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4G LTE PHYSICAL LAYER
Transcript of 4G LTE PHYSICAL LAYER
4G LTE PHYSICAL LAYER
enhanced the Universal Mobile Telecommunication Services
in a set of points on account of the future generation cellular technology needs and growing mobile communication services requirements. Such enhancements are generated due to LTE background requirements, motivations and targets, as presented in section
spectral efficiency of HSPA+ (when MIMO is included) is close to that of LTE when you consider 5Mhz spectrum.
LTE has to satisfy a set of high-level requirements, shown as below :
2- Simple architecture and open interfaces
: Time required to connect to the network is in range of a few hundred milliseconds and power saving states can now be entered and exited very quickly.
LTE introduced to get higher data rates 300Mbps peak downlink and 75 Mbps peak uplink. In a 20MHz carrier, data rates beyond 300Mbps can be achieved under very good signal conditions.
LTE Specification :
To briefly sum up this section, the main advantages of LTE are low latency, high throughput, high speed, great capacity, an enhanced end-user application experience and low operating cost.
LTE Technology Theory
This section presents LTE technology theory, such as :
physical layer for DL
multiple access principles (OFDMA, and SC-FDMA)
Low spectral efficiency From Single carrier
mathematical Definition Of Orthogonalty .
The term Orthogonal Frequency Division Multiplex is due to the fact that two modulated OFDM sub-carriers xk1 (t) and xk2 (t) are mutually orthogonal .
High spectral efficiency compare With conventional- FDM
the basic principles of OFDM modulation and demodulation respectively, these are not the most appropriate modulator demodulator structures for actual implementation. Actually, due to its specific structure and the selection of a subcarrier spacing Δf equal to the per-subcarrier symbol rate 1/Tu .
Up link ( UL )
Down link ( DL )
: 24 bit CRC is the baseline for PDSCH.
Code-Block Segmentation including Possible Per-code-block CRC Insertion.
: Turbo coding based on quadratic permutation polynomial (QPP) inner interleaving with trellis termination.
Rate matching and physical-layer hybrid ARQ functionality.
(QPSK, 16QAM, 64QAM).
7-Mapping to assigned resources and antenna ports.
CRC Insertion Per Transport Block:
n the first step of the physical-layer processing, a 24-bit CRC is calculated for and appended to each transport block. The CRC allows for receiver-side detection of errors in the decoded transport block.The corresponding error indication can, for example, be used by the downlink hybrid-ARQ protocol
as a trigger for requesting retransmissions.
Code-Block Segmentation and Per-Code-Block CRC Insertion:
The LTE Turbo-coder internal interleaver is only defined for a limited number of code-block sizes,with a maximum block size of 6144 bits. If the transport block, including the transport-block CRC,exceeds this maximum code-block size, code-block segmentation is applied before the Turbo coding.
Code-block segmentation implies that the transport block is segmented
into smaller code blocks, the sizes of which should match the set of code-block sizes supported by the Turbo coder.
the QPP (Quadrature Permutation Polynomial) interleaver provides a mapping from the input (non-interleaved) bits to the output (interleaved) bits according to the function:
OFDM ( Orthogonal Frequency Division Multiplexing ) is
Multi-carrier Transmission Technology
. its aims to achieve Higher data rate in the
Multi-path Fading environment of Mobile Communications
In Multi-carrier the high-rate stream of data symbols is first serial-to-parallel converted for modulation onto M parallel sub-carriers as shown in Figure This increases the symbol duration on each sub-carrier by a factor of approximately M, such that it becomes significantly longer than the channel delay spread
In a classical system of parallel transmission (FDM) the total available bandwidth is divided into N parts, separated from each other, one for each sub-channel. Each of them is a separate modulated signal and N sub-channels are multiplexed in the frequency domain. Isolation between bands
where x(kt) is the kth modulated subcarrier with frequency fk =k*∆f and ak(m) is the, modulation symbol applied to the kth subcarrier during the mth OFDM symbol interval, i.e. during the time interval mTu ≤ t <(m+1)Tu. OFDM transmission is thus block based, implying that, during each OFDM symbol interval, Nc modulation symbols are transmitted in parallel.
The high-level network architecture of LTE is comprised of following three main components:
The User Equipment (UE).
The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
The Evolved Packet Core (EPC).
The E-UTRAN (The access network)
The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN) has been illustrated below.
The Evolved Packet Core (EPC) (The core network)
The architecture of Evolved Packet Core (EPC) has been illustrated below. There are few more components which have not been shown in the diagram to keep it simple. These components are like the Earthquake and Tsunami Warning System (ETWS), the Equipment Identity Register (EIR) and Policy Control and Charging Rules Function (PCRF).
Why IFFT ?
OFDM allows for low-complexity implementation by means of computationally efficient Fast Fourier Transform (FFT)processing.
Inter Carrier Interference (ICI) in OFDM due to frequency offset
In OFDM, the uncertainty in carrier frequency, which is due to:
difference in the frequencies of the local oscillators in the transmitter and receiver
Doppler shift in the channel
Assume a delta Fc between the carrier frequency of transmitter and receiver:
QPP:quadratic polynomial permutation
Rate Matching and Physical-Layer Hybrid-ARQ Functionality:
The task of the rate-matching and physical-layer hybrid-ARQ functionality is to extract, from the
blocks of code bits delivered by the channel encoder, the exact set of code bits to be transmitted
within a given TTI/subframe (transport time interval).
LTE downlink scrambling implies that the block of code bits delivered by the hybrid-ARQ functionality is multiplied (exclusive-or operation) by a bit-level scrambling sequence.
By applying different scrambling sequences for neighboring cells, the interfering signal(s) after descrambling is (are) randomized, ensuring full utilization of the processing gain provided by the channel code.
( Orthogonal Frequency Division Multiplexing )
Reduced cost per bit by increasing spectral efficiency
Downlink physical-layer processing chain
Channel coding for DL-SCH is based on Turbo coding. The encoding consists of two rate-1/2, eight-state constituent encoders, implying an overall code rate of 1/3, in combination with QPP-based interleaving.
Students Group :
Dr. Anwar M.Elehlaly
Effect of frequency offset in OFDM receiver
Let us assume that the frequency offset is a fraction of subcarrier spacing
where ( i )is the index of the bit at the output of the interleaver,
c(i) is the index of the same bit at the input of the interleaver, and
K is the code-block/interleaver size
The values of the parameters f1 and f2 depend on the code-block size K . The LTE specification lists all supported code-block sizes, ranging from a minimum of 40 bits to a maximum of 6144 bits,
Why orthogonal ?
Why orthogonal ?
Why IFFT ?
A signal Transmission technology that uses multiple antennas at the receiver and/or the transmitter, in combination with more or less advanced signal processing., is used to achieve improved system performance, including improved system capa city (more users per cell) and improved coverage (possibility for larger cells), as well as improved service provisioning – for example, higher data rate per user.
Single Antenna Beamforming :
This mode uses UE-specific reference signals(RS). Both the data and RS are transmitted using the same antenna weighting .Because the UE requires only UE-specific RS for demodulation of PDSCH. The data transmission for the UE appears to have been received from only one transmit antenna and UE doesn’t see the actual number of transmit antennas. Therefore, this transmission mode is also called” Single Antenna Port”.
The transmission appears to be transmitted from a single “virtual” antenna port 5.
Separate data streams are sent to spatially separated UEs over the same sub-channel ,with each UE serving as one of the multiple Rx antennas.
Involves the transmission of the same informations stream on multiple antennas .The information stream is coded differently using “ Space-Frequency Block code ”.
closed-loop rank-1 spatial multiplexing
The eNodeB sends only one set of data for both Tx using precoding matrix to improve multipath conditions for spatially multiplexed signals . When decoded, these signals contain the same data .
Single Carrier FDMA:
The LTE uplink
OFDM to SC-FDMA
The main difference between OFDM and SC-FDMA transmitter is the DFT mapper. After mapping data bits into modulation symbols, the transmitter groups the modulation symbols into a block of N symbols. An N-point DFT transforms these symbols in time domain into frequency domain.
advantage of SC-FDMA over OFDMA
the lower PAPR (peak-to-average power ratio) of the transmit waveform for low order modulations like QPSK and BPSK, which benefits the mobile users in terms of battery life and power efficiency.
LTE Physical Layer
Carrier Frequency Offset (CFO) Problem Estimation & Correction in OFDM Transmission In LTE
OFDM Basics and
Carrier Frequency Offset
(CFO) problem in OFDM transmission (Simulation)
Downlink physical-layer processing chain
Sc-FDM in up-link transmission
physical Resources in LTE
Open-loop spatial multiplexing
By Sending different data on each antenna ,these Mode can close to multiply peak throughput by the transmission rank . Use a fixed set of precoding matrices to enable multiple layers spatial multiplexing for fast –moving UEs.
sub channel=12 sub carrier
sub carrier = 15 kHZ
sub channel=12x15=180 khz
Frame= 10 ms
Sub fraame = 1 ms
slot= .5 ms
ofdm symbol=Cp +useful time
no of sub carrier = no of resource block x 12
normal: Tcp1=5.21 micro sec.
Tcp=4.7 micro sec.
extended:Tcp=16.67 micro sec
Tu=66.7 micro sec
Tcp > max delay