Qualcomm addresses the issue of 3G macro to femtocell handover

HandoverOne 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.

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#1 Saied said: 
My dear, the Handover problems can be treated as old one..........al so CSG is an access method that aim to reduce the handover between femto and macro. so i dont know what is the new idea that you can give us.........
0 Quote 2012-03-14 22:41
#2 ThinkSmallCell said: 
@Assit Prof: I'm pleased to hear you think that the handover from macro to femto problem is an old one and already solved. I still hear concerns about this from others, and thought I'd highlight this one particular technique. The idea is described in much more detail on the Qualcomm website and in their video - click through to their site to read more detail than the simple summary I gave.

As always, there are many complex issues involved, and my short article above can't address them all in detail.
0 Quote 2012-03-15 07:32
#3 Vichi-apps said: 

Qualcomm is a MSM vendor. Most of the solutions they have proposed (UltraSON, HetNet etc) do not solve the issues caused by small cells. In fact they increase system complexity. Now the reason for increasing MSM complexity is simple.
1.) No one else will be able to make good MSMs (like DO chipsets)
2.) Disturb 3GPP efforts to reduce complexity. (Qualcomm was kicked out of WiMax forum after QCOM bribed consultants to influence 802.16 standard with wrong stuff).

Small cell deployments are eminent. Qualcomm solutions are promising but includes elements which benefit Qcom MSMs. If this conflict of interest is removed, then I guess Qualcomm solutions can be enhanced/built upon to resolve handoff/interfe rence issues.

- Vichi
Learn LTE on the go with vichi-apps
0 Quote 2012-03-18 01:47
#4 ThinkSmallCell said: 
This article certainly stirred some strong feelings! I'm sure that Qualcomm has built up an impressive range of patents in addition to its own products.

The main point I was making was that there are technical solutions to some of the shortcomings of the original 3G specifications, of which this is just one example. Whether this solution is adopted or another is used instead, it's good to see some answers to some of the concerns outstanding.
0 Quote 2012-03-19 22:59
#5 Akanksha said: 
I'm doing thesis in Interference Management in Femtocells. and looking forward to solve the handover problem in Femtocells. Can somebody provide me with some solution for the same.
I need to complete this thesis with its simulation by 27 may.
Looking forward for a favorable reply.
0 Quote 2013-04-29 07:58
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