OFDM Variants: COFDM, OFDMA, f-OFDM, VOFDM, WOFDM, Flash OFDM

This page explores various OFDM (Orthogonal Frequency Division Multiplexing) variants, including COFDM, OFDMA, f-OFDM, VOFDM, WOFDM, OFCDM, and Flash OFDM. It highlights the advantages, disadvantages, and applications of each type.

What is OFDM?

OFDM, or Orthogonal Frequency Division Multiplexing, was introduced in the IEEE 802.11a WLAN standard to increase the bit rate of Wi-Fi systems compared to Single Carrier (SC) systems. It uses densely packed subcarriers to save bandwidth and achieve spectral efficiency.

OFDM is implemented using IFFT (Inverse Fast Fourier Transform) at the transmitter and FFT (Fast Fourier Transform) at the receiver. As shown in the figure, OFDM is used for data modulation, displaying both frequency and time domain views of the signal before and after IFFT. The OFDM module typically follows a data modulation module, which can be QPSK, 16QAM, or 64QAM, among others. The choice of modulation technique determines the number of bits each subcarrier can carry. For instance, QPSK carries 2 bits, while 16QAM carries 4 bits, and so on.

Let’s delve into different OFDM variants. They all utilize the basic OFDM technique, where each subcarrier transmits low data rate signals placed very closely together. At the receiver, these signals are combined to reconstruct the complete signal.

COFDM | Coded OFDM

In COFDM (Coded OFDM), error correction coding is integrated into the OFDM signal. It employs FEC (Forward Error Correction) to transform bits into a larger number of bits, using redundancy to address frequency-selective fading issues in OFDM. COFDM is used in DVB-T.

OFDMA

OFDMA stands for Orthogonal Frequency Division Multiple Access. In standard OFDM, the entire bandwidth of subcarriers is allocated to a single user. However, in OFDMA, the total bandwidth is shared among multiple users. This is achieved by allocating subchannels to each user, with a subchannel consisting of multiple subcarriers that can be contiguous or distributed based on a specific permutation.

Advantages: OFDMA is more robust to fading and interference compared to OFDM.

f-OFDM | Filtered OFDM

In f-OFDM (Filtered OFDM), a filtering module is applied after the IFFT/CP (Cyclic Prefix) insertion. This helps improve the out-of-band radiation of the sub-band signal. Filters used in f-OFDM include Soft-Truncated Sinc Filters (using either a Hann window or an RRC window) and Equi-ripple filters using the Remez exchange algorithm.

Advantages: f-OFDM helps meet the OOBE (Out-of-Band Emission) requirements of the spectrum while leveraging the benefits of OFDM. This makes it more spectrally efficient and offers a throughput gain compared to conventional OFDM. Furthermore, it significantly reduces guard band usage.

VOFDM | Vector OFDM

VOFDM (Vector OFDM) utilizes MIMO (Multiple-Input Multiple-Output) to maximize its advantages. It was developed by Cisco Systems. MIMO enables higher data rates during transmission. The multiple antennas at both the transmitter and receiver leverage multi-path effects, enhancing signal reception.

WOFDM | Wideband OFDM

In WOFDM (Wideband OFDM), the spacing between channels is larger than in standard OFDM. This reduces the impact of frequency offsets between the transmitter and receiver, improving system performance. WOFDM is typically applied in Wi-Fi systems.

OFCDM | Orthogonal Frequency Code Division Multiplexing

OFCDM (Orthogonal Frequency Code Division Multiplexing) is a combination of OFDM and CDMA (Code Division Multiple Access) techniques. In OFCDM, spreading and despreading modules are applied at the OFDM transmitter and receiver, respectively. This variant is also known as hybrid-OFDM.

Flash OFDM

Flash OFDM uses multiple carriers or tones and a fast hopping technique to spread the signal across the band. It was developed by Flarion.