Continued from the last post
The transport channels are mapped to Physical Channels, and after the transport channel processing, as described in last post, the Physical Channel Processing takes place. This is again for LTE Downlink.
Scrambling, Modulation, Layer Mapping, Precoding and Resource
Element Mapping
The baseband
signal representing a downlink physical channel is defined in terms of the
following steps:
- scrambling of coded bits in each of the
code words to be transmitted on a physical channel
- modulation of scrambled bits to generate
complex-valued modulation symbols
- mapping of the complex-valued modulation
symbols onto one or several transmission layers
- precoding of the complex-valued modulation
symbols on each layer for transmission on the antenna ports
- mapping of complex-valued modulation
symbols for each antenna port to resource elements
- generation of complex-valued time-domain
OFDM signal for each antenna port
Scrambling
Scrambling the bits make them less prone to interference as the process make the value of bits to be transmitted to be psuedo random.
The scrambling code used is a length 31 gold sequence which yields 2^31 puseddo random sequences. If the input code word is :
bq(0), bq(1),bq(2) .......... bq(Mqbit - 1); where Mqbit is the number of bits in the code word q.
and scrambling code is:
cq(0), cq(1),cq(2) .......... cq(Mqbit - 1)
the scrambling is done as shown in figure below.
Modulation
Modulation converts the bits in the code words into complex-valued symbols. One symbol can represent n bits depending on the modulation type. (n=2 for QPSK, n=4 for 16 QAM, n=6 for 64 QAM).
The signal s(t) is split into Re and Img parts which are multiplied by the carrier and 90 degree shifted carrier and then summed up to give the compled valued modulation symbol.
The complex-valued modulation symbols for each of the code words to be transmitted are mapped onto one or several layers. Complex-valued modulation symbols
dq(0), dq(1), dq(2) .......... dq(Mqbit - 1),
for code word q shall be mapped onto the n layers (equal to the number of antenna ports), with Mlayersymb is the number of modulation symbols per layer
The following table shows how the bits in the code word are mapped onto different layers of transmission for spatial multiplexing.
For 1 layer and 1 code word (single antenna port and single data stream); the mapping is strainghtforward, with the symbols from the code word added to the layer in order. Note that number of symbols in code word 0 (M(0)symb ) is equal to the number of symbols that the layer can take ( Mlayersymb).
For 2 layer and 2 code words (two antenna ports and dual data stream); one data stream is mapped onto one antenna port and the other data stream on the pther port. Note that number of symbols in code word 0 (M(0)symb ) or code word 1 (M(1)symb ) is equal to the number of symbols that the each layer can take ( Mlayersymb).
For 2 layer and 1 code word (two antenna ports and single data stream); one data stream is distributed over the two layers. Note that number of symbols in code word 0 (M(0)symb ) is twice that of the number of symbols that the layer can take ( Mlayersymb).
Other combinations are shown in the able below:
Precoding
Precoding is done to adapt to the channel conditions. The transmission is precoded according to the frequency response of the channel. The precoding is done in such a way that original signal (before precoding was applied) is received at the Rx end after the transmission undergoes the frequency response of the channel.
In LTE a feedback mechanism is in place for MIMO, where the UE signals back to the eNB one of the three possible precoding matrix to be applied during precoding of the signal.
The scrambling code used is a length 31 gold sequence which yields 2^31 puseddo random sequences. If the input code word is :
bq(0), bq(1),bq(2) .......... bq(Mqbit - 1); where Mqbit is the number of bits in the code word q.
and scrambling code is:
cq(0), cq(1),cq(2) .......... cq(Mqbit - 1)
the scrambling is done as shown in figure below.
Modulation
Modulation converts the bits in the code words into complex-valued symbols. One symbol can represent n bits depending on the modulation type. (n=2 for QPSK, n=4 for 16 QAM, n=6 for 64 QAM).
The signal s(t) is split into Re and Img parts which are multiplied by the carrier and 90 degree shifted carrier and then summed up to give the compled valued modulation symbol.
For each code word, the block of scrambled bits
bq~(0), bq~(1),bq~(2) .......... bq~(Mqbit - 1)
shall be modulated using one of the modulation schemes in table below, resulting in a block of complex-valued modulation symbols
dq(0), dq(1), dq(2) .......... dq(Mqbit - 1).
bq~(0), bq~(1),bq~(2) .......... bq~(Mqbit - 1)
shall be modulated using one of the modulation schemes in table below, resulting in a block of complex-valued modulation symbols
dq(0), dq(1), dq(2) .......... dq(Mqbit - 1).
Physical channel
|
Modulation schemes
|
PDSCH
|
QPSK, 16QAM, 64QAM
|
PMCH
|
QPSK, 16QAM, 64QAM
|
Modulation schemes
The complex valued modulation symbol, onto which the bits in the code word are mapped, is specified by the amplitude of the Real component(I) and the Imaginary component(Q).
Modulation Symbol = I + jQ
For QPSK, the modulation symbols are:
QPSK Modulation
The I, Q for different modulation schemes are given in the tables below
Layer Mapping
The complex-valued modulation symbols for each of the code words to be transmitted are mapped onto one or several layers. Complex-valued modulation symbols
dq(0), dq(1), dq(2) .......... dq(Mqbit - 1),
for code word q shall be mapped onto the n layers (equal to the number of antenna ports), with Mlayersymb is the number of modulation symbols per layer
The following table shows how the bits in the code word are mapped onto different layers of transmission for spatial multiplexing.
For 1 layer and 1 code word (single antenna port and single data stream); the mapping is strainghtforward, with the symbols from the code word added to the layer in order. Note that number of symbols in code word 0 (M(0)symb ) is equal to the number of symbols that the layer can take ( Mlayersymb).
For 2 layer and 2 code words (two antenna ports and dual data stream); one data stream is mapped onto one antenna port and the other data stream on the pther port. Note that number of symbols in code word 0 (M(0)symb ) or code word 1 (M(1)symb ) is equal to the number of symbols that the each layer can take ( Mlayersymb).
For 2 layer and 1 code word (two antenna ports and single data stream); one data stream is distributed over the two layers. Note that number of symbols in code word 0 (M(0)symb ) is twice that of the number of symbols that the layer can take ( Mlayersymb).
Other combinations are shown in the able below:
Precoding
Precoding is done to adapt to the channel conditions. The transmission is precoded according to the frequency response of the channel. The precoding is done in such a way that original signal (before precoding was applied) is received at the Rx end after the transmission undergoes the frequency response of the channel.
In LTE a feedback mechanism is in place for MIMO, where the UE signals back to the eNB one of the three possible precoding matrix to be applied during precoding of the signal.
Mapping to resource elements
For each of the antenna ports used for
transmission of the physical channel, the block of complex-valued symbols yp(0), yp(1), yp(2) .......... yp(Mapsymb - 1) shall be mapped in
sequence starting with yp(0) to resource elements (k,l) which meet all of the
following criteria:
-they are in the physical resource blocks
corresponding to the virtual resource blocks assigned for transmission, and
-they are not used for transmission of
PBCH, synchronization signals or reference signals, and
-they are not in an OFDM symbol used for
PDCCH as described in the post on PDCCH.
The mapping to resource elements (k,l) on antenna port p not reserved for other
purposes shall be in increasing order of first the index k over the
assigned physical resource blocks and then the index l, starting with the first slot in a subframe.