What is TDD mode?

Mobile networks have traditionally operated with separate frequencies used for the basestation to transmit and for the smartphone/mobile device to transmit on. These paired frequencies, also called carriers, can be narrow (200kHz for 2G GSM) or wide (5MHz for 3G WCDMA). By constrast, TDD (Time Division Duplex) shares the same single carrier frequency and alternates the transmission between sender and receiver.

Both 3G and 4G/LTE have TDD modes of operation. 3G TD-SCDMA was championed primarily by the Chinese government, and China Mobile was mandated to adopt it. Despite herculean efforts, takeup has been relatively poor and it has not had success outside China. There is also a strong Chinese drive behind TD-LTE, the TDD mode for LTE, which has much more promise of success. 

TDD mode is used in many other wireless communication standards, the most common of which is Wi-Fi - the IEEE 802.11 standard - used in millions of homes and offices worldwide.

Benefits

Two major benefits of TDD mode are:

a) That it can fit into any single spare piece of spectrum. Since it doesn't require paired spectrum, this opens up previously unavailable frequencies.

b) Since the proportion of time spent sending or receiving can be varied, this allows the system to spend more time downloading than uploading. This makes it much more suitable for data traffic than voice, because data is normally highly asymmetric - most of the usage is viewing content from websites or in the cloud. 

Progress

TD-LTE shares the same system architecture and protocols as the normal LTE standard. It is only the Layer 1 (Physical Layer) which differs. The same software stacks, applications and call processing all apply. Learning from some of the issues with 3G TD-SCDMA, the device chipset industry has been encouraged to incorporate TDD mode from the outset. This should allow smartphone device vendors to be able to design TDD mode capable handset variants quickly and cheaply.

Key markets for TDD mode are likely to include:

  • China, which has championed the technology and where many of the patents have originated
  • India, where the large number of competing operators have very limited amounts of spectrum. It should allow additional spectrum to be provided and made best use of.
  • Japan, where Softbank has already rolled out a proprietary form of TD-LTE at 2.3GHz.
  • US, where Sprint has said that it would invest $1.6Billion to use Clearwire spectrum for TD-LTE mode

TDD Small Cells

For 3G TD-SCDMA small cells, China Mobile has really been the only customer. Our report of the Chinese market indicates that over 10,000 (many residential) TD-SCDMA small cells have been deployed in one region alone. However, the total number when compared with a subscriber base of over 700 million is relatively insignificant.

For TD-LTE, we should expect to see a strong takeup of metrocells in order to deliver the huge forecast traffic capacity. This is most likely in China and India, with other countries perhaps becoming late adopters.

For small cell designers, the work involved to design and build a TDD mode variant would require a slightly different physical component layout - specifically the hardware front end. Timing and synchronisation parameters are strict - phase synchronisation is need to avoid interference. The control of the physical layer, adapting to changing environmental conditions, interference, other users etc. would also require additional design and development work, with field experience being critical to optimise the performance.

Industry Views on TD-SCMDA and TD-LTE small cells/femtocells

We gave a quick recap of the state of the Chinese small cell market, and TDD mode in particular, in August 2012.

We wrote about Sprint's TD-LTE investment announcement (via Clearwire) in December 2011.

In our interview with Dr Doug Pulley, CTO of Mindspeed, he believes TDD mode will have an impact. He explains the technology and points to recent interest in the 3.5GHz band which could become a global TDD standard frequency. Device compatibility is critical.


 

Accelleran foresee huge potential for TD-LTE

Trevor Moore Accelleran 150TD-LTE has had a lot of attention in China (over 300m users) and USA (proposed for the 3.5GHz CBRS band). Trevor Moore, CEO of Belgian small cell vendor Accelleran, notes that many countries already have commercially live independent TD-LTE 3.5GHz services. He explains how these are well positioned to disrupt traditional players through the use of small cells and new business models such as neutral host deployment.

 

Dr Doug Pulley, CTO Mindspeed, highlights progress on TDD mode for small cells

Dr Doug PulleyI met with "Dr Doug" at LTE World Summit last month where Mindspeed were demonstrating their high performance LTE chipset working with commercially available LTE USB dongles. He highlighted a lot of recent activity in the TDD mode for LTE, which he believes will be adopted as a truly global standard. He explained the current market status, how Mindspeed is involved and is excited about a potential new opportunity in the US.

Radisys reveal growing market interest in TD-LTE Small Cells

tddTDD mode, which shares the same frequency band for both uplink and downlink, hasn't been widely adopted in 3G cellular systems to date. The new 4G technology LTE actively embraces and adopts this mode, which can make use of a lot of otherwise wasted spectrum, especially at frequencies suitable for small cells.

We spoke with Renuka Bhalerao, Senior Product Line Manager at Radisys, who supply software solutions for small cells, to understand the latest market developments.

Sprint to use Clearwire LTE for high traffic hotspots; would femtocells be better?

CableThe cable TV companies have been in the news a lot recently, with some significant decisions announced that change or reconfirm industry direction. The past week has seen major announcements from Comcast, Time Warner and Cox communications as they release spectrum and wireless network assets. Meanwhile Sprint made some major changes at Clearwire (which they own 50% share of), while Verizon is well placed to take advantage of closer ties with the cable companies. Sprint plans to use Clearwire as part of their LTE solution - we ask if this is the most cost effective approach and consider the implications. 

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