In the previous chapters we learned about LTE Frame Structure. In this chapter we shall learn about OFDMA and Resource Block.
First of all what is Modulation?
It is a way of converting data into radio waves.
What are the modulation techniques used my 4G LTE?
For downlink we use OFDMA
For uplink we use SC-FDMA
Few points on OFDMA
* OFDMA is used for downlink
* OFDM stands for Orthogonal Frequency Division Multiplexing
* OFDMA stands for Orthogonal frequency-division multiple access
* OFDM splits the carrier frequency bandwidth into many small subcarriers spaced at 15 kHz, and then modulates each individual subcarrier using the QPSK, 16-QAM, or 64- QAM digital modulation formats.
* OFDMA uses OFDM multiple users are sharing the bandwidth at each point in time.
* Increases spectral efficiency due to orthogonality between sub-carriers.
* One of the dis-advantages is high peak to average power ratio. It means, the peak value of the system is very high when compared to average of the whole system.
* This can be overcome by using high compression point power amplifiers and amplifier linearization techniques.
*These hardware can be used in Base Station, but when used in UE it will become expensive. Hence LTE uses SC-FDMA.
Few points on SC-FDMA
* SC-FDMA stands for Single Carrier FDMA with cyclic prefix.
* SC-FDMA is used for uplink.
* It uses lower PAPR (Peak to Average Power Ratio)
* It consumes less battery
* Requires simpler amplifier design
* It improves uplink coverage and cell-edge performance.
The above image shows the relation ship between slot, symbol and resource blocks.
What is a resource bock?
As we saw in above OFDMA diagram, it has been divided into time slots in time axis and frequency subcarrier on frequency axis.
A resource block is a smallest unit of resource allocation in LTE.
Resource element is the smallest unit of resource block.
It is 0.5ms duration with 12 subcarriers in 1 OFDM symbol. 1 OFCM symbol is equal to one slot.
One time slot is equal to 7 OFDM symbol in normal cyclic prefix and 6 OFDM symbol in extended cyclic prefix.
One full resource block is equal to 12 sub carriers by 7 symbol in notmal CP
Channel bandwidth is the width of the channel as measured from lowest channel edge to the highest channel edge.
So in conclusion:
A Radio frame/System frame is of 10 msec. It has 10 sub frames.
A subframe is of 1 msec, A slot is of 0.5 msec. Hence each subframe has 2 slots.
One resource block is 0.5ms and contain 12 subcarriers for each OFDM symbol in frequency domain.
A slot consists of 7 ofdm symbols (if normal cyclic prefix is used) separated by cyclic prefix.