CDD (cyclic delay diversity)
Cyclic delay diversity (CDD) is a technique used to improve the performance of wireless communication systems. It is based on the principle of diversity, which involves the use of multiple paths for transmitting the same signal. CDD works by introducing a time delay to the transmitted signal and then repeating it at regular intervals. This creates multiple copies of the signal that arrive at the receiver at different times, thus providing diversity.
The main idea behind CDD is to mitigate the effects of multipath fading. Multipath fading occurs when a transmitted signal travels through different paths with different lengths and properties. These paths can cause the signal to experience phase shifts, attenuation, and other distortions. As a result, the received signal can become weaker and distorted, leading to errors in the transmitted data.
CDD works by creating multiple copies of the transmitted signal with a time delay between them. The delay is typically less than the duration of the symbol, which is the smallest unit of information in a communication system. By introducing a delay, CDD creates a set of time-shifted copies of the signal that have different phase and amplitude characteristics. These copies can then be combined at the receiver to reduce the effects of fading.
The time delay introduced by CDD is achieved by inserting a cyclic prefix (CP) into the transmitted signal. The cyclic prefix is a copy of the end of the symbol that is inserted at the beginning of the symbol. The length of the cyclic prefix is typically equal to the length of the channel impulse response (CIR). The CIR represents the response of the wireless channel to a unit impulse signal. By inserting a cyclic prefix, the transmitted signal is effectively delayed by the length of the CIR.
At the receiver, the copies of the signal with different delays are combined using a linear combiner. The linear combiner weights each copy of the signal by a complex coefficient that depends on the channel characteristics. The coefficients are typically calculated based on the channel state information (CSI), which represents the current state of the wireless channel. The CSI can be estimated using various techniques, such as pilot symbols or channel estimation algorithms.
The linear combiner effectively sums the copies of the signal with different delays, resulting in a combined signal with reduced distortion and improved signal-to-noise ratio (SNR). The SNR is a measure of the quality of the received signal, which is defined as the ratio of the signal power to the noise power. By improving the SNR, CDD can increase the data rate and reliability of the wireless communication system.
CDD can be implemented in various wireless communication systems, including cellular networks, Wi-Fi, and digital broadcasting. In cellular networks, CDD is typically used in conjunction with other diversity techniques, such as space diversity and frequency diversity. Space diversity involves using multiple antennas at the transmitter and receiver to exploit the spatial diversity of the wireless channel. Frequency diversity involves transmitting the same signal on different frequency bands to exploit the frequency diversity of the wireless channel.
CDD can also be used in Wi-Fi systems, which use orthogonal frequency division multiplexing (OFDM) as the modulation scheme. OFDM involves dividing the frequency band into multiple subcarriers, each of which carries a portion of the data. CDD can be applied to each subcarrier independently to improve the overall performance of the system.
In digital broadcasting systems, CDD can be used to improve the reception of digital audio and video signals. Digital broadcasting systems use different modulation schemes, such as quadrature amplitude modulation (QAM) and vestigial sideband modulation (VSB), to transmit audio and video signals. CDD can be applied to each modulation symbol to improve the reception quality of the signals.
In conclusion, CDD is a powerful technique for improving the performance of wireless communication systems. By introducing a time delay to the transmitted signal and creating multiple copies of the signal with different delays, CDD can reduce the effects of multipath fading and improve the signal quality at the receiver. CDD can be implemented in various wireless communication systems, including cellular networks, Wi-Fi, and digital broadcasting. It can be used in conjunction with other diversity techniques, such as space diversity and frequency diversity, to further improve the performance of the system.