SINR BASED VERTICAL HANDOFF STRATEGY

Maximum achievable data rate for the given carrier bandwidth and SINR can be determined with the help of Shannon capacity formula, the maximum achievable data rate R is given by:    

Where:

• W is the carrier bandwidth

• γ is SINR received at user end when associated with WLAN or WCDMA

• Г is the dB gap between uncoded QAM and channel capacity, minus the coding gain.

Let RAP and RBS be the maximum achievable downlink data rate while user connected with WLAN and WCDMA.

From Shannon capacity, we have:

Where, γAP and γBS are the receiving SINR from WLAN and WCDMA respectively. We are interested in the relationship between required γAP and γBS while offering the same downlink data rate to the user by WLAN and WCDMA.

Letting RAP = RBS , we can solve the equation and get the relationship between γAP and γBS as:

The parameters in (4) are:

• The carrier bandwidth for WLAN WAP is 1MHz , and 5MHz for WCDMA WBS .

• ГAP equals to 3dB for WLAN , and ГBS equals to 16dB for WCDMA.

Having the relationship between the maximum achievable data rate and the receiving SINR from both WLAN and WCDMA makes the SINR based vertical handoff method applicable, in which the receiving SINR from WCDMA γBS is being converted to the equivalent γAP required to achieve the same data rate in WLAN, and compared with the actual receiving SINR from WLAN.

With the combined effects of both SINR being considered, handoff is triggered while the user is getting higher equivalent SINR from another access network. It means that given the receiver end SINR measurements of both WLAN and WCDMA channel, the handoff mechanism now has the knowledge of the estimated maximum possible receiving data rates a user can get from either WLAN or WCDMA at the same time within the handover zone, where both WLAN and WCDMA signal are available. This gives the vertical handoff mechanism the ability to make handoff decision with multimedia QoS consideration, such as offer the user maximum downlink throughput from the integrated network, or guarantee the minimum user required data rate during vertical handoff.

A handover Scenario

A scenario in which a user travels through an area with overlapping coverage of a 3G network, WLAN cell and WiMAX is used to explain the three main components of the VHD and their contributions.

Performance evaluation metrics for VHD algorithms

VHD algorithms can be quantitatively compared under various usage scenarios by measuring  the mean and maximum handover delays, the number of handovers, the number of failed handovers due to incorrect decisions, and the overall throughput of a session maintained over

a typical mobility pattern. These metrics are further explained below:

Handover delay refers to the duration between the initiation and completion of the handover process. Handover delay is related to the complexity of the VHD process, and reduction of the handover delay is especially important for delay-sensitive voice or multimedia sessions.

Number of handovers: Reducing the number of handovers is usually preferred as frequent handovers would cause wastage of network resources. A handover is considered to be superfluous when a handover back to the original point of attachment is needed within a certain time duration, and such handovers should be minimized.

Handover failure probability: A handover failure occurs when the handover is initiated but the target network does not have sufficient resources to complete it, or when the mobile terminal moves out of the coverage of the target network before the process is finalized. In the former case, the handover failure probability is related to the channel availability of the target network, while in the latter case it is related to the mobility of the user.

Throughput: The throughput refers to the data rate delivered to the mobile terminals on the network. Handover to a network candidate with higher throughput is usually desirable.