Barriers exist to Wimax-to-LTE conversion

The wireless broadband market has been divided into the two main groups of wireless operators: the established mobile operators with or without 3G spectrum, and the new entrants, mainly greenfield operators and other fixed line operators. The Wimax Forum and 3GPP have been pursuing two separate avenues to reach the same market.

All-IP Wimax, coming from the IT world, hoped to disrupt the established reign of the dominant mobile operators by providing new entrants with a technology that was not the evolution of past cellular technologies and spectrum characteristics, namely FDD. Wimax was for greenfield players or operators with TDD spectrum that did not need backward-compatibility with legacy cellular technologies supported by LTE. 

On the other hand, LTE was developed by mobile operators - along with their vendors - with little likelihood they would embrace a new disruptive standard. LTE had never been appealing to Wimax operators as an FDD-only technology... at least not until TD-LTE appeared from the ruins of China's TD-SCDMA efforts. There is plenty of TDD spectrum available, and in most cases it is cheaper and under-utilized. Even 3G licenses frequently have TDD allocations and upcoming 2.5 GHz auctions in most cases contemplate TDD bands.

Now that TD-LTE is on the horizon, the question of if and how Wimax operators should transition to LTE has become more relevant. Two years ago several Wimax vendors started to offer a roadmap from Wimax to LTE, thus breaking the taboo that Wimax was losing ground to LTE as a dominant mobile technology.

In response to these trends, the Wimax forum initiated the Wimax 2.0 upgrade that offers most of the same features as LTE-Advanced while providing backward compatibility to Wimax 1.0. Wimax 2.0 is purportedly set to become available towards the end of 2011, but there is little evidence of vendor investment support in the development, especially in these difficult times when resources are limited and vendors have to make a choice.

But what choices do operators have in terms of TD-LTE spectrum?

The Global Certification Forum (responsible for LTE equipment certification) is starting its work in the 2.3 and 2.6 GHz bands, giving a migration path to many mobile Wimax operators operating in those bands. However, due to high signal attenuation, 3.5 GHz band is not seen as suitable for mobile deployments, which is why it is being mostly used today for fixed or nomadic services.

Therefore, LTE has initially left 3.5 GHz band out of its target bands. 3.5 GHz spectrum holders represent the bulk of Wimax operators, and do not have the option of migrating to TD-LTE, at least for now - the Release 10 table has not been released and it is very possible that 3.5 GHz will be included in the profile bands as a way for LTE to enter the fixed/portable market. Other semi-licensed bands such as TV white spaces and 3.65 GHz in the US do not have yet an LTE roadmap.

Hard realities

The major problem with conversion between Wimax and LTE is the cost of device conversion. If the operator has relatively few subscribers already and plans to convert to LTE and move into higher volume mobile service, or otherwise has a large market opportunity to move into, then they might justify the move from Wimax to LTE soon. The first aspect of such a conversion is what to do with the Wimax customers?

These customers have made an investment in the carrier - buying a device, signing a contract, etc. They are not going to be happy if their device can no longer connect to the network. Furthermore, there is the aspect of retention and growth - it's difficult to convince customers to join your Wimax network if they know you're switching to LTE. Current Wimax customers will be more difficult to keep happy as they wonder how they will fare during the transition.

Then there is the problem of RAN. Some vendors' RAN equipment will support both Wimax and LTE concurrently in the same base station, which can reduce the cost of running dual networks - but even these have restrictions about how this can be done. Motorola is one of the vendors claiming it can support both Wimax and LTE in the same base station with the LTE channel turned on as overlay to the Wimax channel.

Of course a new card on the rack or a new rack can also be installed, but everything that involves a visit to the site will be expensive. Another challenge is whether the base station has enough processing power to support two channels, as there may not be enough spectrum to run two parallel networks. 

On the device side, these are expensive and the carrier has the option of a natural migration of users to LTE as they upgrade devices, but this can take a couple of years. If carriers wish to speed up the process it will involve device subsidies. For example, the average lifetime of a USB dongle is 18-24 months; but a Wimax-embedded laptop presents more difficulties than a simple dongle.

Add to the challenge the fact that the core equipment is different for each standard. Despite the commonality in high-level goals, the respective core network architectures developed independently by the Wimax Forum Networking Task Group and by the 3GPP have many differences both in how specific functional and performance goals are achieved, user and control interfaces and protocols, and how the specific functions are partitioned between the key network elements.

There is a small percentage of the core network that is suitable in both networks, such as the AAA and packet router, but mostly the carrier will buy separate core equipment for Wimax and for LTE. In the case of a migration, 80% of the core infrastructure would have to be duplicated, the biggest expense being to replace the Wimax ASN gateway with the LTE equivalent.

Finally and perhaps most importantly, we have the question of spectrum. Turning off the Wimax network to change to LTE is not likely to be acceptable, so carriers who are going to make the change will need to have sufficient spectrum to provide overlay coverage of both access standards. Newer carriers may find that, due to a lower number of customers per base station, they can utilize smaller channel bandwidths for the Wimax network to make room for LTE channels.

As Wimax subscribers migrate to the LTE network, the Wimax network can be migrated to smaller channels and lower frequency reuse patterns to free up spectrum for LTE. Another thing to note is that the availability of TD-LTE was a necessity for most carriers to even consider such a migration, since TDD Wimax and FDD LTE would not play well together.

As a result, the technical, financial, and resource-related issues will keep many carriers from attempting such a conversion. In fact, converting probably only makes sense if the carrier expects to be in direct competition with an LTE network or if roaming is a necessity for its subscribers. 
Adlane Fellah is research director for Maravedis