01-06-2013, 12:29 PM
Analysis of BLER Performance for LTE Uplink Baseband Simulation System
Analysis of BLER.pdf (Size: 634.63 KB / Downloads: 406)
Abstract
SC-FDMA(Single Carrier Frequency Division Multiple Access) is the Uplink Multiple Access scheme for
the standard LTE (Long Term Evolution). In this paper a baseband simulation system model is proposed
for analyzing the BLER (block error rate) performance of various algorithms in LTE uplink. The BLER
performance of the proposed system is studied under five different channels (AWGN, Pedestrian A
channel, Pedestrian B channel, Vehicular A channel and Vehicular B channel)including two level CRC
(Cyclic Redundancy Check) encoding, modified Turbo encoding based on QPP (Quadratic Polynomial
Permutation) internal interleaving, Turbo decoding and rate matching models. Simulation results show
that under four different channels, the BLER of LTE system is effectively reduced by using two level CRC
encoding, improved internal interleaver Turbo encoding and the LogMapMaxLogMap Turbo decoding
used. Compared to the MaxLogMap Turbo decoding method, with the increasing of the SNR, the
LogMap Turbo decoding has better BLER performance, but the decoding complexity increases as well.
Introduction
Long Term Evolution is the largest communication research program of 3GPP(The 3rd Generation
Partnership Project) organization. LTE is the 3.9G communication technology with the
core technology based on OFDM/FDMA. Compared to the 3G technology, the transmission performance
has been greatly improved. Under the 20MHz spectrum bandwidth, it can achieve
downlink 100Mbps, uplink 50Mbps peak rates. As the multiple access scheme for standard LTE
uplink, SC-FDMA has been researched more and more. SC-FDMA can be also considered as a
kind of DFT spread OFDM technology which is a special form of OFDM.
System Model
The Figure 1 shows the simulation system structure of LTE uplink baseband, Two level CRC
encoding, code block segmentation, Turbo encoding, rate matching, scrambling, modulation,
SC-FDMA modulation, antenna transmitting, channel, SC-FDMA demodulation, demodulation,
descrambling, rate de-matching and Turbo decoding are included.
Following, the procedure of LTE uplink baseband simulation system is introduced. First, the
users give data(also called transmission block) that they want to transmit in the channel .According
to the LTE physical layer specifications, 24 bits CRC encoding is added to the transmission
block; if the length of transmission block is more than 6144 bits, code block segmentation needs
to be done, 24 bits CRC encoding is added to the code block, that is how two level CRC check
encoding model works. Then the encoded CRC code blocks are sent to Turbo encoder based on
QPP interleaver whose encoding rate is 1/3. In order to match the channel transmission rate,
data needs to be punched and retransmitted, that’s why the rate matching is important. In code
block concatenation model the separated code blocks are put together for transmission. Because
the SC-FDMA modulation is used in LTE uplink, the encoded data is modulated into SC-FDMA
symbols. Then the symbols are transmitted by antennas, the maximum number of the receive
antennas is 4. Then the signal is transmitted in channels(AWGN channel,Pedestrian A channel,
Pedestrian B channel,Vehicular A channel and Vehicular B channel). The work mechanism
of the receiver in the base station is reverse procedure of transmit procedure, including antenna
receiving, SC-FDMA demodulation, demodulation, code block segmentation, rate de-matching,
Turbo decoding, code block concatenation and CRC check. After the base station receiving the
user data of users, we can get the BLER from CRC check results.
Two level CRC coding and code block segmentation technology
CRC check coding is used, in order to transmit the symbol with error detection ability. Some
certain bits are added after the transmission bits. In receiver, we can use the CRC remainder to
judge whether the transmission symbol is right. The received bits are divided by the known CRC
code generation polynomial to get the remainder. If the remainder is not equal to zero, the bits
must be transmitted again.
LTE physical layer of the transmission system specify the largest transport block length as 6144
bits, to prevent the length of the transport blocks too long. Two level CRC check coding and
code block segmentation are introduced to follow this principle. The data block which is called
transport block will be transmitted in channel. First, the 24 bits CRC check code will be added
to transport block. If the length of transport block is greater than 6144, then the code block
segmentation technology is necessary. The transport block is divided into many code blocks.
Then, 24 bits CRC check code will be added after each code block[8]. Finally, the separate code
blocks are concatenated.
Turbo encoder and decoder
The improved interleaver Turbo encoder is adopted in LTE channel coding. A new permutation
polynomial based deterministic interleaver is adopted, which is called QPP interleaver and antiinterleaver
with many advantages beyond other interleavers. This encoder has simple structure,
flexible operation and the most important is that parallel Turbo decoding and register contention
problem is solved successfully, which effectively increases the efficiency of the high-speed block
parallel Turbo decoding[9][10].
LTE Turbo Code encoder structure is shown in Figure 4, the output is Turbo coding data with
1/3 code rate. Turbo encoder is mainly consists of two RSC encoders and an inner interleaver.
Each RSC encoder outputs system information bits and encoding bits whose verification information
exists crossly. the system information bits and verification information bits from RSC#1,
the verification information from RSC#2, and 12 tail bits from output compose three parallel
data stream xk , zk and z′
k of output.
Rate matching
Rate matching means the bits in transport channel are retransmitted or punched to match the
carrying capacity of physical channel. Punching means the current bits are removed while the
following bits shift forward in sequence; retransmission means the current bits is transmitted
again before the next bit.
The rate matching unit special designed for 1/3 code rate Turbo decoder channel for LTE
system shows in Figure 6. The main process is elaborated as follows. After the input data passes
Turbo encoding model, the turbo encoding generates a system bit and two parity bits. Then, the
system bits and parity bits are interleaved by matrix interleave respectively. When performing
matrix interleave, according to LTE system specification, the number of columns of interleave
matrix must be 24, so we can decide the number of rows of the interleave matrix can be decided
from the number of code bits. Normally, we have to add padding bits have to be added in front of
the symbol, then performs interleave and bits mapping. System bits maps directly, while parity
bits 1 and parity bits 2 maps crosswise[13], that is called bit collection, the block of Turbo decoder
bit collection for LTE system is shown in Figure 7.
Channel model
The channel adapt Pedestrian A, Vehicular A, Pedestrian B, Vehicular B and AWGN channels
which are ITU(International Telecommunications Union) specified for multi-path channel in LTE
simunation system[7], the channel parameter is base on 3GPP TS 25.101.
Table 1 shows the channel delay profiles of ITU Pedestrian A, Vehicular A, Pedestrian B and
Vehicular B channels. As can be seen from the table that different channel type have differnet
number of multipaths and attenuation, for example, Pedestrian A channel have three multipaths,
Vehicular A channel have five multipaths. The delay of Pedestrian A channel is longer than
Vehicular A channel, the frequency selective fading of Vehicular A channel is more serious than
Pedestrian A channel[1]. And the frequency selective fading of Pedestrian B channel is more
serious than Pedestrian A channel, so is Vehicular B channel and Vehicular A channel.
Conclusion
In this paper, the LTE uplink baseband system simulation model is proposed and verified. In five
typical LTE channels, two level CRC check code, Turbo encoder based on QPP interleaver and
rate matching technology can reduce the system BLER effectively. Furthermore, in receiver, the
BLER performance of the LogMap algorithm is better than the MaxLogMap algorithm when the
SNR is high, but the decoding complexity is relatively higher at the same time.