This section presents an overview of communication blocks involved in the physical layer of
‘spectrum efficient’ SH-A (satellite handheld A) mode of DVB-SH communication standard, as shown in Fig. 2.1.
2.2.1 Transmitter
Transmitter of the DVB-SH physical layer consists of ‘turbo encoding & QPSK/QAM modula- tion’ and ‘OFDM (orthogonal frequency division multiplexing) framing & transmission’ blocks, as shown in Fig. 2.1. The DVB-SH frame of 12282 bits from ‘transmitter data link layer’ is fed to PCCC (parallel concatenated convolutional code) ‘turbo encoder’ that consist of two convolu- tional encoders and a turbo interleaver [26]. The transfer function for ‘turbo encoder’, compliant with DVB-SH standard, is given as
S(D) = µ
1, 1 +D+D2
1 +D2+D3,1 +D+D2+D3 1 +D2+D3
¶
. (2.1)
ARP (almost regular permutation) turbo interleaver [31] for the block length of 12282 bits is used and the mathematical expression for interleaved address Π(i) is given as
Π(i) = (P0×i+Q0)mod Ndvb. (2.2)
In the above expression, Ndvb=12282 bits, P0=6125, Q0=1225 and i={1, 2, 3, 4 ... Ndvb}.
At the transmitter, ‘puncturing unit’ processes the turbo encoded bits to achieve different code rates of 1/5, 2/9, 1/4, 2/7, 1/3, 2/5 and 1/2 for an efficient utilization of channel bandwidth [32].
The punctured data is bit interleaved for different code rates compliant to DVB-SH standard [19]. In order to perform the mapping optimization on the DVB-T (digital-video-broadcasting- terrestrial) frame purpose, the ‘rate adaptation unit’ is used for puncturing the bit interleaved block. After the rate adaptation process, the bit interleaved block is fed to a ‘convolutional interleaver’ which mitigates the burst error incurred by long term fading of mobile satellite channel that may immensely degrade the quality of service [30]. The ‘bit demux’ unit maps input bit stream forM-ary modulation schemes. DVB-SH in a SH-A mode of operation, incorporates modulation schemes such as QPSK (quadrature phase shift keying) and 16-QAM (quadrature amplitude modulation), thereby ‘bit demux’ unit maps the input bit stream into n=log2M=2 for QPSK (∵M=4) and n=4 for 16-QAM (∵ M=16), as shown in Fig. 2.1. The ‘OFDM framing
Chapter2: Performance and Throughput Analysis of Turbo Decoders for the Physical Layer of
DVB-SH Standard 12
SYMBOL INTERLEAVER
PILOT SYMBOL INSERTION MODULATION (QPSK/16-QAM) TRANSMITTER
DATA LINK LAYER
TURBO ENCODER
PUNCTURING UNIT DVB-SH FAME
BIT DEMUX QPSK(n=2) 16-QAM(n=4) CONVOLUTIONAL
INTERLEAVER BITWISE
INTERLEAVING
& RATE ADAPTATION
UNIT TURBO ENCODING & QPSK/QAM MODULATION
I F F T CP INSER -TION W
I N D O W I N G P/S CONV.
D A C RF SECTION
OFDM FRAMING & TRANSMISSION
TRANSMITTER
WIRELESS COMMUNICATION
CHANNEL
CONVOLUTIONAL DE-INTERLEAVER
BITWISE DE-INTERLEAVRING TIMING/FREQUENCY
SYNCHRONIZATION
&
CHANNEL ESTIMATION UNIT
SOFT DEMODULATION
(QAM/OFDM) A
D C
CP REMOVAL
S/P CONV.
F F T
CHANNEL
EQUALIZATION P/S CONV.
RECEIVER DATA LINK LAYER
DE- PUNCTURING
UNIT TURBO
DECODER 1
0
DE-PUNCTURING & TURBO DECODING RF
SECTION
RECEIVER
CYCLIC PREFIX REMOVAL & SOFT DEMODULATION
Figure 2.1: System level architecture for the physical layer of DVB-SH-A wireless communi- cation standard.
& transmission’ block performs IFFT (inverse fast Fourier transform), DAC (digital to analog conversion) and RF (radio frequency) transmission. Various IFFT sizes of 1K, 2K, 4K and 8K for OFDM multi carrier system are supported by DVB-SH standard depending on the bandwidth utilization [19]. The ‘symbol interleaver’ unit is fed with QPSK or 16-QAM modulated symbols and is used for mapping these modulated symbols with pilot symbols for different IFFT sizes.
‘Symbol interleaver’ unit incorporates pilot symbols with the modulated symbols to produceNf parallel symbols, whereNf is the size of IFFT. Cyclic prefix is concatenated and windowed into different OFDM frames. The OFDM frames are fed to ‘parallel to serial conversion’ unit, then transformed to analog signals using DAC and finally, transmitted via RF transmitting antenna.
2.2.2 Receiver
In this work, we have simulated the physical layer model of DVB-SH standard in frequency selective fading environment. The faded analog signals from the channel are received at the antenna of ‘RF receiver’ unit and Gaussian noise is added to these analog signals, as shown in
Fig. 2.1. These faded plus noisy analog signals are converted into discrete values using ADC (analog to digital converter) and fed to the receiver base-band system. Timing recovery and channel estimation are being performed to estimate the frequency response of faded channel that can be used for channel equalization process to mitigate the effects of ISI (inter symbol interference). The CP (cyclic prefix) from each of the OFDM symbol is removed by ‘CP removal’
unit and then the serial stream of OFDM symbols are converted into parallel stream by ‘serial to parallel conversion’ unit in ‘cyclic prefix removal & soft demodulation’ block, as shown in Fig. 2.1. Nf-point FFT is performed for parallel symbols to extract the transmitted symbols which are modulated using multiple sub-carriers. In the ‘channel equalization’ block, Fourier transformed frequency domain symbols are equalized using the estimated frequency response of channel to mitigate the effect of ISI. Finally, the ISI free symbols are parallel to serial converted and soft demodulated using QPSK or 16-QAM demodulation scheme. The soft demodulation process generatesLLR(logarithmic likelihood ratio) of a-priori probabilities for the transmitted bits. These LLR values are time and bit de-interleaved to produce an input bit stream for de-puncturing unit. The ‘de-puncturing & turbo decoding’ block constitutes turbo decoder as an error-correcting channel-decoder followed by de-puncturer unit. De-puncturedLLRvalues of a-priori probabilities of the transmitted bits are fed to turbo decoder which is subjected to an iterative decoding process to generate the final LLRvalues of a-posteriori probabilities. Turbo decoder comprises of SISO (soft input soft output) units based on MAP algorithm, interleaver and de-interleaver [21]. Decoded a-posteriori probabilityLLRvalues of the transmitted bitsUk
can be computed using the received a-priori probabilityLLRvalues of systematic and parity bits as well as logarithmic a-priori extrinsic information generated in every iteration of the decoding process [2], and is given as
LLRk= ln
P
(s0,s)=Uk=+1
b
αk−1(s0)×bγk(s0, s)×βbk(s) P
(s0,s)=Uk=−1
b
αk−1(s0)×bγk(s0, s)×βbk(s)
, (2.3)
where,αbk(s),βbk(s) andbγk(s) are forward-state, backward-state and branch metrics, respectively, of each states at kth trellis stage. Finally, the turbo decoded LLR values are fed to the hard decision unit, which produces a sequence of 12282 bits for every DVB-SH frame. These decoded frames are passed to the upper data link layer of receiver side.
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