Monday, 21 September 2015

LTE Radio Interface (OFDM,OFDMA,SC-FDMA)



The Rectangular Pulse
 



Advantages:
+ Simple to implement: there is no complex filter system required to detect such pulses and to generate them.
+ The pulse has a clearly defined duration. This is a major  advantage in case of multi-path propagation environments as it simplifies handling of inter-symbol interference.

Disadvantage:
- it allocates a quite huge spectrum. However the spectral power density has null points exactly at multiples of the frequency fs = 1/Ts. This will be important in OFDM.

OFDM Basics
        
 Transmits hundreds or even thousands of  separately modulated radio  signals using orthogonal subcarriers spread across a wideband channel


-         Data is sent in parallel across the set of subcarriers, each subcarrier only transports a part of the whole transmission
-         The throughput is the sum of the data rates of each individual (or used) subcarriers while the power is distributed to all used subcarriers
-         FFT ( Fast Fourier Transform) is used to create the orthogonal subcarriers. The number of subcarriers is determined  by the FFT size ( by the bandwidth)
 



Multiple Access Methods Comparison

OFDM is the state-of-the-art and most efficient and robust air interface

 



OFDM is the state-of-the-art and most efficient and robust air interface

 

OFDM Signal
 
OFDM VS OFDMA

 


         The basic idea is to assign subcarriers to users based on their bit rate services. With this approach it is quite easy to handle high and low bit rate users simultaneously in a single system.
           But still it is difficult to run highly variable traffic efficiently.
           The solution to this problem is to assign to a single users so called resource blocks or scheduling blocks.
           such block is simply a set of some subcarriers over some time.
           A single user can then use 1 or more Resource Blocks.

OFDM Weaknesses
Inter-Carrier Interference (ICI) in OFDM

         The price for the optimum subcarrier spacing is the sensitivity of OFDM to frequency errors.
          If the receiver’s frequency slips some fractions from the subcarriers center frequencies, then we encounter not only interference between adjacent carriers, but in principle between all carriers.
          This is known as Inter-Carrier Interference (ICI) and sometimes also referred to as Leakage Effect in the theory of discrete Fourier transform.
          One possible cause that introduces frequency errors is a fast moving Transmitter or Receiver (Doppler effect).



 




Leakage effect due to Frequency Drift: ICI