ODFM modulation basics
OFDM is a technique used in modern broadband wireless communications systems. To mitigate the effect of dispersive channel distortion in high data rate OFDM systems, CP is introduced to eliminate inter-symbol interference (ISI). It copies the end section of an IFFT packet to the beginning of an OFDM symbol. Typically, the length of the CP must be longer than the length of the dispersive channel to completely remove ISI.
OFDM modulation in a transmitter includes inverse fast Fourier transform (IFFT) operation and CP insertion. In an OFDM receiver, the CP is removed before the packet data is sent to FFT for demodulation.
Next generation wireless systems feature highly dynamic configurations, where the CP length changes according to the transmission mode, frame structure (shown in Figures 1 and 2), and higher level protocol. For instance, the CP configuration for 3GPP LTE changes within each slot.
The CP length is based on an OFDM symbol with 2048 time intervals. Similar variable
CP structures in WiMAX systems can be observed.
Figure 1: 3GPP LTE Frame Structure 1, Applicable to TDD and FDD.
Figure 2: 3GPP LTE Frame Structure 2, applicable to TDD.
Implementing OFDM modulation
The following sections discuss implementing cyclic prefix insertion and removal for OFDM modulation and demodulation.
FFT and inverse FFT operations
The most computationally intensive operation of OFDM modulation is IFFT, and similarly, the core of OFDM demodulation is FFT. High FFT throughput is essential in broadband systems, especially when FFT is shared between multiple data paths.
In modern scalable wireless systems such as WiMAX and 3GPP LTE, run-time reconfigurability is also an integral part of system requirements. The FFT MegaCore function in variable streaming mode targets specifically reconfigurable wireless communications and is a suitable candidate for designing OFDM systems.
The FFT MegaCore function is configured in the variable streaming mode, which allows FFT size and direction change on a packet-by-packet basis. The FFT MegaCore function also takes advantage of the memory-efficient engine-only mode of the FFT core, which outputs bit-reversed symbols directly from the FFT butterfly engines. Bit reversal with cyclic prefix insertion outside the FFT core can be combined. The overall OFDM modulation saves a single buffer.
FFT module reuse
To reduce logic usage, the FFT module is often clocked much faster than the rest of the baseband modules and reused. The FFT module can be shared by different sources; for instance, by multiple antennas, transmission and reception in time division duplex (TDD), and frequency division duplex (FDD) systems.
The FFT module can also be shared with other functional modules, such as crest factor reduction or channel estimation. However, such reuse depends on the user-specific algorithms and may not be a general design consideration. The focus of this article is on the most common wireless communications scenarios: MIMO techniques and TDD and FDD communications.