One of the useability issues with 3G small cells has been that handover (or handoff) from an outdoor macrocell onto a femtocell hasn’t been implemented extensively. Calls typically remain on the outdoor macrocell until they complete. A handset feature has been specified in the standards which helps, but this doesn't quickly fix the problem for existing phones. Here we highlight one innovative technical solution from Qualcomm that addresses this together with a video they've produced which clearly explains it.
Normal handover operation
In simple terms, handover normally works by the cellsite broadcasting a neighbour list of other suitable nearby cellsites. The handset scans for these and reports the signal strength and quality for the best alternatives. When a neighbour cell can provide a better service than the current cell, the system executes a handover. The new cell broadcasts its own neighbour list, and the process continues again.
This is a gross simplification of course. There are many factors and complex algorithms which determine when and where a handover is best; soft-handover allows multiple signals from different cellsites to be combined; a range of recovery procedures and other actions are defined to handle signal loss and other failure conditions.
The problem for femtocells
The specific problem relating to femtocells concerns handover from an outdoor macrocell to a femtocell, for example when entering your home or office. The basic issue is that there may be many femtocells located within the same footprint of a macrocell – too many to be included in a neighbour list broadcast message, and too many for the handset to individually scan for.
There could also be an operational issue to configure each macrocell with the relevant neighbour lists including femtocells.
One solution is in the handset
3GPP Release 9 included a feature called Closed Subscriber Group which allowed the handset to remember which femtocell(s) it should be looking out for in addition to the neighbour list broadcast by the macrocell. If implemented on every handset, this would solve the problem overnight. The CSG feature can also restrict access to specific femtocells, for example in an office environment only to office staff.
However, not all features included in standards make it into the handset. I'm not aware that this is commonly included in the latest phones today. And of course there are a lot of old handsets still in circulation which won’t ever have this new feature.
Furthermore, I'm not sure that CSG would solve the problem for public access small cells because of the large number of different areas each device might use. It was designed more for home and office use, where users are frequently going to the same places.
Qualcomm propose using a timing difference
This video on the Qualcomm site explains how they adjust the timing parameter for each femtocell, which allows the network to identify which one the handset is receiving the stronger signal from. This allows seamless call continuity as the user moves inside and between different femtocells.
The technique is designed to work with the common 3G UMTS system and any regular 3G handset.
It is based on their UltraSON Mobility Management feature of Active Hand-In using Qualcomm's OTD mechanism for PSC disambiguation and Beacon Based Mechanism for Inter Frequency cell reselection. While this solution may not be suitable for every case, it’s nice to see a documented technical approach that solves an issue for some.
This idea was original published in December 2011, but I only just came across it.