OFDM was selected for the downlink because it can
- Improved spectral efficiency
- Reduce ISI effect by multipath
- Provide better Protection against frequency selective fading
OFDM is a scheme that offers good resistance to multipath and is now widely recognized as the method of choice for mitigating multipath for broadband wireless. It can be straightforwardly extended to a multi-access scheme called OFDMA, where each user is assigned a different set of subcarriers.
Frequency Spectral Efficiency Improvement
OFDM increases spectral efficiency by incorporating multiple carriers in the same frequency space as a single carrier.
Reducing the Impact by Inter Symbol Interference (ISI)
Improvement of frequency spectral efficiency requires the reduction of Inter symbol interference (ISI). This is achieved by tighter frequency roll off and alignment of nulls and peaks between different frequencies.
Better Protection against Frequency Fading
Smaller subcarrier and resource block bandwidth increase robustness against frequency related fading With this smaller carrier bandwidth, the frequency coherence bandwidth is much smaller than 3G systems while and correlation factor is much higher.
As a result, it will also be much easier to implement scheduling algorithm based on Frequency Selective Scheduling to improve system throughput in the manner shown below.
Single Carrier-FDMA is a recently developed single carrier multiple access technique which has similar structure and performance to OFDMA. SC-FDMA can be viewed as a special OFDMA system with the user’s signal pre-encoded by discrete Fourier transform (DFT), hence also known as DFT-pre-coded OFDMA or DFT-spread OFDMA.
One prominent advantage of SC-FDMA over OFDMA is the lower PAPR (peak-to-average power ratio) of the transmit waveform for low order modulations like QPSK and BPSK, which benefits the mobile users in terms of battery life and power efficiency.
OFDM signals have a higher peak-to-average ratio (PAR)—often called a peak-to-average power ratio (PAPR) than single-carrier signals do. The reason is that in the time domain, a multicarrier signal is the sum of many narrowband signals. At some time instances, this sum is large and at other times is small, which means that the peak value of the signal is substantially larger than the average value.
This high PAR is one of the most important implementation challenges that face OFDM, because it reduces the efficiency and hence increases the cost of the RF power amplifier, which is one of the most expensive components in the radio. The figure below shows the relationship between OFDM and SC-FDMA in LTE.
The major difference between the downlink and uplink transmission scheme is that each subcarrier in the uplink carries information about each transmitted modulation symbol as shown in figure below, whereas in downlink each subcarrier only carries information related to one specific modulation symbol. As a result, the uplink power level due to SC-FDMA also need to be increased by 2~3dB to compensate for the extra noise due to more spreading.