LTE PHY Specification

Orthogonal Frequency Division Multiplexing (OFDM) is a kind of multi-carrier transmission, which achieves high data rate and combat multi-path fading in wireless networks. The main of OFDM is that data is divided into several parallel data streams or channels. Its ability to cope with several data streams simultaneously, thus achieve a high spectral efficiency. In addition, one for each sub-carrier carries individual data, and the equalizer can be implemented with low complexity in frequency domain.

2.1.1. Transmitter

Fig. 2.1. depicts that the architecture of MIMO-OFDM transmitter in LTE specification proposal. The initial scrambling stage serves in the purpose of interference rejection. Scrambling sequence can be generated by a shift-register implementation, and then the scrambled bits pass through modulation block to generate complex-valued modulated symbols.

The layer mapping is the function to establish a mapping of the complex-valued modulation symbols onto one or several transmission layers. Basically a layer corresponds to a spatial multiplexing channel.

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The pre-coding extracts exactly one modulation symbol from each layer, jointly process these symbols, mapping the result in the frequency domain and antenna domain.

The resource-block mapped divides the transmitted symbols into the resource elements of the set of resource blocks on each antenna. The resource block is assigned by the MAC scheduler for transmission of transport block(s). To transform the signal after modulator in frequency domain constellations into time-domain constellations, Inverse Fast Fourier Transform (IFFT) is used. There are 1024 frequency entries for each IFFT, or 1024 sub-carriers in each OFDM symbol. 600 of them are data carriers, the rest 423 are null carriers. Finally, the time domain signals, appended to the Cyclic Prefix (CP) of 144 or 160 sub-carries, are transmitted by RF modules.

Scrambling

element mapper OFDM signal generation Layer

mapper Precoding

N layers M antennas

Fig. 2.1 LTE MIMO transmitter

2.1.2 Receiver

The receiver block diagram is shown as Fig. 2.2. Assume the dipole antenna is used to receive signals from the RF module, the first step is synchronization to recognize each OFDM symbol, and then transport block(s) are transformed signal streams by resource element mapper. Through the digital beamforming function block, the proposed algorithm can control the signals to received large energy. After the proposed algorithm, each OFDM symbol is transformed to frequency domain by the Fast Fourier Transform (FFT). If the OFDM symbol belongs to reference signal (described in section 2.1.4), then it is used for channel estimation.

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In order to restore the original signal shape, the equalization is used to adjustment and is designed to compensate for the unequal frequency response of some other signal processing circuit or system. The signal streams are through de-precoding and equalization, and then these combine signal streams by layer mapper.

After separation by decoder, the signal streams are de-modulated to bit-level streams. These bit-level data streams merge into the two data streams after layer de-mapper and de-modulation. Finally, the data streams are de-scrambler.

Fig. 2.2 LTE receiver

2.1.3 Receiver moving

In communication system of daily life, receiver and transmitter are as well mobile and base station respectively. The mobile device has own position by mobility, the bean pattern would trace desired signal to adjust AOA of detection. In order to simulate the mobility, AOA of signal increases 10^o in every 5 subframes.

2.1.4 Frame Format

The frame and sub-frame format are shown as Fig. 2.3. and Fig. 2.4. The radio frame is comprised of 10 sub-frames. A sub-frame consists of Physical Downlink Share Channel (PDSCH), Reference Signal (RS) and Physical Downlink Control Channel (PDCCH). PDSCH is used for all user data, as well as for broadcast system information. Data is transmitted on the PDSCH in units known as transport blocks.

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PDCCH carries a message known as Downlink Control Information (DCI), which includes resource assignments and other control information for a UE or group of UEs.

RS(s) are known signals which do not carry any data. It can be used to do channel estimation. Each sub-frame has 14 OFDM symbols, and then each symbol has 12 sub-carries. Fig. 2.5 describes that Sub-carries be filled in resource block in turn.

Especially, first and sixth sub-frame for downlink is that these sub-frames include the LTE synchronization signals (SS). The SS (s) are transmitted on downlink of each cell and are intended to be used for initial cell search as well as for neighbor-cell search.

Fig. 2.3 LTE frame format

Fig. 2.4 LTE sub-frame format

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Fig. 2.5 LTE resource block format

2.2. Channel Model

There are many imperfect effects during transmitted signals through channel, time invariant multipath energy factor and Additive White Gaussian Noise (AWGN), and so on. The block diagram of channel model is shown in Fig. 2.6.

Fig. 2.6 Block diagram of channel model

2.2.1. Channel Noise Model

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Additive white Gaussian noise (AWGN) comes from many natural sources, such as the thermal vibrations of atoms in antennas, "black body" radiation from the earth and other warm objects, and from celestial sources such as the sun. The AWGN channel is a good model for many satellite and deep space communication links. On the other hand, it is not a good model for most terrestrial links because of multipath, terrain blocking, interference, etc. The signal distorted by AWGN can be derived as

( ) ( ) ( ) r t =s t +n t

where ( )r t is received signal, s t( ) is transmitted signal, ( )n t is AWGN.

2.2.2 Energy Factor

Digital beamforming is able to enhance the received signal form the transmitter and minimize unwanted interferences. The digital beamforming uses beam pattern on the receiver end to get the distinct strength of signal on the different directions of arrival. In this paper, the energy factor is used to simulate the digital beamformer behavior.

The energy factor is defined as the channel strength of each multipath. For example, every path has 6 taps which the individual strength is differently. In the receiver, the assumption is that all antennas have the same angle of arrival AOA. Laplacian distribution is used to generate the power angular spectrum. Each channel tap exhibits Laplacian power azimuth spectrum (PAS) in the domain and the angular spread (AS) is the second moment of PAS. The shape is 1 ^{2 /}

( ) 2

p φ e ^{φ σ}

σ

= − where Φ is azimuth

angle and σ is Angular Spread (AS). Fig. 2.6 is the example of Laplacian distribution.

The horizontal axis denotes arrival degree and the vertical axis is the power of signal.

In the received end, the beam pattern is used to scan transmission environment. If the direction-of-arrival (DOA) of the signal-of-interest (SOI) is searched, the beam

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pattern has adjusted to focus on the SOI while turning the SOI has large signal strength.

Fig. 2.6 Example of Laplacian AoA (AoD) distribution, cluster, AS = 30