CP-OQAM (Cyclic prefix-based offset quadrature amplitude modulation)

CP-OQAM, or cyclic prefix-based offset quadrature amplitude modulation, is a modulation scheme used in wireless communication systems. It is an extension of the OFDM (orthogonal frequency division multiplexing) modulation scheme that is widely used in modern communication systems. CP-OQAM has become increasingly popular due to its ability to mitigate inter-symbol interference (ISI) and inter-carrier interference (ICI) caused by multipath fading.

To understand CP-OQAM, it is important to first understand OFDM. OFDM is a modulation scheme that divides a high-speed data stream into several parallel subcarriers, each carrying a low-speed data stream. These subcarriers are orthogonal to each other, meaning that they do not interfere with each other. This property is achieved by ensuring that the subcarriers are separated by integer multiples of the subcarrier spacing.

OFDM is susceptible to ISI and ICI because it uses a rectangular pulse shape to modulate the subcarriers. This means that the pulse shape extends beyond the symbol period, causing ISI and ICI. CP-OQAM addresses this problem by using a pulse shape that is not rectangular, but instead has a cyclic prefix.

The cyclic prefix is a copy of the end of the OFDM symbol that is added to the beginning of the symbol. This copy is typically a fraction of the symbol period and is known as the guard interval. The guard interval allows the receiver to remove any ISI and ICI that may have been introduced during transmission.

CP-OQAM also introduces a new concept called offset QAM. In QAM, the data is modulated onto the amplitude and phase of a carrier wave. Offset QAM adds a small offset to the carrier frequency of each subcarrier. This offset is typically half of the subcarrier spacing and is designed to make the subcarriers non-orthogonal to each other.

By making the subcarriers non-orthogonal, the ICI caused by multipath fading is reduced. This is because the non-orthogonality causes the subcarriers to have a small overlap in frequency, which allows the receiver to better handle the interference.

To further reduce the impact of ICI, CP-OQAM uses a filter bank to process the received signal. The filter bank separates the signal into different frequency bands, each with its own set of filters. These filters are designed to match the pulse shape of the transmitted signal, which reduces the ICI even further.

One of the benefits of CP-OQAM is its low computational complexity. The pulse shape used in CP-OQAM is simple to implement, and the filtering process used in the receiver is also relatively simple. This makes CP-OQAM a good choice for low-power devices with limited processing capabilities.

Another benefit of CP-OQAM is its ability to achieve high spectral efficiency. The use of non-orthogonal subcarriers allows CP-OQAM to transmit more data per unit of bandwidth compared to traditional OFDM. This makes CP-OQAM a good choice for applications where high data rates are required.

In conclusion, CP-OQAM is a modulation scheme that uses a cyclic prefix and offset QAM to mitigate ISI and ICI caused by multipath fading. It achieves high spectral efficiency and has low computational complexity, making it a good choice for modern communication systems. CP-OQAM has been proposed for various wireless communication systems, such as 4G LTE and 5G NR (New Radio). In 4G LTE, CP-OQAM is used in the downlink (i.e., from the base station to the mobile device) to improve the channel robustness and data rate. In 5G NR, CP-OQAM is used in both the uplink and downlink to achieve higher spectral efficiency and support a wide range of use cases, such as ultra-reliable low-latency communication (URLLC), massive machine-type communication (mMTC), and enhanced mobile broadband (eMBB).