Wimax standards and Security The Wimax

Macro Diversity Handover and Fast BS Switching

In addition to the HO operation discussed above, there are two optional HO techniques: MDHO and FBSS. The purpose of both HO schemes is to provide a diversity gain that increases cell coverage and QoS at a cell boundary, as well as a fast HO.
MDHO performs the diversity combining both DL and UL, since two or more BSs transmit the same DL data to the MS and receive the same UL data from the MS in the same time interval. FBSS HO utilizes selection diversity and a fast switching mechanism to improve link quality. In FBSS, the MS only transmits/receives data to/from its serving BS (called the anchor BS when this technique is in operation) at any given frame. The anchor BS can change, frame by frame, according to a scheme for selecting BSs.
There are several requirements that enable MDHO and FBSS to occur between the MS and a group of BSs, as follows:

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  The BSs involved in MDHO/FBSS are synchronized based on a common time source.
  
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  The frames sent by the BSs involved in MDHO/FBSS at a given frame time arrive at the MS within a predetermined interval.
  
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  The BSs involved in MDHO/FBSS have a synchronized frame structure.
  

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  The BSs involved in MDHO/FBSS have the same frequency assignment.
  
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  The BSs involved in MDHO/FBSS are required to share or transfer a MAC context.
  

In the case of MDHO, the following two conditions are required additionally:

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  The BSs involved in MDHO use the same set of CIDs for the connections that are established with the MS.
  
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  The same data are sent to the MS by all BSs involved in MDHO.
  

1. Macro Diversity Handover

In MDHO, an MS and a BS manage a diversity set, which is a list of BSs that are involved in MDHO with the MS. Among the BSs in the diversity set, an anchor BS is defined. When operating in MDHO, the MS can communicate with all BSs in the diversity set for UL and DL traffic. For DL MDHO, two or more BSs provide synchronized transmission of MS DL data such that the diversity combining can be achieved by the MS. For UL MDHO, the transmis- sion from an MS is received by multiple BSs such that the selection diversity can be achieved by multiple BSs.
Figure 6.9 shows a procedure of MDHO. A BS that supports MDHO or FBSS broadcasts the DCD message that includes the H_Add and H_Delete thresholds. These thresholds are used by an MS with FBSS/MDHO capability to determine when the MOB_MSHO-REQ should be sent. When the long- term CINR of a neighboring BS is higher than the H_Add threshold, the MS sends the MOB_MSHO-REQ to require that this neighboring BS be added to a diversity set. When the long-term CINR of a serving BS is less than the H_Delete threshold, the MS sends the MOB_MSHO-REQ to require that this serving BS be removed from the diversity set.
In Figure 6.9, an MS that communicates with its serving BS (BS1) transmits an MOB_MSHO-REQ message if the CINR of a neighboring BS is higher than the H_Add threshold. The MOB_MSHO-REQ message contains not only a possible list of BSs to be included in the MS’s diversity set, but also their chan- nel quality evaluated using previous channel measurements. When sending an MOB_BSHO-RSP, the BS provides a list of BSs recommended for the MS’s diversity set. In Figure 6.9, a BS2 is added into the diversity set. Moreover, the BSs can provide a recommended list of BSs by sending an MOB_BSHO-REQ in an unsolicited manner. If the MS receives the MOB_BSHO-RSP message, it chooses the actual update by considering the received diversity set and sends an MOB_HO-IND message that contains the type field of confirm diversity set update. Finally, the MS can receive DL-MAP/UL-MAP from BS2 as well as from BS1 (which is the anchor BS in the diversity set), so it can communicate with BS1 and BS2 simultaneously.