Introduction to OFDM
Before entering into OFDM, the need of OFDM should be understood; the reason why orthogonality is preferred in the signals. In frequency division multiplexing, users are separated from one another spectrally with multiple users using separate frequency channels and channel bandwidth being equal to the transmission bandwidth.
A simple Frequency Division Multiple Access (FDMA) arrangement would be that multiple frequency channels are arranged serially. For lower adjacent channel interfer-ence, guard band is necessary which eventually increases the bandwidth of the system and lowers the spectral efficiency.
Basic idea is to use a large number of narrow-banded orthogonal sub-carriers simulta-neously. Figure 3.1 shows the orthogonal sub-carriers placed together overlapping each other in such way that the interference experienced in each sub-carrier due to the neigh-bouring sub-carriers during the sampling is minimum. The sub-carrier spacing in the above figure is 15 kHz i.e. the spacing between the peaks of each sub-carrier is 15 kHz.
The phenomenon that haunts most of the radio access technologies is the Inter Symbol Interference (ISI). ISI is caused by multipath propagations which causes the elongation of the received signals in the time domain. This causes the bits to interfere each other and degrade the received signal. To prevent this, a Cyclic Prefix (CP) is added to the symbol which is simply a copy of the tail of the same symbol added at the start of the symbol. CP is preferred to Guard Interval (GI) which is the separation of the symbols in time domain by a time interval to neutralize the delay spread caused by the multipath. This is because with the use of GI, the receiver filter has to consider the delay added to the delay spread. With the use of CP, the data stream becomes continuous and this shortens the receiver filter delay.
There are effectively two sets of CP used based on their duration; Long CP with a dura-tion of 16.67 μs and short CP with a duration of 4.67 μs. Long CP is used in the chal-lenging multipath environments where the delay spread of the received signal is much longer. The inherent properties that make OFDM a better choice for radio access are:
• Better tolerance against frequency selective fading due to the use of multiple sub-carriers
• Link adaptation and frequency domain scheduling
• Simpler receiver architecture based on Digital Signal Processing (DSP)
Figure 3.1 shows the bunch of 12 sub-carriers that are placed orthogonally to each other. These blocks of 12 sub-carriers form a Physical Resource Block (PRB) with a band-width of 180 kHz. In time domain, these sub-carriers are allocated for duration of 0.5 ms.
Single Carrier FDMA
Single Carrier Frequency Division Multiple Access (SC-FDMA) is the preferred uplink multiple access technology over OFDMA in LTE. The problem associated with the OFDMA in the uplink direction is its high Peak to Average Power Ratio (PAPR). This means that the operating point of the power amplifiers in the transmitter needs to be lowered off which in turn lowers the amplifier efficiency. This is not much of an issue in the downlink as the power is much more abundant at the eNodeB side compared to the battery operated UEs. Hence for a longer battery life at the UE end, SC-FDMA is used.
SC-FDMA is also referred as Discrete Fourier Transform (DFT) based OFDMA as it uses DFT mapper to generate frequency domain symbols which would be mapped throughout the different sub-carriers with the subcarrier mapping techniques such as localized mapping or interleaved mapping. Thus the main difference between OFDMA
and SC-FDMA in data wise perspective is that in the OFDMA, the symbols are carried by individual sub-carriers while in the SC-FDMA, the symbols are carried by a group of sub-carriers simultaneously.
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