LTE - basic PHY structure [Under
LTE]
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First we looked at LTE requirements and discussed the challenges for HSPA+ and LTE. Then we noted the technology (and technique) selections made for HSPA, HSPA+, and LTE in article "so we choose". We started study with HSPA (Rel 5 feature HSDPA, Rel 6 features - HSUPA and MBMS) and continued with HSPA+ (Rel 7 features - CPC, MIMO, Higher modulation techniques etc.). Now we will look at LTE PHY access.
As noted in article "so we choose": OFDM was chosen for downlink and DFTS-OFDM (SC-FDMA) was chosen for uplink. Both FDD and TDD configurations are supported. Bandwidth support for LTE range from 1 MHz to 20 MHz (which is 4 times of 3G BW of 5 MHz). Individual sub-carrier bandwidth is 15 kHz (12 sub-carriers) (or 7.5 kHz/24 sub-carriers for MBSFN operation). We will start with downlink OFDM scheme and concentrate mainly on FDD configuration/15kHz spacing.
802.16 (Mobile WiMAX) use grid structure made of subchannels and OFDM symbols (refer article Elements of 802.16 - 1). These subchannels are not directly mapped to subcarriers but rather subchannels are logical arrangement of subcarriers. This also mean a subchannel may not necessarily contain consecutive subcarriers.
Let us look at how the subcarriers are structured in LTE for multiple access. You may want to compare LTE PHY access with that of WiMAX as we go along; it would be an interesting exercise.
Below is basic LTE PHY structure:
Above arrangement is for normal CP (Cyclic Prefix); for extended CP, instead of 7 resource elements (per subcarrier per resource block), there would be 6 elements.
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Cyclic Prefix (CP) Cyclic
Prefix In OFDM, last part (certain amount) of the symbol is copied and inserted at the beginning of the symbol. CP helps in reducing both inter-symbol interference inter-subcarrier interference. Note that perfect orthogonality between subcarriers require perfect signal conditions, so safe-guard is needed to take care of inter-subcarrier interference. Due to nature CP (copy of end), it helps in reducing this inter-subcarrier interference. |
The basic structure is same for both FDD and TDD with following difference. In FDD, DL/UL subframes are in sync. In TDD, after 2 DL subframes, there is special subframe (having DL part, guard period, and UL part), then 2 UL subframes, then immediately DL subframes.
1 subframe (i.e. 2 consecutive slots i.e. 2 resource blocks) forms the basic unit for (dynamic) scheduling. This mean, 84 resource elements for normal CP and 72 for extended CP. Extended CP is meant for MBSFN (Multicast-Broadcast over Single Frequency Network) operation. We will not cover MBSFN, but note down few points if needed.
If you recall, UMTS/WCDMA too use 10 ms frame structure. This of course going to help developments of multi-mode handsets. We will also see one more encouraging timing relation between UMTS/WCDMA PHY and LTE/OFDM PHY.
Basic unit for LTE is 1/30720000 seconds - basis of which is sampling rate for 2048 FFT size implementation of OFDM for subcarrier spacing of Δf = 15 kHz:
Sampling frequency for above is fs = 15000 x 2048 = 30720000 Hz. Ts = 1/30720000.
You will see that fs is integer multiple WCDMA chip rate (3.84 Mcps). The same is true if we take other FFT sizes as 1024 and 512.
Tf (time for each frame) would then be = 307200 Ts = 10 ms. Tslot = 15360 Ts.
Usable symbol time is 66.7 μs (= 1/Δf). TCP = 160 Ts (for 1st symbol) and 144 Ts (for rest of the symbols in slot). TCP for extended CP is 512 Ts.
WiMAX also use grid structure of subchannels, but subchannels are logical arrangement of subcarriers. This logical arrangement aim to randomise selection of subcarriers in a subchannel. Recall that signal conditions differ smoothly from one frequency to another consecutive frequency (that is if one frequency is facing bad signal condition, next consecutive frequency is likely to have only little more bad or only little better signal condition rather than hugely different signal conditions). The random distribution ensures consistent (and predictable) signal response among subchannels. LTE use consecutive subcarriers, so does that makes WiMAX better than LTE ? or how does LTE takes care of above scenarios, is it through frequency hopping ? We need to note this down to answer it later.
We will continue LTE PHY discussion in next article.
References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming and LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker.
| Copyright © Samir Amberkar 2010-11 | § § |
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