Wrong number

LETTERS

This refers to “Dialling more collusion”(September 15). In the article it is alleged that the ministry of communications has inducted the TDD mode in the Broadband Wireless Access (BWA) guidelines to favour some companies. But this is not true. The decision was based on the current availability of spectrum for the purpose of auction, globally-harmonised standards and other technical reasons, as explained below, which show that TDD technology has an edge over the FDD mode:

 

 

FDD mode is best suited for applications, such as voice, that generate symmetrical traffic, while TDD is best suited for bursty, asymmetric traffic, such as Internet or other data-centric services. It may be noted that as per ITU, the BWA service is fundamentally an internet protocol-centric data based service.

 

 

In TDD, both the transmitter and receiver operate on the same frequency but at different times. Therefore, TDD systems reuse the filters, mixers, frequency sources and synthesizers, thereby eliminating the complexity and costs associated with isolating the transmit antenna and the receive antenna. An FDD system uses a duplexer and/or two antennas that require spatial separation and, therefore, cannot reuse the resources.

 

TDD utilises the spectrum more efficiently than FDD. FDD cannot be used in environments where the service provider does not have enough bandwidth to provide the required guardband between transmit and receive channels. FDD system requires pairing of frequencies for go-and-return channels.

 

TDD is more flexible than FDD in meeting the need to dynamically reconfigure the allocated upstream and downstream bandwidth in response to customer needs; especially Internet/web based applications.

 

TDD allows interference mitigation via proper frequency planning. It requires only one interference-free channel compared with FDD, which requires two interference-free channels.

In summary, TDD is a more desirable duplexing technology that allows system operators to receive the most from their investment in spectrum and telecom equipment, while meeting the needs of each individual customer.

However, the government always maintains a technology-neutral position and does not favour any company or technology. In case any company wishes to provide BWA services using the FDD mode, the government shall consider it subject to availability of spectrum in the frequency bands for BWA services.

R P Agrawal, wireless adviser to the Government of India

Sunil Jain replies: The asymmetric traffic you allude to was more typical of Web 1.0, but with peer-to-peer communication more the norm in Web 2.0, the advantage of TDD over FDD may not be as clearcut as you make out. In any case, most technical experts say CDMA is more efficient than GSM, yet GSM is the global market leader — so you have to leave the choice of technology to the user. While each country is free to choose its band structure, it is true that global standards (ITU) show that the 2500-2700 MHz paired with 2620-2690 MHz is an FDD band while the one in between (2570-2620 MHz) is a TDD one — you have, in contrast, kept the whole band for TDD. I am glad you have, however, said you will consider allowing FDD as well, perhaps during the auction process.