Multicarrer Modulation

The philosophy of multicarrier modulation is: a large number of subcarriers (L) are used in parallel, so that the symbol time for each goes from T-->LT. In other words, rather than sending a single signal with data rate R and bandwidth B, why not send L signals at the same time, each having bandwidth B/L and data rate R/L ? In this way if B/L is less carrier BW, each signal will undergo approximately flat fading, and the time dispersion for each signal will be negligible. As long as the number of subcarriers L is large enough, the condition B/L is less than carrier BW can be met.

This elegantidea is the basic principle of orthogonal frequency division multiplexing (OFDM).

Cellular System problems !!

One of the more intriguing aspects of wireless channels is fading. Unlike pathloss or shadowing, which are large-scale attenuation effects owing to distance or obstacles, fading is caused by the reception of multiple versions of the same signal.

The multiple received versions are caused by reflections that are referred to as multipath.

Because of multiple paths between transmitter and reciever, there will be phase difference between arriving signal, and the intereference can be either constructive or distructive, which causes a very large observed difference in the amplitude of the received signal even over very short distances.

In other words, moving the transmitter or the receiver even a very short distance can have a dramatic effect on the received amplitude.This happens because each frequency harmonic of transmitted signal goes through different level of attenuation and fading.Which ultimatly results in delay spread and in turn ISI (Inter symbol Interference). It is very complex to implement equilizer for time varying wireless channel.

OFDMA is the technique which can help a lot to mitigate Inter Symbol Intereference (ISI).

Cellular System

In cellular systems, the service area is subdivided into smaller geographic areas called cells, each served by its own base station.In order to minimize interference between cells, the transmit-power level of each base station is regulated to be just enough to provide the required signal strength at the cell boundaries.Therefore, the same frequency channels can be reassigned to different cells, as long as those cells are spatially isolated.

The rate at which frequencies can be reused (freq-reuse ratio) should be determined such that the interference between base stations is kept to an acceptable level. In this context, frequency planning is required to determine a proper frequency-reuse factor.The frequency-reuse factor f = 1 means that all cells reuse all the frequencies. Accordingly, f = 1/3, implies that a given frequency band is used by only one of every three cells.

Above figure shows a hexagonal cellular system model with frequency-reuse factor of 4, where cells labeled with the same letter use the same frequency channels. In this model, a cluster is outlined in boldface and consists of four cells with different frequency channels.

But this will create a problem of CCI (Co-channel Interference) between the adjecent cell using the same frequency. And also it wastes much bandwidth and power by radiating power in complete cell.

Cell Sectoring is the best way to tackle this situation. Using directional antennas instead of an omnidirectional antenna at the base station can significantly reduced the cochannel interference. So it increases both bandwidth and system capacity by the number of time sectoring is done !!

PHY layer deails

 

Physical Layer (PHY) processing

Scrambler: It is nothing but a randomizer, which randomizes incoming data stream of continuos 0's and 1's. This helps in AGC and timing recovery circuit

Channel Encoder: Its a FEC scheme, which adds extra redundent bits to data in order to increase the error correcting capabilities. Convolution codes (CC) and Convolution Turbo Codes (CTC) are example of channel encoder.

Interleaver: Protect burst errors by spreading incoming bits in different channels, which helps in recovering data even after burst errors. It ensures that adjacent code bits are mapped to non adjacent subcarrers, which provides frequency diversity and improves the performence of decoder

Symbol Mapper: The sequence of binary bits are converted into sequence of complex valued symbols. QPSK, 16QAM and 64QAM are defined in WiMax

Space Time Coding: A space–time code (STC) is a method employed to improve the reliability of data transmissionn in wireless communication systems using multiple transmit antennas. STCs rely on transmitting multiple, redundant copies of a data stream to the receiver in the hope that at least some of them may survive the physical path between transmission and reception in a good enough state to allow reliable decoding

Subcarrer Mapping/IFFT: Depending on the type of subcarrer allocation scheme data will be mapped on the different subcarrers. There are basicaly 2 different types of schemes: Adjacent subcarrer permutation and distrubuted subcarrer permutation scheme. For 1024 FFT (subcarrers) 768 subcarrers are used for data, 82 for pilot and rest are unused. IFFT converts frequency domain signal to time domain and maps data to no of IFFT subcarrers.

D/A: Digital to Analog converter converts digital data to analog for the transmission on the air.

RF Card: Analog data will be modulated with the 2.3 Ghz range of frequency (depending on the system freq), power amplified and will be feed to antenna through TDD switch.