WLAN cell,
N = N ∪ {n} , if P Wn > P W (W LAN ) , ρn+ ∆L < ρth , and
cell n is currently used or WLAN RF module is available. (3.4)
The second case is about the handoff request which uses the WLAN RF module, and the consideration is given by,
For handoff request which uses WLAN RF module, For cell n is a WLAN cell,
N = N ∪ {n}, if P Wn> P W (W LAN ), and ρn+ ∆L < ρth. (3.5)
For cell n is a WCDMA cell,
N = N ∪ {n}, if P Wn > P W (W CDM A) , ρn+ ∆L < ρth , and
cell n is currently used or WCDMA RF module is available.(3.6) The last case is the consideration of new service request, which is shown as,
For new service request,
For cell n is a WCDMA cell,
N = N ∪ {n}, if P Wn > P W (W CDM A) , ρn+ ∆L < ρth , and
cell n is currently used or WCDMA RF module is available.(3.7) For cell n is a WLAN cell,
N = N ∪ {n}, if P Wn > P W (W LAN ) , ρn+ ∆L < ρth , and
cell n is currently used or WLAN RF module is available. (3.8) In each case, the cell n is grouped into the candidate cell group N when it complies with the signal strength constraint, cell admission constraint, and terminal capability constraint.