LTE - basic PHY structure DL [Under LTE]
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In last article, we looked at basic PHY structure on which LTE works and noted timing relations with UMTS PHY. Smallest time unit, a OFDM symbol, in LTE is 0.5ms (one slot=15360 Ts ). If you recall, UMTS R99 DCH use 10 ms TTI, HSDPA use TTI of 2ms. So LTE can provide better granularity than UMTS.
Specification use NRB to denote number of resource blocks; N RB-min = 6 and NRB-max = 110. 1 resource block contain 12 subcarriers with frequency spacing of Δf = 15 kHz, so if we multiply NRB with Δf, we will get correspoding BW. Minimum supported BW would be 1.080 MHz and maximum would be 19.800 MHz !
We looked at basic PHY structure, let's see how further organisation is done.
We know (GSM PHY/UMTS PHY) that we will need ways for synchronisation (which include OFDM frequency/time, LTE frame/slot based PHY structure), ways to communicate system parameters, common control (group or all)information, user or group specific UL/DL allocation information, and so on.
LTE use initial 1 to 4 symbols of (1st slot of) each subframe for carrying control information (called control region) as shown below. This looks similar to 802.16 (check Elements of 802.16 - 1 ) !
Control information is meant for PHY/Access control signaling like information about control region (size), information about rest of the region (like downlink assignments, uplink allocations), and HARQ acknowledgements. Primary and Secondary synchronisation signals (PSS & SSS) are kept the fixed places (subframes 0 & 5 for FDD, subframes 1 & 6 for TDD). PSS & SSS are meant for slot and frame synchronisation respectively. In addition to synchronisation, PSS + SSS gives cell identity.
Note that every subframe will have control region, so UE has to "wake up" every 1ms (1ms - 7 multiplied by OFDM symbol time of CP+66.7 μs to be bit more exact) to check control information or continue to listen (for changed system information, downlink assignments for itself etc.) as needed.
Above diagram also shows two reference signals (there would be more). LTE has three types of reference signals, most important being cell-specific reference signal. LTE gives 504 cell-specific reference sequences, which means support for 504 cells and identities. These 504 cell identities are divided in 168 groups (3 each). PSS decoding reduce cell identity determination from 504 to 168, and SSS decoding gives exact cell identity out of this 168. Cell-specific reference signals are meant for channel estimate (as the name suggests).
TS 36.211 gives details about subcarrier location, timing, recurrence etc. information about PHY channels, reference signals etc.
The physical channels are based on above PHY structure and organisation. Below is list of downlink physical channels, against each a keyword is noted to help recall function of the channel.
- Physical Control Format Indicator CHannel (PCFICH): size of
control region
- Physical Downlink Control CHannel (PDCCH):
DL/UL allocation or control info
- Physical Harq Indicator
CHannel (PHICH): HARQ acks of course
- Physical
Downlink Shared CHannel (PDSCH): DL user data
- Physical
Broadcast CHannel (PBCH): System information
- Physical
Multicast CHannel (PMCH): MBSFN control/data
Note that for the sake of "getting the basics right first", we have not covered MIMO. In next article, we will look at PHY structure from uplink point of view.
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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