iPhone 3G, Cell Breathing, And Intersystem Handoffs

So everybody is talking now about the dropped calls on the 3G iPhones. Well... I wrote a piece on this subject two years ago: http://headworx.slupik.com/2006/11/disconnected-umts.html. So let us try to understand once again what is happening.

1. 3G iPhone is a UMTS device. This measns it works on 3G/UMTS networks known as WCDMA networks (Wideband CDMA or Wideband Code Division Multiple Access). WCDMA is an evolutionary generation step for GSM networks, but the two standards differ so much, this can hardly be named evolution. It would be like saying iPhone is an evolution of a rotary phone. Sure, both allow people to talk to each other, but are so far away technology - wise...
2. WCDMA networks generally have much smaller cells. This means the distance between the mobile terminal (iPhone) and the base station is relatively short. And this means there have to be many base stations to cover a given area. Many more than 2G GSM base stations. Network planning has to be very careful, especially in urban areas, where buildings greatly affect the coverage. The cells created around base stations must overlap each other. This means when a phone is moving out of the coverage of one base station, it has to be moving in the range of another base station. This process is called a handoff and is relatively seamless between two 3G (or UMTS) base stations. Or at least it should be seamless.
3. The trouble starts when a terminal (like an iPhone) has less sensitive 3G/WCDMA radio than assumed by the network planners. There is an unconfirmed rumor iPhone has some problems in that area. iPhone prototypes the networks were tested with, supposedly were more sensitive compared to production units. This means they passed network interoperability tests but now fail to comply. With less sensitive antenna / receiver, the iPhones are simply falling out of one base station coverage before they are able to attach to another. The reason may be somewhere else. There are opinions the problems originate in immature 3G chipsets inside iPhones. If it takes too long for the Infineon chipset that powers the iPhone to negotiate with the second base station, the connection may be dropped because the user moves out of the coverage of the first base station.
4. There is a chance the problem has always been there but simply it was too difficult to prove any correlation. Before iPhone there was not a single model of a 3G phone so closely examined by the community and the analysts (ans lawyers!). But let us consider the facts:
   - There are many more handoffs in 3G (due to smaller cell sizes) than in 2G
   - Each handoff may fail (simply because the cell the user is moving into is already full and cannot accomodate another extra call)
   - The coverage of 3G base stations is less predictable due to the characteristics of the frequency bands of 3G networks (especially the 2100MHz that has so many problems penetrating concrete walls, so you may experience multiple back and forth handoffs simply walking around a room in a building, sometimes moving closer, sometimes further from a window)
5. The above means iPhone users are very likely to experience so called inter - system handoffs, or handoffs from 3G/UMTS/WCDMA to 2G/GSM networks. Such handoffs are very likely to fail for several reasons:
   - First: the capacity of a 2G cell is usually much smaller than the capacity of a 3G cell, so simply there may be no free slot to accommodate new user.
   - Second: usually the 2G cells are part of a 2G sub-network and 3G cells are part of a 3G sub-network, and the entire conversation context has to be moved from one network to the other. The success rate of that handoff is never 100% and varies depending on the infrastructure equipment vendor. Nortel and Nokia are generally considered poor in that area while Ericsson and Huawei usually work fine.
6. Intersystem handoffs are even more difficult when there is an active data connection (in parallel to the voice connection). Even more. Intersystem handoffs fail more often when there is data session in the background. Now imagine, due to its functional design, how likely it is an iPhone has an active data session during a call?
7. And if that was not enough, WCDMA cells are prone to the "breathing" phenomena. A cell may contract (means: drop the users at the edge), when the overall signal-to-noise ratio gets worse. And this may happen just because another handset / terminal enters the cell.
   

Considering all of the above, I would not count on fast remedy Apple may have to the problem. Dropped calls are the inherent nature of 3G networks. As long as the 3G cells are not overlapping each other by a wide margin (minimizing the number of handoffs and eliminating intersystem handoffs all together), we will be experiencing dropped calls. Apple has probably done nothing wrong here, they just brought everybody's attention to the problem.