5-1 Zone Concept Sub-channelization Method
Fractional frequency reuse (FFR) cell structure and two kinds of sub-channelization processes in WiMAX system have been introduced in Chapter 2. To implement FFR cell structure, the BS must arrange different frequency reuse sub-channels for data transmissions within a frame.
We consider that FFR is applied and all BSs have the same carrier frequency. The process of forming different sub-carrier sub-channelizations is explained as follows:
1. The distributed sub-carrier sub-channelization process is to divide all usable sub-carriers into 120 physical clusters, and renumbers all of the physical clusters into logical clusters based on DL_PermBase which is provided by the serving BS. Afterwards, the logical clusters are grouped, assigned pilot location, and then take all remaining sub-carriers to form a sub-channel based on DL_PermBase. Owing to the different values of DL_PermBase for each BS, we cannot segment on the frequency for allocating different sub-channels to different BSs. As shown in Fig. 30, the possible way is to segment a frame into many time zones in which the different time zones are allocated to the different BSs for data transmission to achieve FFR.
Time
RF=3 RF=7
Fig. 30 Frame structure for fractional frequency reuse
2. The adjacent sub-carrier sub-channelization is formed in the same way for all BSs;
therefore, it can be segmented either by the time domain or the frequency domain to complete FFR cell structure.
5-2 FBSS Initiation Algorithm for FFR Cell Structure
This section explores how the fast handoff mechanism implements the special FFR cell structure to achieve seamless handoff. In the traditional cell planning for meeting SINR’s need at the cell border, the minimum frequency reuse factor is regarded as K. Under the system structure of the factional frequency reuse, a BS contains a variety of sub-channels with different frequency reuse factor within a cell’s coverage. When close to the BS, the user can use the sub-channels with the reuse factor smaller than K to increase the system capacity.
Besides, planning a small portion of sub-channels with reuse factor larger than K is called handoff sub-channels that can improve the poor signal quality during the handoff. The concept of cell planning is shown in Fig. 31.
Fig. 31 Cell planning for fractional frequency reuse cell
Three kinds of handoff mechanisms have been introduced in Chapter 3. To design the fast base station switching algorithms suitable for the fractional frequency reuse cell structure, we redefine the following parameters for the proposed handoff algorithms by referring to FBSS in IEEE 802.16e-2005, the previous hard and soft handoff mechanisms, and the white paper proposed by WiMAX Forum.
1. Candidate set: the set of the neighboring BSs in which its preamble can be received by the MS.
2. Active BS: An active BS is informed of the mobile station capabilities, security parameters, service flows, and full MAC context information.
3. Diversity Set: The diversity set contains a list of active BSs to the MS. The diversity set is managed by the mobile station and base station. The diversity set is applicable to
macro diversity handover (MDHO) and fast BS switching (FBSS).
4. Serving BS: For any mobile station, the serving base station is the base station with which the mobile station has most recently completed registration at initial network entry or during a handover.
5. Target BS: The base station that a mobile station intends to be registered with at the end of a handover.
6. Maximum Diversity Set Size: The maximum allowable number of BSs in the diversity set.
Fig. 32 Modified FBSS initiation algorithm under FFR cell structure
Fig. 32 is the modified FBSS initiation algorithms under the FFR cell structure. To implement the algorithm, we define the following parameters and initiate some actions based on the received signal strength indicator (RSSI) of the preamble.
1. Th_add: If (RSSI of Serving BS) – (RSSI of Best Candidate Set BS) < Th_add, the network pre-registration process will be started. This process adds the Best Candidate BS into the diversity set and set up the data path, so that the user’s packets can be delivered to the new BS correctly.
2. Th_change: If (RSSI of Best Diversity Set BS) – (RSSI of Serving BS) > Th_change and
the data path of the network layer has been completely established, the link layer handoff will be launched to turn the Best Diversity Set BS into the new Serving BS.
3. Th_replace: If the Diversity Set size is equal to the Maximum Diversity Set size and (RSSI of Best Candidate BS) – (RSSI of Worst Diversity Set BS) > Th_replace, the BS Candidate BS replaces Worst Diversity Set BS to become one of the diversity set and to start the establishment of the data path.
4. Th_remove: If (RSSI of Serving BS) – (RSSI of Worst Diversity Set BS) > Th_remove, the Worst Diversity BS is removed from the Diversity Set to reduce the waste of the network resources.
5. Th_use: If (RSSI of any Diversity BS) – (RSSI of Serving BS) > Th_use, the Serving BS serves the users so as to improve their signal quality by using the pre-arranged handoff sub-channels.
Shown by a random distribution, the time for establishing data path depends on the network state, network topology, and routing path. Besides, when the Th_add and Th_change is fixed, as shown in Fig. 33, we define it into Tc the time change from Th_add to Th_change which is caused by RSSI’s differential values between the Serving BS and the Best Candidate Set BS.
The value of Tc is also a random distribution which depends on the moving speed of the user and the shadow fading.
T
CFig. 33 The definition of Tc
5-3 Signaling Procedure
The signaling procedures of network layer handoff, scanning, initial ranging, fast handover ranging have been introduced in Chapter 4. We describe signaling procedure for fast base station switching under fractional frequency reuse cell structure in this section.
Fig. 34 FBSS signaling procedure
Fig. 34 is the signaling procedure for FBSS. It can be divided into two parts. The first part is the signaling procedure to add the target BS into the diversity set. As the serving BS receives
MOB_MSHO-REQ message from the MS to add the neighboring BS into the diversity set, it negotiates with the target BS through backbone networks message exchanges such that the user’s information can be delivered to the target BS. The other part is to change the serving BS. As the serving BS receives MOB_MSHO-REQ message from the MS to change serving BS, the serving BS can inform the target BS to allocate a dedicated ranging code and slot to the handoff user. The handoff user can perform contention-free fast handover ranging to expedite the handoff such that service disruption time is minimized. On the other hand, the serving BS might be requested by the MS to serve him/her with especially-arranged handoff sub-channels to increase the signal quality if the Th_use condition is met.