Telecom Tutorials by Samir Amberkar

LTE - DL PHY Resource Element Group [Under LTE]
» E-UTRA; physical channels and modulation - 36.211, Rel 8 «

Best way to understand PHY organisation would be to make an attempt in mapping physical channels onto frequency/time coordinates. Steps that can be followed are: 1) map cell specific reference signals, 2) understand and note down resource element groups, and then 3) map PSS, SSS, PBCH, PFICH, PHICH, PDCCH, and PDSCH (in that order).

Cell specific reference signals mapping works directly on resource element coordinates. Below is an example for single antenna port (p = 0) cell of ID 6.

l 0 1 2 3 4 5 6 0 1 2 3 4 5 6 1 2                                                 r     R     r       R     1                         1                         R     r     R       r     0                                                 r 0     R     r       R     0                                                 R     r     R       r     11 k 10 9 8 7 6 5 4 3 2 1 0

 R  are reference signals on antenna port 0 and  r  are reference signals on port 1. Note that even if cell has only one antenna port (like in our example), resource elements as per second antennna port configuration are kept unused (36.211:6.2.4).

Next step is to note down resource element groups. Resource elements other than those for reference signals are grouped as resource element groups, 4 REs form one group. In above example, control region is of 2 symbols with 2 REGs in first symbol and 3 in second symbol. Similarly if control region occupy 3 symbols, 3 REGs would be present in third symbol as well. If there are 4 antenna ports, four more REs from second symbol would go for reference signals reducing one REG from second symbol.

Each REG is represented as (k', l') where k' is lowest subcarrier number. l', symbol number is same for all REs within a REG. For example, REG #1 in second symbol would be represented as (4, 1).

This representation of REG is used for mapping PCFICH and PHICH.

Third step would be to start mapping channels as per equations specified in 36.211. In next article, we would look at examples of 6 RB and 15 RB configurations. Note that there are six configurations possible, represented as n6, n15, n25, n50, n75, and n100 (36.331:6.6.2/MIB), which correspond to channel bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz respectively (36.101:5.6).

Copyright © Samir Amberkar 2010-12

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