OFDM (Orthogonal frequency division multiple access)
OFDM (Orthogonal Frequency Division Multiplexing) is a modulation technique widely used in modern wireless communication systems. It provides efficient transmission of data by dividing the available frequency spectrum into multiple narrow subcarriers that are orthogonal to each other. This allows simultaneous transmission of multiple data streams, increasing the overall system capacity.
The fundamental idea behind OFDM is to convert a high-rate data stream into a set of parallel low-rate data streams, each transmitted on a separate subcarrier. These subcarriers are closely spaced and overlap in frequency, but their orthogonality is maintained by carefully designing their waveforms. The orthogonality ensures that there is no interference between the subcarriers, allowing them to be simultaneously transmitted and received without causing mutual interference.
The process of converting a high-rate data stream into parallel low-rate data streams involves two main steps: modulation and multiplexing.
In the modulation step, the input data stream is divided into smaller blocks, each representing a symbol. These symbols are then mapped onto a specific subcarrier using a modulation scheme such as Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK). The choice of modulation scheme depends on the desired trade-off between data rate and robustness against noise and interference.
Once the symbols are modulated onto the subcarriers, the next step is multiplexing. In OFDM, the multiplexing is achieved by using the Inverse Fast Fourier Transform (IFFT). The IFFT takes the modulated subcarriers as input and generates a time-domain signal composed of multiple parallel subcarriers. This time-domain signal is then converted into an analog signal and transmitted over the channel.
At the receiver side, the received signal is first converted back to the digital domain and then undergoes the reverse process. The received signal is split into parallel subcarriers using the Fast Fourier Transform (FFT). Each subcarrier is then demodulated to recover the original symbols, which are then combined to reconstruct the original data stream.
One of the key advantages of OFDM is its robustness against frequency-selective fading channels. Since the frequency spectrum is divided into narrow subcarriers, each subcarrier experiences flat fading over its bandwidth. This means that even if some subcarriers experience deep fading, others may still remain unaffected. This makes OFDM particularly suitable for wireless channels with multipath propagation, where the transmitted signal arrives at the receiver through multiple paths with different delays and attenuations.
Furthermore, the orthogonality of the subcarriers in OFDM allows for efficient spectrum utilization. The subcarriers can be closely spaced, enabling a high spectral efficiency and accommodating a large number of users simultaneously. This makes OFDM an attractive choice for systems that require high data rates and support multiple users, such as cellular networks and digital broadcasting systems.
To mitigate the effects of inter-symbol interference (ISI) caused by multipath propagation, OFDM systems typically employ a guard interval or cyclic prefix. The guard interval is a period of zero padding inserted between successive OFDM symbols. It allows the receiver to separate the delayed replicas of the transmitted signal and eliminates the ISI. The length of the guard interval depends on the maximum delay spread of the channel and should be longer than the channel impulse response.
In addition to its robustness and spectral efficiency, OFDM also provides flexibility in managing system resources. By adjusting the allocation of subcarriers to different users, OFDM can support multiple access schemes, such as Orthogonal Frequency Division Multiple Access (OFDMA). OFDMA extends the benefits of OFDM to multiple users by dividing the available subcarriers among different users or groups of users, allowing simultaneous transmission and reception.
Overall, OFDM is a versatile modulation technique that has revolutionized wireless communication systems. Its ability to efficiently utilize the frequency spectrum, robustness against fading, and support for multiple users make it a fundamental building block of many modern communication standards, including Wi-Fi, 4G LTE, and 5G. By dividing the frequency spectrum into orthogonal subcarriers, OFDM enables high data rates, reliable transmission, and efficient use of available resources, making it an essential technology in today's wireless world.