TDD or FDD Wireless: That is the Question!

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TDD, or Time Division Duplex, where a single radio channel is used to send and receive data, has been a common technique employed in unlicensed microwave transmission bands, such as 2.4 and 5.8GHz. The advantage of TDD is a simplified and lower cost design, often based upon 802.11 standards. In contrast, FDD, or Frequency Division Duplex, where data is transmitted in one frequency channel and received in another (separated by anywhere from less than 100 to more than 1,000 MHz) has been the staple of licensed frequency bands between 2 and 38 GHz worldwide.

Now, a number of the CEPT recommendations for the new point to point bands over 40GHz contain provisions for TDD operation. TDD is accommodated either as an alternative band plan or a mixed TDD/FDD band plan, in addition to the more common FDD band plan. However, CEPT recommendations are only just that—recommendations. How these bands will be implemented in each country will be determined by the individual national regulatory authority.

Recently, we asked a number of European national regulators about if and how they would introduce TDD operation in these new bands. The general response was that they were not opposed to the introduction of TDD in principle, and that such operation would have to be worked into existing or revised band plans. One complication raised was that spectrum would have to be reserved for guard bands between TDD and FDD segments within the same band. Regulators usually try to avoid having to waste valuable spectrum in this way. Also, once a band plan is established and the spectrum allocated to users, efforts to introduce TDD operation at a later date is extremely difficult.

Some regulators have already issued new national band plans at 42GHz and above, and to date none of these allow for TDD operation. Furthermore, for countries that have allocated new bands through spectrum auction, there we see the usual FDD style symmetric band approach.

Despite the appeal of TDD operation from a cost perspective, early indications are that although provision for TDD operation is being made in these higher bands, practical complications and concerns over maximizing the use of new bands may prevent its widespread introduction.

What are your thoughts on using TDD more in national band plans? Leave a comment, if you’d please.

Ian Marshall
Regulatory Manager, Aviat Networks

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Comments

    Sreco June 13, 2011/ Reply

    TDD systems have higher latency and packet delay variation, since they cannot transmit continuously. That has an impact on the Quality of Service.

    aviatnetworks June 20, 2011/ Reply

    Dear Sreco,

    We agree with your observation regarding higher latency and packet delay variation on TDD systems vs. FDD. For this reason, neither PTP nor PMP are suitable for LTE backhaul, which has much more stringent requirements on PDV compared to other mobile access standards.

    Regards,
    Aviat Networks


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