Orthogonal Frequency Division Multiplexing (OFDM) is a fundamental modulation technique used in 5G, LTE networks, particularly for its downlink. The concept of orthogonality in OFDM ensures that subcarriers are mutually orthogonal, which minimizes interference and enhances spectral efficiency.
Here, we delve into the technical aspects of orthogonality, OFDM, and its related techniques in the context of 5G.
Orthogonality in OFDM
OFDM modulation consists of modulating orthogonal subcarriers as shown in below figure.
Two subcarriers ( c1(t) ) and ( c2(t) ) are orthogonal if their scalar product over a symbol duration (T) is zero:
This condition imposes that the subcarrier spacing Δf must be the inverse of the symbol duration ( T ):
Orthogonality allows multiple subcarriers to coexist without interference, which is critical for maximizing spectral efficiency and mitigating the effects of frequency-selective fading in the propagation channel.
Benefits of OFDM
- Spectral Efficiency: Due to orthogonality, OFDM modulation makes efficient use of the available spectrum.
- Resilience to Fading: OFDM provides robustness against frequency-selective fading, a common issue in wireless communication.
OFDMA in Downlink
In the downlink, OFDM is used in conjunction with Orthogonal Frequency Division Multiple Access (OFDMA) to support multiple access capabilities. OFDMA divides the spectrum into numerous subcarriers that are assigned to different users, enabling efficient and dynamic allocation of resources.
- Zero-Padding Technique: This technique helps reduce the Peak-to-Average Power Ratio (PAPR) and minimizes power leakage into neighboring bands. The PAPR is defined as the ratio of the peak power to the average power of the OFDM signal within a given bandwidth.
Uplink Transmission: OFDM and DFT-S-OFDM
Uplink transmission in 5G can use either OFDM or Discrete Fourier Transform Spread OFDM (DFT-S-OFDM):
- DFT-S-OFDM: Derived from OFDM with transform precoding, DFT-S-OFDM is applied over a broader frequency band. It uses either OFDMA or Single Carrier-FDMA (SC-FDMA) for multiplexing.
- Power Efficiency: DFT-S-OFDM reduces power consumption by lowering PAPR, making it suitable for uplink transmission where power efficiency is crucial.
- Spectral Efficiency: While DFT-S-OFDM is less spectrally efficient than OFDM, it is advantageous for lower power consumption in uplink transmissions.
Multiplexing Techniques in 5G
1. OFDMA and SC-FDMA:
- OFDMA: Allocates time-frequency resources to users dynamically, ensuring efficient utilization of the spectrum.
- SC-FDMA: Offers lower PAPR compared to OFDMA, making it suitable for uplink transmissions.
2. Spatial Division Multiple Access (SDMA):
- Also known as Multi-User MIMO (MU-MIMO), SDMA allows simultaneous transmissions on the same frequency band to multiple users using multiple antennas, enhancing overall system capacity.
3. Non-Orthogonal Multiple Access (NOMA):
- Introduced in Release 16 of 5G NR, NOMA allows multiple users to share the same time-frequency resources simultaneously.
- Multi-User Detection (MUD): At the receiver, MUD techniques such as Successive Interference Cancellation (SIC) are employed to separate the signals of different users. This improves the overall system capacity and efficiency.
Conclusion
OFDM and its associated techniques play a crucial role in the efficient functioning of 5G networks. The orthogonality of subcarriers ensures minimal interference, high spectral efficiency, and resilience to frequency-selective fading. Techniques like OFDMA, SC-FDMA, SDMA, and NOMA further enhance the capability of 5G to meet the diverse and demanding requirements of modern wireless communication systems.