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Tdd Versus Fdd


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Why are most operators in LTE fixated on FDD? This presentation shows advantages that TDD provides over FDD.

Tdd Versus Fdd

  1. 1. FDD vs TDD Comparison:Why TDD LTE Improves Battery Life & Lowers Costs <br />Dr. Darcy Poulin<br />October 15, 2009<br />
  2. 2. Overview<br />One easy way to mitigate complexities of “40 bands” is to eliminate the duplexer<br />The easiest way to do this is to switch to TDD<br />TDD will result in lower power consumption, lower cost, and smaller size<br />
  3. 3. Introduction<br />FDD/TDD overview and comparison<br />In the absence of losses, FDD and TDD are identical in terms of data throughput and energy consumption<br />Practical considerations<br />FDD duplexer (+ switch?).<br />TDD switch<br />Side by side comparison taking losses into account shows FDD transmitter consumes 55% more energy than TDD <br />Other factors also favoring TDD<br />
  4. 4. TDD versus FDD<br /><ul><li>Frequency division duplexing (FDD) transmits and receives on two frequency bands simultaneously
  5. 5. 2x5MHz so total BW is 10 MHz
  6. 6. Total data transmitted is 5Mbps x 5ms
  7. 7. Total data received is 5Mbps x 5ms
  8. 8. Time division duplexing (TDD) only uses one 10 MHz frequency band
  9. 9. Receives for half the frame, transmits for half the frame
  10. 10. Total data transmitted is 10Mbps x2.5ms
  11. 11. Total data received is 10 Mbps x 2.5ms</li></li></ul><li>TDD versus FDD<br />In terms of data throughput, FDD and TDD are identical<br />FDD uses half the bandwidth for twice the time compared to TDD, so overall data transmitted is the same whether TDD or FDD is used.<br />Neglecting component losses, TDD and FDD are identical in terms of data throughput.<br />
  12. 12. TDD versus FDD: Power and Range<br />FDD systems transmit data over half the bandwidth, for twice as long as TDD systems<br />2x5 MHz FDD system transmits data over 5 MHz bandwidth for the entire frame<br />10 MHz TDD system transmits data over a 10 MHz bandwidth for about half of the frame<br />If both systems transmit at 200mW, power is more concentrated for FDD (40mW/MHz for FDD, 20mW/MHz for TDD), so the range of an FDD system will be larger than TDD<br />To achieve identical range, the power at the antenna of a TDD system needs to be 2x (3 dB) higher than the power at the antenna of an FDD system<br />….BUT<br />An FDD system transmits 2x longer than a TDD system, so the net energy (power*time) transmitted from the antenna is identical for both cases<br />Again, neglecting component losses, FDD and TDD are identical in terms of energy consumption<br />
  13. 13. TDD versus FDD: Duplexing<br />Duplexing refers to transmitting and receiving voice/data simultaneously<br />A TDD system <br />receives data for about the first half of the frame, and then transmits data for about the last half of the frame<br />Uses the same frequency for both Rx and Tx, and uses time to duplex<br />Uses a T/R switch that can be integrated into the RF front end<br />An FDD system<br /> transmits and receives continuously and simultaneously<br />Transmits and receives data on different frequencies<br />Requires a large and lossy duplex filter (required to protect the sensitive receiver from Tx noise). Very difficult to integrate the duplexer into an RF front end.<br />
  14. 14. Duplexer Technology<br />Duplexer insertion loss depends on duplex spacing<br />Typical duplexer insertion loss is ~2.5 dB<br />Typical duplexer size is 2.5x3mm<br />
  15. 15. TDD versus FDD comparison for LTE<br />Putting it all together:<br />A TDD LTE PA needs to be ~1 dB (25%) bigger than an FDD LTE PA to achieve the same range. <br />However, due to duplexer losses, PA power consumption of the TDD system is about 55% better than for the FDD system<br />
  16. 16. Other considerations: Asymmetric data<br />Mobile traffic is highly asymmetric<br />Asymmetric data is difficult to handle with FDD, since the FDD bands have all been allocated with equal UL/DL splits. <br />This leads to inefficient use of bandwidth for FDD<br />With TDD, asymmetry is trivial; simply change the DL/UL ratio<br />
  17. 17. Other considerations: MIMO<br />MIMO requires precise knowledge of the channel. The channel characteristics vary significantly with frequency<br />FDD MIMO is much more difficult to implement, since it is more difficult to do accurate channel estimation<br />TDD MIMO uses the same frequencies for UL and DL, and channel estimates made in the UL can be applied directly to the DL<br />
  18. 18. Other considerations: Radio design<br />FDD systems require that the transmitter and receiver are both on at the same time, on different frequencies<br />Simultaneous operation on different frequencies requires a more complex synthesizer and more complex radio; it may even force users to separate receiver and transmitter Ics for isolation reasons<br />TDD systems have a single synthesizer, and Tx/Rx radios can be on the same die as there is no issue with isolation <br />Baseband chipsets are larger and more complex for FDD; they require independent Tx and Rx chains to handle simultaneous Tx and Rx<br />TDD systems can reuse most of the baseband processor, as it is shared between Tx and Rx (although it does have to run at twice the speed)<br />
  19. 19. Other considerations: Integration<br />QSC6270<br /><ul><li>For multi-band devices, you need a duplexer for each band and a SPxT switch for band selection
  20. 20. Each duplexer is 2.5x3mm and costs ~$1
  21. 21. The SP7T switch die is 1x1.2mm (PE42672)
  22. 22. You need the switch for either TDD or FDD, so it is almost free for TDD.
  23. 23. Duplexer adds cost, size, and complexity to the overall radio</li></li></ul><li>Where did FDD come from?<br />FDD seems to have arrived for historical reasons<br />When higher peak-to-average signals started to arrive in late 1990’s (for UMTS), people were worried that power amplifiers could not be designed to deliver higher powers required for TDD<br />Today, PAs can easily meet the TDD power requirements (~1 dB larger than FDD PAs)<br />GSM uses a TDD approach; UMTS does not?<br />3G and 4G spectrum has continued to be allocated for FDD even though this is the wrong choice for mobile devices!<br />Only China has TDD LTE plans (Europe has TDD band as well, but no plans to deploy?)<br />
  24. 24. Conclusions<br />In this analysis, we have compared TDD and FDD devices<br />While a TDD system must transmit at higher power to achieve the same range as an FDD system, a TDD system will always consume less power than an FDD system.<br />Typically, TDD systems will have &gt;50% better battery life than FDD systems<br />For mobile devices, TDD is preferred. Battery lifetime is longer, and the FDD duplexer is larger than the TDD switch<br />There are also other considerations that favor TDD as well<br />Asymmetry of UL and DL traffic is easier with a TDD system<br />MIMO is better suited for TDD<br />TDD radios are easier to build, and overall sizes are smaller<br />I urge everyone to push regulators to consider TDD for next generation wireless technologies!<br />
  25. 25. Wish list<br />LTE runs exclusively on TDD basis, and duplexers are relegated to the same fate as isolators (this is a wish list after all). <br />But seriously…<br />Awareness of TDD advantages would be a great start…<br />List of band priorities so that I know which order to design PAs in. Should we do 700MHz first? 2.1 GHz first? 2.5 GHz first?<br />