Softbank will implement TD-LTE, developed by China Mobile, as opposed to the original standard, FDD-LTE deployed by Docomo, US Verizon, and others. The primary difference between the two standards is the use of time-division duplexing (TDD) instead of frequency-division duplexing (FDD).
I'll avoid getting into details, but TDD uses the same same channel for up and downlink while FDD uses two slightly offset frequencies. There are pluses and minuses to each. Because TDD uses the same channel for uplink and downlink, bandwidth can be actively allocated between the two as needed. However, TDD increases network complexity and requires more planning with respect to base station locations.
FDD has traditionally been used for voice and data, while TDD just for data. The first demonstration of voice over a TD-LTE network was just performed by Ericsson.
Now, you're probably think CRAP! just what we need is another standard to make things incompatible. Fortunately, the same chipset can easily support both FDD and TDD. The only stumbling block is the one we trip over every day: different frequencies.
NTT Docomo is using 2100 MHz for their "Xi" LTE, while softbank will be using 2500 MHz, an allocation of which it received when pumping cash into willcom.
But with quad band W-CDMA phones common, I imagine that once LTE handsets mature, the radios will have similar frequency coverage.
Testing is slated to begin in Japan in the second quarter of 2011. Softbank CEO Son plans to have the neXt Generation PHS (XGP) picked up from Willcom 100% compatible with TD-LTE. In addition, he says he is "committed" to having a business-oriented LTE XGP service up and running within "this year," which I assumes means this fiscal year, so by April 2012. No word on a consumer oriented service. I imagine that will be a few years down the line, so the best softbank mobile customers can hope for is their DC-HSDPA network.
Reasons for interest in TD-LTE
To finish, an excerpt from a good article Monica Paolini:
here are four main factors driving a growth in support for TD-LTE:
- The FDD LTE and TD-LTE versions of the 3GPP standard are very similar. As a result, devices can support both the FDD and TDD interfaces through a single chipset--i.e., without any additional cost. This is a hugely important new development: TD-LTE will benefit from the wide availability of FDD LTE devices that will be able to support TD-LTE as well. Unlike WiMAX, TD-LTE does not need to prove to have a substantial market share to convince vendors to develop devices. Vendors do not need to develop new devices, they simply need to add TD-LTE support to the existing ones.
- There is a lot of TDD spectrum available, and in most cases it is cheaper and under-utilized. 3G licenses frequently have TDD allocations and upcoming 2.5 GHz auction in most cases contemplate TDD bands.
- The increasing availability of base stations that can be cost-effectively upgraded will make it possible and relatively inexpensive for WiMAX operators to transition to TD‑LTE using the same spectrum allocation. The transition will still require substantial efforts and be justified only in some cases, but it will make it easier for WiMAX operators to have roaming deals and to have access to the same devices that LTE operators have.
- Industry commitment to WiMAX 16m, the ITU-Advanced version of WiMAX and successor to the current WiMAX 16e, is still limited.