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Why PCCW is laughing all the way to the bank

TDD mode is not just data delivery

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Letter The article PCCW opens kimono (a little) on UK broadband wireless plans has a statement that is significantly misleading in it. In absence of anywhere else to send it I thought I'd send my comments to you. The article says "We say WiMAX class because initial intelligence on PCCW had it that its new Netvigator service will in fact use the IP Wireless UMTS TDD technology that is a data delivery extension of 3G.", and this is incorrect.

To understand why I need to go back a little while. Back many years ago, the ITU founded a group called IMT2000 - this group was to decide on the single universal standard that would be the 3G system globally. The idea was that this would allow easy migration for PCS, TDMA, GSM and CDMA operations so that a phone (a UPT - Universal Personal something or other - can't remember) would work anywhere. The IMT groups asked the industry for suggestions as to what should be used as the radio interface for these services (given the requirements of the system, i.e. high bandwidths).

As far as I remember from what I've read there were between 10 and 20 proposals given and the IMT2000 decided on just 1? No it didn't it decided on 5. The 5 are FDD = Frequency Division Duplex, TDD = Time Division Duplex, TD-SCDMA = Time Division Synchronous CDMA, CDMA 1x and CDMA 3x. CDMA 1x is a relatively trivial upgrade from existing CDMA operators, and CDMA 3x is not a massive upgrade from there. By the way the 1x and 3x refer to the number 1.25MHz channels that are used to communicate.

TDD and FDD mode both use 5MHz channels, so 3x actually uses .25MHz bandwidth less. Obviously the Chinese are going to use TD-SCDMA for IP reasons. Note that all systems use spread spectrum techniques and so Qualcomm gets paid whatever.

As Japan had the most urgent need for 3G services (due to capacity problems) it order from Nokia and Ericsson FDD mode test equipment. This then led to Nokia and Ericsson (a) being most advanced with development and subsequently being able to cream the opposition in the market and (b) Nokia and Ericsson pushing the FDD mode systems globally. TDD mode remained the poor cousin.

A few companies, notably Interdigital in the US and IP Wireless in the UK, picked up TDD and started to develop it. In the UK, of the 5 licences made available 4 included 1 band of "unpaired" spectrum (i.e. spectrum suitable for TDD mode use).

I spoke to IP Wireless last year about providing TDD mode equipment into rural areas to bring broadband as part of the EEDA's (not) Connecting Communities competition. In my view TDD mode provided a really good way of dealing with rural broadband as WiMAX was really just a bit of a glimmer at that stage. This was what I proposed. What we got was Trilogy Telecoms LLU solution - but that's a whole other story.

Anyway there is nothing that necessarily requires TDD mode to work in the 2.1 GHz range and indeed IP Wireless were working on the 3.4GHz spectrum as well. The reason the above statement is wrong relates to the whole discussion about which standards were selected. TDD mode is NOT "the data delivery extension of 3G". TDD is capable of providing everything that the 3G operators can provide.

What this means is that the TDD should support 144kbps in all radio environments (up to 500km/h); TDD uses USIMs to authenticate and has all that security and (in PCCWs case) is fully capable of supporting telephony services - it's built into the standards. Whether the suppliers of PCCWs kit support the voice services is another issue, but the TDD mode system fully supports as a first class bearer (it's not VoIP).

TDD mode systems will also support the accelerated emergency call set-up as GSM does. I hope this explains why the statement is misleading.

Of course the 3G operators are pissed about it, if they could have bought a nationwide licence for 3G service for £14million instead of £5billion they'd be laughing all the way to the bank - that is what PCCW has done.

Dominic Binks

Internet Security Threat Report 2014

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