In the cellular mobile communication systems, the movement of users causes the variations of channel quality so that the service quality may become worse. In order to provide the users with better communication qualities and uninterrupted services, the handoff mechanisms should be initiated to switch them to other BSs with better channel quality. The goals to perform handoff are presented as follows: [4, 21, 22]
1. When users are in the vicinity of cell border, the handoff mechanisms should be initiated to keep the services uninterrupted.
2. Users can choose the BSs with better channel quality for communications to reduce the power consumption and the interferences to other users.
3. Performing handoff can achieve the load balancing of the systems.
4. Performing handoff maintains the Quality-of-Services (QoS) of the users.
The initiation of the handoff mechanisms may result from the changes of the connection quality, the service requirement, the users’ moving speed, the network conditions, and so on.
In the IP-based OFDMA system, like WiMAX system based on IEEE 802.16e-2005, both of the network and the link layer handoffs should be considered in designing handoff mechanisms. The service disruption caused by the handoff depends on the processing time of the two layers handoff. To achieve seamless handoff, therefore, it would be better to shorten the processing time.
In the WiMAX system, there are three kinds of the handoff mechanisms, the hard handoff, the macro diversity handoff (MDHO), and the fast base station switching (FBSS).
3-1 Hard Handover
As shown in Fig. 21, the user periodically scans the signal qualities of the serving and the neighboring BSs for determining the requirement of handoff when being served. If the signal
quality of the neighboring BS is better than that of the serving BS by a threshold Th_change, the user can inform the serving BS to initiate the handoff mechanism. The user would first perform the link layer handoff for the initial ranging including the clock adjustment, the power adjustment, and the carrier frequency adjustment. After completed, the network reentry process will be initiated. However, the network and the link layer handoffs cause service interruption because they take time.
Referring to the hard handoff mechanism, it has to disconnect the old connection and then reestablish the new one, which is called break-before-make (BBM) mechanism. The benefits of the hard handoff include that it does not occupy the network and the link layer resources, and the signaling procedure is simpler. However, the user’s packets are not correctly delivered to the new BS before the network layer handoff completion. Consequently, the service is interrupted which may not satisfy the quality-of-service (QoS) of real-time services when the users perform the handoff. Moreover, the hand handoff has the ping-pong effect which is related to the setting of the value of Th_change. If the setting value of Th_change is too low, it might leads to wrong handoff decisions which make the user be switched between the new and old BSs back and forth, and cause the resource waste and the service interruption. If the value is higher, it can avoid the problem of making the wrong handoff decision and reduce the occurrence of the ping-pong effect, but the signal quality of the user might become worse.
Fig. 21 Hard handoff initiation algorithm
3-2 Macro Diversity Handover
Different from hard handoff, soft handoff [21-22] allows users to establish connections with multiple BSs. As shown in Fig. 22, the user periodically scans the signal qualities of the serving and the neighboring BSs for determining the requirement of handoff when being served. If the signal quality of the neighbor BS has the trend to become better, and when the differential value of signal quality between the serving BS and the neighboring BS is smaller than the threshold Th_add , as shown in Fig. 22, the user informs the serving BS to add this neighbor BS into the diversity and establish the network layer data path through signaling exchanges such that the user’s packets can be forwarded to the new BS in advance. After this operation is completed, the user can communicate with all the BSs in the diversity set. When the user receives data from all the BSs in the diversity set, it can perform the combining to get the diversity gain.
There are two kinds of combining methods [23]. One is called radio frequency combining (RFC) and the other one is called maximum ratio combining (MRC). Without loss of generality, we assume that there are two BSs in the diversity set. As RFC is performed, the two BSs transmit the same data on the same sub-carriers to the user. There is a delay between the two signals from the two BSs, but the user treats this delay as the results of multi-path channel effect. Consequently, the user doesn’t experience that there are two BSs transmitting the data to him/her and decodes packets as communicating with only one BS. When the MRC is performed, the two BSs send the same data on their own sub-channels to the user, and the user combines the soft bits to decode the data.
Different from the hard handoff, macro diversity handoff allows the user to communicate will multiple BSs, and the problems of ping-pong effect and the long service disruption time are solved. In macro diversity handoff mechanism, the serving BS forwards the user’s data to all the BSs in the diversity set, and the user communicates all of them to get the better signal quality. However, the macro diversity handoff consumes not only the network layer resources but also the link layer resources.
Th_add
Th_replace
Th_remove
Communicate with BS1
Handoff Processing
Time
time RSSI
BS2
BS3
BS1
Communicate with BS2
Handoff Processing
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Fig. 22 Macro diversity handover initiation algorithm
3-3 Fast Base Station Switching
For the hard handoff, the service disruption takes too long so that its QoS can not be guaranteed. On the contrary, the macro diversity handoff can solve this problem, but it occupies the radio resources of the link layer. Therefore, fast base station switching (FBSS) in IEEE 802.16e-2005 [12] adopts the same network layer handoff with MDHO in which the user’s packets will be early forwarded to all BSs in the diversity set. On the other hand, FBSS utilizes the same link layer handoff with the hard handoff in which the user can only communicate with one BS at a time. In so doing, the radio resources of FBSS can be saved, and the QoS can be guaranteed as well.
When the differential value of signal quality between the serving BS and the neighboring BS is smaller than the threshold Th_add, as shown in Fig. 23, the mobile adds the neighboring BS into the diversity set, initiates the network layer handoff, and establishes the data path between the serving BS and the newly-added BS. Therefore, the user’s packets belonging to MS can be transferred to BS2. Different from the time of network reestablishment for the hard
handoff, the FBSS would establish the network connections in advance which can help solve the problem of packet loss for the network disconnection. When communicating with the BS, besides, the user can request the serving BS to allocate the scanning intervals in order to let it perform initial ranging with the neighboring BSs which can adjust the power, the timing, and the carrier frequency in advance to reduce the time required for the link layer handoff. The more detailed signaling procedure in this part will be explored in Chapter 4.