Thursday, December 8, 2011

SC-FDMA Modulation

Modulation symbol mapping

The transmitter of an SC-FDMA system converts a binary input signal to a sequence of modulated subcarriers. Todo so, it performs the signal processing operations shown in the Figure. Signal processing is repetitive in a few different time intervals. Resource assignment takes place in transmit time intervals (TTIs). In 3GPP LTE, a typical TTI is 0.5 ms. The TTI is further divided into time intervals referred to as blocks. A block is the time used to transmit all of subcarriers once.

At the input to the transmitter a baseband modulator transforms the binary input to a multilevel sequence of complex numbers Xn in one of several possible modulation formats including Binary Phase Shift Keying (BPSK), quaternary PSK (QPSK), 16-level Quadrature Amplitude Modulation (16-QAM) and 64-QAM. The system adapts the modulation format, and thereby the transmission bit rate, to match the current channel conditions of each terminal.

The type of modulation format used often depends on the signal-to-noise level of the received signal and the receiver ability to decode them correctly. These modulated symbols are then mapped to subcarriers. An inverse-FFT (IFFT) is used to transform the modulated subcarriers in frequency domain to time domain samples.

In general, the same modulation format is used in all the subcarriers to keep the control information overhead small. However, it is possible to have different modulation formats over multiple subcarriers, and it is in fact advantageous in harsh and time varying channel conditions. In a broadband system, the channel is frequency selective over its large system bandwidth, meaning the signal fading on each subcarrier is independent. The interference level on each subcarrier can also be different and vary uniquely with time. It results in a different signal-to-impairment level on each of the subcarriers. Hence, having an appropriate modulation format on these subcarriers would help to maximize the overall system throughput. OFDM system inherits an adaptation of modulation formats to each of the subcarriers depending on channel conditions, and this is called Channel-dependent scheduling.

A cyclic prefix block copies a portion of the samples at the end of the time domain samples block (at the IFFT output) to the beginning. Since the DFT/FFT outputs are periodic in theory, copying the samples to the beginning will make the signal continuous. The length of the cyclic prefix depends on the channel delay spread, and is preferably longer than the length of the channel response. At the receiver, the prefix part of the symbol is thrown away as it may contain ISI from its previous symbol. Hence, it removes the effect of ISI caused by the multipath signal propagation. However, the prefix is the overhead in an OFDM system, as it does not carry any useful information.

PAPR analysis SC-FDMA offers similar performance and complexity as OFDM. However, the main advantage of SC-FDMA is the low PAPR (peak-average-power ratio) of the transmit signal. PAPR is defined as the ratio of the peak power to average power of the transmit signal. As PAPR is a major concern at the user terminals, low PAPR makes the SC-FDMA the preferred technology for the uplink transmission. PAPR relates to the power amplifier efficiency at the transmitter, and the maximum power efficiency is achieved when the amplifier operates at the saturation point. Lower PAPR allows operation of the power amplifier close to saturation resulting in higher efficiency. With higher PAPR signal, the power amplifier operating point has to be backed off to lower the signal distortion, and thereby lowering amplifier efficiency.

As SC-FDMA modulated signal can be viewed as a single carrier signal, a pulse shaping filter can be applied to transmit signal to further improve PAPR. PAPR comparison between OFDM and SC-FDMA variations such as interleaved SC-FDMA and localized SC-FDMA has been done in [2]. With no pulse shaping filters, interleavedSC-FDMA shows the best PAPR. Compared to OFDM PAPR, the PAPR of interleaved SCFDMA with QPSK is about 10 dB lower, whereas that of localized SC-FDMA is only about 3 dB lower. With 16-QAM, these levels are about 7 dB and 2 dB lower respectively. Therefore, interleaved SC-FDMA is a preferred modulation technique for lower PAPR. Pulse shape filtering of SC-FDMA in fact degrades the PAPR level of interleaved SC-FDMA whereas it shows no effect with localized SC-FDMA.

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