TDD UL and DL Ratios and Uplink Speeds

An interesting technical detail came to my attention today concerning Time Division Duplex (TDD) wireless systems such as WiMAX: Since uplink and downlink transmission is done in the same frequency band, uplink and downlink capacity can be adjusted based on demand. In theory this is an advantage over FDD (Frquency Division Duplex), used by most cellular 2G and 3G systems today. Here, uplink transmissions use a seperate frequency band which is just as large as the downlink frequency band (e.g. 5 MHz for UMTS). This means that FDD systems always have a 1:1 ratio between uplink and downlink. With TDD systems, this ratio can be changed, for example to 2:1, 3:1, etc. to give more capacity to the downlink. But there is one important thing to remember: The efficiency of uplink transmissions is much lower than in the downlink due to the lower transmission power and small antennas of the mobile device. Thus, even with a 1:1 ratio, uplink data rates are far lower than data rates in the downlink despite the using the same amount of bandwidth. I estimate that the maximum overall speed achieved in uplink direction is only 1/3 or 1/4 of the downlink. With rising uplink requirements of web 2.0 applications (picture, video uploads for example) I wonder if it will even make sense in practice to configure a 2:1 or 3:1 ratio in TDD systems as the uplink capacity would then be only a tenth of that of the downlink!? Opinions, anyone?

5 thoughts on “TDD UL and DL Ratios and Uplink Speeds”

  1. TDD is generally preferred over FDD for 3 reasons:
    1) the one you said: operators can tune their UL/DL ratio
    2) receiver architecture is generally simpler: RF front-end is used for a single frequency
    3) benefits for smart antenna technics: the uplink and downlink are using exactly the same channel, which can simplify some smart antenna tricks. E.g. pilots are valid for both UL and DL, multipath are the same (and can be evaluated on a single link), etc.

    As for the UL rate limitations, well there is no mysteries. If you want rate, you need power and bandwidth. A solution would be to use a WiMAX TDD 2:1 20MHz channel compared to the 3G 2×5 MHz channel.

    I see this flexibility in UL/DL ratio could be a solution to provide UL capacity. While UL rates are fixed with FDD, systems using TDD could precisely increase UL capacity if needed…

  2. The observations made are correct. The simple solutions could be envisaged within cilent sets (mobile handsets etc.) which could take care of uplink scheduling policies so as to first serve “heavier” traffic better and then others. Integrating battery consumption (interrupts etc) should also be a goal while serving uplink.

  3. There are no doubt benefits of TDD. That is why you mostly find it in “less complicated” systems like WLAN, DECT or PHS.
    There are however problems when you want to make a cellular system. All major cellular systems are FDD today GSM, C2k, WCDMA etc.

    So where are the problems? Let me just give two short examples

    1. Inter cell interference. Base stations and mobiles will transmit their power not only in the “dedicated cells” There will be problems at the cell edges.

    2. Stricter timing requirements. You have to make sure that base stations are synchronized, and you need to work with time alignment between the mobiles.

  4. It is true that the mobile device generally has a lower transmission power and a smaller antenna than the base station.

    On the other hand, the base station can be equipped with more advanced receive antennas with diversity. This will help to compensate and balance UL and DL transmission.

  5. good observation.

    avg UL/DL achievable bw ratio is true but for 2:1 partition (already), keep in mind there can be significant control overhead in DL.

    I don’t fancy consumers will get symmetric bw in broadband anytime soon simply bcz operator won’t let you (not necessarily for technical reasons, but for commercial and political reasons).

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