A-BFT (Associated Beam Forming training Time)

Introduction

Quadrature Amplitude Modulation (QAM) is a modulation scheme used to transmit digital information over a radio frequency carrier wave. QAM is a combination of two key modulation techniques: Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK). The 8 Quadrature Amplitude Modulation (8QAM) is an extension of the basic QAM, which uses 8 different states to encode digital information. This article will provide a detailed explanation of 8QAM, including its encoding and decoding techniques, advantages, and limitations.

Encoding Techniques

In 8QAM, each symbol is represented by a combination of eight different amplitude and phase states. These states are arranged in a 2D constellation. The horizontal axis represents the in-phase component (I) of the signal, and the vertical axis represents the quadrature component (Q) of the signal. Each point on the diagram represents a different combination of amplitude and phase states. The eight points on the diagram represent the eight possible states in 8QAM.

To encode a digital symbol using 8QAM, the symbol is first converted into a 3-bit binary sequence. This sequence is then mapped to one of the eight possible states in the constellation diagram. The mapping can be done using a variety of techniques, such as Gray coding or natural binary coding. Gray coding is commonly used because it ensures that adjacent states in the diagram differ by only one bit, which reduces the likelihood of errors due to noise and interference.

The eight states in 8QAM are arranged in a circular pattern on the diagram, with four states on the outer circle and four states on the inner circle. The four states on the outer circle have equal amplitude, but different phases, while the four states on the inner circle have equal phase, but different amplitudes. The outer circle states are used to transmit symbols with high amplitude, while the inner circle states are used to transmit symbols with low amplitude.

Decoding Techniques

To decode a received 8QAM signal, the receiver must first estimate the amplitude and phase of the signal. This is typically done using a technique called maximum likelihood detection. In this technique, the receiver compares the received signal to each of the eight possible states in the constellation diagram and selects the state that is closest to the received signal in terms of amplitude and phase.

The receiver then maps the detected state back to the original 3-bit binary sequence using the same mapping technique used by the transmitter. This results in the recovery of the original digital symbol.

Advantages of 8QAM

8QAM has several advantages over other modulation schemes. One of the main advantages is its higher spectral efficiency compared to other modulation schemes such as Binary Phase Shift Keying (BPSK) or Quadrature Phase Shift Keying (QPSK). Spectral efficiency refers to the amount of digital information that can be transmitted per unit of bandwidth. 8QAM achieves higher spectral efficiency by using more states to encode digital information.

Another advantage of 8QAM is its robustness to noise and interference. The use of multiple states in 8QAM allows for a higher signal-to-noise ratio (SNR) compared to other modulation schemes. This means that 8QAM can transmit digital information over longer distances and in environments with high levels of noise and interference.

Limitations of 8QAM

One of the main limitations of 8QAM is its susceptibility to phase noise. Phase noise is a type of distortion that can occur in a communication system due to variations in the carrier wave's phase. Phase noise can cause errors in the detection of the amplitude and phase of the received signal, which can result in errors in the recovered digital symbol.

Another limitation of 8QAM is its complexity compared to other modulation schemes such as BPSK or QPSK. The use of more states in 8QAM requires more complex encoding and decoding techniques, which can increase the overall complexity of the communication system.

Applications of 8QAM

8QAM is used in a wide range of applications, including digital television, satellite communication, and wireless communication. In digital television, 8QAM is used to transmit high-definition television (HDTV) signals. In satellite communication, 8QAM is used to transmit digital information over long distances.

In wireless communication, 8QAM is used in 802.11n and 802.11ac wireless standards, which are used in Wi-Fi networks. These standards use 8QAM in combination with other modulation schemes such as QPSK and 16QAM to achieve higher data rates and better spectral efficiency.

Conclusion

In summary, 8 Quadrature Amplitude Modulation (8QAM) is a modulation scheme that uses eight different amplitude and phase states to encode digital information. 8QAM provides higher spectral efficiency and better robustness to noise and interference compared to other modulation schemes such as BPSK or QPSK. However, 8QAM is susceptible to phase noise and is more complex than other modulation schemes. 8QAM is used in a wide range of applications, including digital television, satellite communication, and wireless communication.