Sunday, June 29, 2014

OFDM overview and LTE

In this section the block diagram of OFDM will be described. From figure at first the bits are passed through a serial to parallel converter. The output of the converter then passed through an encoder. From here we got the N sub channels. Then the signal passed through IFFT for inverse Fast Fourier Transform. From the output of N-IFFT the signal cyclic prefix added. Then the signal is passed through parallel to serial converter. After that the serial data passed through Digital to analog filter and then to antenna port.

OFDM overview image

In the receiver part analog signal is taken via receive filter which is actually an analogue to digital converter. From this digital signal cyclic prefix removed. Then the signal passed through N-FFT for Fast Fourier Transform. Then the signal is passed through a frequency domain equalizer for inverting the channel. This signal is passed through a QAM decoder. From the output of QAM decoder Parallel to serial converter takes the input. Ultimately the bits found from the output of parallel to serial converter.

The LTE physical layer implements a number of technologies to deliver on requirements for high data rates and spectral efficiency. OFDMA with MIMO allows the downlink to provide as high as 100 Mbps in link throughput while SC-FDMA on the uplink reduces design complexity for the user terminals by reducing peak-to-average power ratio or PAPR. The design of the physical layer and system parameters are well matched with the characteristics of mobile propagation channel to allow optional downlink and uplink frequency selective scheduling thereby enhancing throughput performance. Adaptive modulation and coding maximizes throughput to individual subscribers and increases overall cell capacity.
The physical layer provides low latency. The 1 ms sub frame0 duration provides low latency through small scheduling intervals while maintaining low overhead related to higher layer protocols. The PHY is also well designed to provide high cell-edge performance with specific features such as dynamic bandwidth allocation to users and the design of reference signals and control channels which take into account more challenging path loss and interference environment at the cell edge.

The LTE physical layer incorporates several physical channels both in the uplink and the downlink. The following sections provide a brief description of the channels.