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Orthogonal principle in details :-
Signals are Orthogonal if they don’t interfere with each other.  This interference can be avoided by controlling the transmission of the signals in one of the several domains.  The most prominent domains are frequency, time and space.   

So the OFDM is achieved under following guidelines.

A) Separate the sub-carriers within a OFDM symbol by inverse of modulation rate.
B) Make sure modulation rate is consistent for all sub-carriers within a modulation symbol.

LTE RF link:
In LTE radio link between transmitter and receiver its done by following approach. Synchronize UE local frequency oscillator with the eNodeB and hence insure Orthogonal relation in-between sub-carriers. This results in theoretical zero inter-symbol-interference.  This is very important requirement of OFDM systems.
OFDMA - Orthogonal Frequency Division Multiple Access & SC-FDMA - Singe Carrier Frequency Division Multiple Access
SC-FDMA - Single Carrier Frequency Division Multiple Access
SC-FDMA is still a multiple access scheme like OFMDA but avoids multiple carriers in transmission. With only one carrier at transmitter the signal has comparatively low peak-to-average power ratio (PAPR).

OFDMA is OFDM based Access. Where OFDM is a modulation technique.  

OFDM Principle for Wireless:- OFDM - Orthogonal Frequency Division Multiplexing

Its multi-carrier modulation scheme which uses  a large number of channels to transmits data over air. These channels are also called sub-carriers.  The most important property of OFDM is the orthogonal relationships of its sub-carriers.

 

Limitation of OFDMA: Its high Peak to Average Power Ratio or PAPR reduces the efficiency of RF power amplifier on Uplink. This is a significant limitation for Mobile Equipment (ME) like a cell phone. So we will next discuss details on PAPR and steps to reduce PAPR limitations. In LTE OFDMA, PAPR can be reduced by using single carrier pre-coding technique which is also known as SC-FDMA for Uplink (or transmit).

Peak-to-Average Power Ratio (PAPR) -

Its a ratio of the peak amplitude of the waveform divided by the Root Mean Square Value (RMS) of the waveform.

With low PAPR the mobile terminal can increase transmit power efficiency.

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