Handset development
In a 3G/ 2.5G world, not all handsets handovers are alike. Graham Carter describes the inherent complexities of dual-mode handover designs, and says tighter integration between the two modes can increase satisfaction of users and operators.
GSA, 169 networks in 91 countries are deploying EDGE and 68 networks are already in commercial operation. That is more than 50 percent of the world’s commercial GPRS networks upgrading to EDGE. But significantly more than 44 of those EDGE networks are being deployed in parallel with WCDMA – that is, they are dual-mode or W-EDGE networks.
To support the delivery of 3rd generation services, there is a clear established need for dual-mode 3G handsets for operators and consumers alike. A dual-mode handset is one that allows the user to make calls on 3G WCDMA networks and 2.5G EDGE/GPRS/GSM networks. Agere supports the view that a combined 3G WCDMA and 2.5G EDGE (W-EDGE) solution offers the most efficient dual-mode experience. However, whether or not mobile operators adopt W-EDGE or just simply WCDMA-GPRS the complexities of dual-mode are the same.
This article highlights one of the key issues of a dual-mode handset and what designers should look for in selecting a dual-mode platform. It raises and answers the question of what is a true dual-mode phone and suggests that there is something better that handset manufacturers should be striving for: a maximal dual-mode handset.
The issue with combined 3G and 2.5G support is not so much being dual-mode but how the handset handles switching from one mode to the other – commonly called handover. Even with the increasing publicity and awareness of data services, we should remember that voice calls are still the biggest source of revenue (80 percent even for 3G networks). As a result, the operator has the best chance of keeping this revenue and avoiding dissatisfied customers by ensuring that dual-mode handover is of the highest quality and dropped calls are a thing of the past.
Background
If you look at the high level architecture of a dual-mode 3G handset, at one level it is a 2.5G and 3G handset side-by-side. That is how the early dual-mode handsets were designed which provided a very basic dual-mode implementation. It works. However, the chances of a clean handover when moving from 3G coverage into 2.5G coverage are slim.
The key to better dual-mode implementations is tight integration. In fact, it is essential for two reasons.
First, consumer expectations of mobile phones are now very high and the competition for the handset market forces the bar constantly higher. Consumers have developed a keen awareness of how much a phone should cost, as well as its physical size and weight, and in particular, they expect the handset to last a considerable time between recharges especially in standby. Second, consumers expect high voice quality and that calls will not be dropped.
Central to achieving these consumer expectations is the design and quality of the handover. Only then will the consumer accept the phone as something that simply works which is essential for mass-market acceptance. It is only possible to achieve these goals of near perfect handovers when moving from 3G to 2.5G through synchronization of the protocol stack, Layer 1 software and physical layers of both the 2.5G and 3G protocol stacks.
Handover & Dual-mode
Handover is the sequence of messages and actions that take place when a handset and base station determine, for whatever reason, that it is best for the handset to connect with another base station. This has been around since the beginning of mobile and works effectively for 2.5G and 3G handsets and base stations.
A dual-mode phone is one that allows the user to make calls on both W-CDMA networks and EDGE/GPRS/GSM networks. Early dual-mode phones had the 2.5G and 3G communications stacks loosely or poorly integrated and tended to drop the call as users left a 3G coverage area. This forces consumers to make a new call once they are within the 2.5G area. Given the fact that users will not tolerate having conversations interrupted due to call drops between networks, it is quite reasonable to say that unless a phone can handover a voice call from 3G to 2.5G, it is not really dual-mode at all.
Handover types
Obviously, we have raised the questions that not all handovers are alike. The question is how good is a phone at handover from 3G to 2.5G? To understand this we need to go back to some basics.
At the outset, 3G networks weren’t designed to handover to 2.5G networks since the original concept was to provide blanket 3G coverage. Over time, as it became clear this was not going to happen, various forms of handover were implemented between 3G and 2.5G – blind, asynchronous and synchronous in ascending order of quality and complexity. The differences between the three center on what measurements are taken before the handover and the resultant quality and reliability of the handover.
Blind handover makes no measurements, asynchronous handover measures power, while synchronous measures power and is synchronised before the handover. The chances of a successful handover increase respectively and the speech gap decreases to the point where it is as small as it can be with synchronous handover.
Blind Handover
A handover occurs when the 3G mobile phone signal is getting weak and the 3G base stations indicate to the phone to try a 2.5G base station on channel X. This is referred to as blind because the network does not know if a mobile can see a 2.5G base station on channel X. The mobile drops the 3G call and tries to find the 2.5G base station. If it does, the handover is successful. This is called a “flying trapeze” as the phone jumps from one and hopes the other is there. Because the mobile phone has never seen the base station on channel X before, there is a high probability of not finding it, which is why blind handovers are of limited value. Combining the time it takes to find the base station along with the time it takes to synchronize with it also results in a large speech gap.
Asynchronous Handover
Ericsson networks invented the concept of compressed mode where the 3G Mobile phone signal is compressed in time (data rate is boosted to keep the same connection speed) and a gap is created. In this gap, the 3G mobile phone can measure the power of nearby 2.5G base stations on, say, channels X, Y, Z and send a measurement report to the 3G network. On this basis, the 3G network can give a handset a command to handover to base station Z, which is the best candidate of all the three that the mobile can see. Blindness is gone as the mobile has already seen X, Y and Z allowing it to select the best choice improving the chance of success. When the mobile phone goes to the 2.5G base station, it still has to synchronise before restarting the voice and this process takes time resulting in a small speech gap. Another consideration is that although channel Z had the “best” power of the 2.5G base stations it may have been power from interference and not necessarily the best choice. This is termed as rogue interference.
Synchronous Handover
This is the same as asynchronous mode, but with the added feature that in the compressed mode gaps the 3G mobile tries to find the power and synchronization information from the 2.5G base station, which amounts to positively identifying the existence of the 2.5G base station. In this case, the handover is a much more reliable option because the 2.5G base station “unquestionably” exists and the mobile has locked on to it. Further, when the mobile phone is on the 2.5G side, the mobile phone can go straight into speech mode because the synchronization info is already known. This means that speech gaps during synchronous handover are the smallest possible or more specifically that the handover is virtually seamless.
True Dual-mode
Our premise is that a phone should only be defined as true dual-mode when it is physically possible to seamlessly handover from 3G to
2.5G networks and vice versa. By definition, it must be able to execute the best implementation of 3G to 2.5G handovers supported by the network.
There are some dual-mode designs that only provide blind handover. The better ones provide blind and asynchronous handovers. There are precious few that provide all three types of handovers. True dual-mode handsets provide all three allowing the network to choose to do synchronous handover for maximum reliability.
Synchronous handover is a more complex process and requires running 2.5G modes inside 3G compressed mode gaps. Without correct system design and good understanding of 2.5G deployment in live commercial networks, this is easy to get wrong.
Maximal Dual-mode
Adopting synchronous handover, however, is not all that can be done to provide the best handover capability. The specifications for absolute frequency accuracy of 3G and 2.5G networks imply that the maximum relative frequency error is 500Hz. A system design for 3G to 2.5G handover could assume this was the case. In reality, the error can be much bigger and it is especially prevalent in the roll-out phases of mobile networks.
With good system design and deep technical understanding, frequency errors much larger than 500Hz can be managed. Specifications provide a basis, but phones still work when the network is out of specification and other phones are failing to deliver a much more acceptable user experience. So there is a method better than true dual-mode and that is what we call a maximal dual-mode implementation.
Idle Mode Handover
So far, we have considered dual-mode handover from the perspective of the user and therefore, the quality of that handover when the handset is in an active call. Of course, a handset spends most of its life waiting to make or receive a call. This is known as idle mode and the handset is still going to have to make 3G to 2.5G handovers in this state.
The quality of the handover here though is not measured directly from a user’s perception of the in-call experience. Instead, it is measured by the length of standby time of the handset. Users have an expectation of the standby time without making the handset too cumbersome with a large battery. Mobile operators include this as a key performance indicator.
Standby times are affected by the quality of the system implementation of idle mode handovers between 3G to 2.5G and vice versa. This is because an idle-mode handover is technically a 3G/2.5G network selection and takes place when the user is not on a speech call and just roaming from 3G to 2.5G areas. In this case, the challenge for the handset is that looking at 3G and 2.5G networks in idle mode can cause a drain on the battery and result in lower standby times. Particular attention must be given to this area. There is a skill to designing support of idle mode handover whilst maintaining low power consumption. Proven systems experience in 2.5G idle mode network selection is key to delivering the same for a dual-mode handset. Some of the early handsets that are out on the market have delivered less than acceptable standby times.
Summary
We have identified that operators’ and consumers’ expectations will demand some fundamental implementation characteristics in dual-mode handover. These are the key areas in which to provide differentiation for a handset which are critical for achieving success with a design in the competitive market place. The two key areas of differentiation will come from:
1. Quality of handover during a call.
2. Power-efficiency of handover in idle mode.
A handset should offer a maximal implementation of dual-mode. That is, it is the best that can practically be done supporting all that the network is capable of achieving, while still interoperating even when the network is out of specification. Further, the 3rd generation handset standby times must match those of the existing 2.5G handsets.
Both of these characteristics mandate that the platform must be built from a proven 2.5G base where handover quality and power efficient idle mode have been a key performance indicator for several years.
Whether the user device is a voice centric mobile handset or an application centric device, such as a music player, gaming deck or enterprise email client, this article has identified that great consideration must be given to the quality of the dual-mode implementation for both in-call handover (voice quality) and idle-mode handover (standby time) in delivering the user experience mobile subscribers have grown to expect from their mobile phones or wireless data devices.
At Agere, we have more than 10 years of experience in developing GPRS technology, including 2.5G and 3G platforms. Those platforms having been the basis for hundreds of handsets being used by every major operator and network in the world. With eight generations of platforms each proving and improving on the whole system approach, we believe there is no platform better to develop dual-mode 2.5/3G handsets upon.
