Introduction:
According to 3GPP specifications, a UE is required to support many different kinds of measurements. The most commonly used are intra-frequency measurements, inter-frequency measurements and traffic volume measurements. In this article we will look at Inter-Frequency measurements. If every aspect of that is considered then we would need to write a big fat book. We will look at the need and how and why these measurements are performed. Also it would be important to mention here that all the measurement types are independent of each other.
Why are Inter-Frequency Measurements needed?
Inter-Frequency measurements are not a must for a mobile handset to support. If and only if the UE is able to support more than one frequency than this measurements will be used. Some of the reasons why we need these measurements are as follows:
- There could be presence of Hotspots. Say there could be one cell in a large area. But that area has a race course. During the racing season it is full of people using their mobile phones. Thus this one cell might not be able to handle all these calls. For this particular race course, during the racing season there might be a small cell (technically known as micro cell while the cell that covers the big area is known as macro cell). The users would be handed over to these micro cells during the season so other users calls wont be much affected.
- If a lot of people are camped on a same frequency and they move to the same area then the traffic on this particular frequency will increase. To evenly balance the traffic between different frequencies some of the users would be forced to do Inter-Frequency measurements and hence perform Inter-Frequency handover.
- The user might move to an area where the current frequency coverage is about to end. At this particular point the network would ask the UE to perform the Inter-Frequency measurement and move to the new frequency as soon as possible to avoid the loss of call.
How are Inter-Frequency Measurements performed?
RNC UE ----- ---- | | | Measurement Control Message | | ----------------------------------------> | | | | | | Measurement Report | | <---------------------------------------- | | |
The network decides that inter frequency measurements need to be perfoirmed and sends the MEASUREMENT CONTROL MESSAGE with Measurement type set to Inter-Frequency measurements. Generally it will set an Event as well along with the measurements. The following are list of Events that can trigger Measurement Report.
- Event 2a: Change of Best Frequency
- Event 2b: The estimated quality of the currently used frequency is below a certain threshold and the estimated quality of a non-used frequency is above a certain threshold
- Event 2c: The estimated quality of a non-used frequency is above a certain threshold
- Event 2d: The estimated quality of the currently used frequency is below a certain threshold
- Event 2e: The estimated quality of a non-used frequency is below a certain threshold
- Event 2f: The estimated quality of the currently used frequency is above a certain threshold
The most commonly used events from the above list are events 2b and 2d. In case when UE enters 'End of Coverage' area, network will send MCM with event Id set to event 2b and 2d. Event 2d performs the same function as event 2b for the current frequency but the threshold set in case of 2d is much lower. When event 2b is triggered a hand over to new frequency is performed without any problems. In case if event 2b is not triggered while event 2d is triggered that means that the other frequency is not strong enough but the current frequency has deteriorated very much and the only option is to handover to new frequency. In this case the results will not definitely be success.
Hierarchical Cell Structures (HCS):
No discussion about inter-frequency measurements would be complete without discussing HCS. In UMTS, Node B's will be capable of supporting multiple frequencies, though this is not mandatory (and the cost of these Node B's would be more). Thus the cells with different frequencies would be arranged in a hierarchical structures as hown in the figures below.
Figure 1 shows a scenario when there are two identical layers of Macro cells. The network can decide depending on the load which frequency the UE should camp on. Figure 2 shows a Macro and Micro layer. The Micro cells would be available in hot spots where there is extremely high traffic compared to other areas. Figure 3 shows a combination of the first two cases.
References:
[1] 3GPP TS 25.331: RRC Protocol Specification
[2] 3GPP TS 25.304: UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected mode
[3] H. Holma, A. Toskala: WCDMA for UMTS, John Wiley