I was going through an article on channel modelling for 5G communication systems where I encountered the following phrase:

The time selectivity of radio channels is much faster so that TDD technologies are preferable.

  • Now what does time selectivity actually means here?
  • Why is it faster for high frequency signals & How does it make TDD favorable over FDD for > 6GHz signals?

Now what does time selectivity actually means here?

I take it as implying that at higher carrier frequencies, the speed of time division duplexing is also enhanced but, I'm not convinced that TDD does actually benefit much because, in FDD (frequency division duplexing), a higher carrier frequency also means a faster synthesizer lock time....

For a frequency synthesizer's PLL (phase locked loop) to lock, it needs to determine an "error" so that it can drive to the right frequency. That error is the difference between where the output frequency currently is and, where it needs to drive to (the target). Clearly, if the target frequency is higher, the error is "calculated" more often and this can "drive" the feedback loop more regularly and obtain lock more quickly. Take a look at this: -

enter image description here

There are three responses. The slowest has a loop bandwidth of 1 kHz and the fastest has a bandwidth of 20 kHz. For a low target frequency you need to have a low loop bandwidth so that the error (the signal that drives the VCO) does not contain significant ripple artefacts that can cause frequency jitter when locked.

Hence, if the target frequency is high, the loop bandwidth can be high for the same jitter as the low target frequency. A high loop bandwidth is quicker at responding as per the picture above.

Maybe ultimately the claim about TDD benefiting when higher carrier frequencies are used will be reset by changes in technology (beneficial to FDD) which we are not yet aware of.

  • \$\begingroup\$ "and this can "drive" the feedback loop more regularly and obtain lock more quickly." Shouldn't it be the other way around? When frequency difference is higher the loop is driven more often & hence takes more time to lock? \$\endgroup\$ – Tarandeep Kalra Nov 4 '16 at 9:23
  • \$\begingroup\$ Unless I'm misunderstanding you, no. \$\endgroup\$ – Andy aka Nov 4 '16 at 9:39
  • \$\begingroup\$ Okay i researched a bit more & conclude that the higher the loop bandwidth the faster the PLL can lock.So to make FDD viable the PLL should have a higher loop bandwidth which can be achieved by using a higher cutoff loop filter. Comparing this to the ease of enhancing the speed of TDD at higher frequencies, where do we stand now? Which can be considered better? \$\endgroup\$ – Tarandeep Kalra Nov 4 '16 at 9:57
  • \$\begingroup\$ Personally, I think it's a bit of a drawn game. Increasing frequency has benefits on both sides. \$\endgroup\$ – Andy aka Nov 4 '16 at 10:00

At high Carrier bands and high data rates with shared RF Synthesizers, it is faster for the RF Synthesizer settling time to use the same channel for TDD rather switching synthesizer freq from demod to mod with FDD.

The time slots do not have to be equal for each side and asymmetry of bandwidth for each side is allowed. Also spectrum for FDM is double so as spectrum costs rise this doubles the cost, but range limitations will require more cell towers. Like Ethernet return times and distance are limited. In Telecom we call this " Line build out " or return delay times which must be critically controlled for mobile to be received with the same delay while moving with short time slots so it is not a power limitation, rather a time slot sync doppler limitation.

enter image description here

  • \$\begingroup\$ Thank you for the answer.So the crux of the selection between TDD & FDD here lies in the settling time of LO.Since for higher frequencies the settling time of LO will be a limitation TDD is preferred over LTE.Is my understanding correct? Moreover more questions have come up in my mind:- 1)One of the primary features expected in 5G will be extremely low latency(~1ms) but using TDD & incorporating guard bands & switching will only add up to latency worries.Am i right? 2)Range for TDD is less and hence micro cells can incorporate TDD? 3)Why MIMO & beamforming is easier for TDD? \$\endgroup\$ – Tarandeep Kalra Nov 3 '16 at 8:53
  • \$\begingroup\$ TDD is used in WiFI and time slots are variable depending on data rate and SNR and Ricean Fading at fringe. Beam phase forming is possible with a dedicate band for both Rx and Tx but requires more phase stability ie better freq stability. Time slots are shorter , thus lower latency. TDM is used in optical links as well. all the way up in data rates.Gbps.. with different wavelengths for duplexing. \$\endgroup\$ – Tony Stewart EE75 Nov 3 '16 at 14:53

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