Requirements and challenges - 4 [Under
LTE]
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E-UTRA/EUTRAN requirements - 25.913, Rel 8 «
4) Spectral efficiency
When we use two carrier frequencies, we maintain a certain gap between the two to avoid interference. If we calculate bits per second per Hz, we arrive at spectral efficiency. Due to limited nature of frequency spectrum, it is advisable to use as many frequencies in alloted frequency spectrum as possible. Better spectral efficiency would lead to better utilisation of radio resources ! It is clear that CDMA based solutions gives better spectral efficiency than FDMA based solutions (as CDMA use entire bandwidth). But this become disadvantage in multi-user environment as information for one becomes interference for other ! So can we have in-between approach ? Can we choose closer frequencies (least guard band) and also have almost zero interference ? OFDM is a system like that. OFDM use orthogonal frequencies; being orthogonal, the frequencies provide theoretically zero interference to each other.
Looking at above 4 point discussion, it seems OFDM is preferred choice for E-UTRA (i.e. LTE) ! OFDM - though realisable now due to technological breakthroughs - is a complex technique and require more and more complex RF design if more bandwidth is to be supported. That puts limit on per device capability. OFDM transmission also result into more variations in an instantaneous power (more peak-to-average power ratio) per user. Though this is not an issue for base station which transmission for several users, it is certainly not desirable for mobile device from power amplifier design point of view.
To avoid these problems, LTE use OFDM for downlink transmission and DFTS-OFDM (Discrete Fourier Transform Spread-OFDM), also known as Single Carrier-FDMA for uplink transmission. DFTS-OFDM is a variation of OFDM, which use DFT operations similar to OFDM, but also add certain number of constant subcarrier components to make resulting waveform smoother (i.e. better peak-to-average power ratio).
We also came out with various techniques - Adpative modulation and coding (AMC), HARQ, MIMO, Beam forming - most of all can be applied to HSPA+ and LTE radio access.
In following articles, we will see which techniques have been chosen for HSPA+ and LTE and related parameter values.
References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker, and Release 8 document at 3gpp.
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