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View Full Version : Question Help me with Ts = 1/30720000s of LTE



bienbk
2014-01-08, 10:53 AM
Please explain to me more detail that why LTE uses Ts = 1/30720000s (fs=30.72MHz) in bandwidth of 20MHz.
Because with the bandwidth of 20MHz, the sample rate frequency should be fs >= 2.20 = 40MHz to satisfy Nyquist rate. So fs min should be 40MHz.

Thanks alot.

incoincov
2014-01-10, 04:26 AM
Hi bienbk,
The actual FFT size and sampling frequency for the LTE downlink are not specified.However, the above parameterizations are designed to be compatible with a samplingfrequency of 30.72 MHz. Thus, the basic unit of time in the LTE specifications, of whichall other time periods are a multiple, is defined as Ts = 1/30.72 μs. This is itself chosen for backward compatibility with UMTS, for which the chip rate is 3.84 MHz – exactly one eighth
of the assumed LTE sampling frequency.
In the case of a 20 MHz system bandwidth, an FFT order of 2048 may be assumed forefficient implementation. However, in practice the implementer is free to use other DFT sizes. Lower sampling frequencies (and proportionally lower FFT orders) are always possible to reduce RF and baseband processing complexity for narrower bandwidth deployments: for example, for a 5 MHz system bandwidth the FFT order and sampling frequency could be scaled down to 512 and fs = 7.68 MHz respectively, while only 300 subcarriers are actuallymodulated with data.

Hope, it's clearer now

KR!