FD-CDM (Frequency Domain Code Division Multiplexing)

Frequency Domain Code Division Multiplexing (FD-CDM) is a multiple access technique used in wireless communication systems to transmit multiple signals simultaneously over a single channel. FD-CDM is a combination of two well-known techniques, namely, Code Division Multiplexing (CDM) and Frequency Division Multiplexing (FDM). In FD-CDM, the frequency domain is divided into several subcarriers, and each subcarrier is assigned a unique code. The coded subcarriers are then transmitted simultaneously over the channel, and the receiver uses the corresponding codes to separate the transmitted signals.

The basic idea behind FD-CDM is to use a spreading code to modulate each subcarrier in the frequency domain. The spreading code is a unique sequence of values that is multiplied with the data signal to create a coded signal. The coded signal is then transmitted over the corresponding subcarrier in the frequency domain. At the receiver, the transmitted signal is multiplied with the corresponding spreading code to recover the original data signal. The process of multiplying the coded signal with the spreading code is called despreading, and it is used to separate the signals transmitted by different users.

The use of a unique spreading code for each subcarrier in FD-CDM makes it possible to separate the signals of different users at the receiver. This is because each user's signal is spread using a different code, which makes it orthogonal to the signals of other users. The orthogonality of the signals allows them to be separated using simple filtering techniques.

FD-CDM has several advantages over other multiple access techniques such as Time Division Multiplexing (TDM) and Frequency Division Multiplexing (FDM). One of the main advantages of FD-CDM is its ability to provide high spectral efficiency. This is because the frequency domain is divided into several subcarriers, each of which can be used to transmit data simultaneously. The use of multiple subcarriers allows FD-CDM to achieve higher data rates than TDM and FDM.

Another advantage of FD-CDM is its ability to provide good resistance to multipath fading. Multipath fading occurs when the transmitted signal arrives at the receiver via multiple paths, resulting in signal interference and degradation. FD-CDM uses spreading codes to spread the transmitted signal across multiple subcarriers, which makes it less vulnerable to multipath fading. The use of multiple subcarriers also provides diversity, which improves the quality of the received signal.

FD-CDM also has a low peak-to-average power ratio (PAPR) compared to other multiple access techniques. PAPR is a measure of the difference between the maximum and average power levels of a signal. High PAPR values can lead to distortion and reduced signal quality. FD-CDM uses spreading codes to spread the transmitted signal across multiple subcarriers, which reduces the peak power level of the signal.

FD-CDM can be implemented using different spreading codes such as Gold codes, Kasami codes, and Walsh codes. Gold codes are a type of binary sequence that has good auto-correlation and cross-correlation properties. Kasami codes are a type of sequence that has good correlation properties and low cross-correlation between different codes. Walsh codes are a type of orthogonal code that is widely used in digital communication systems.

FD-CDM can also be combined with other multiple access techniques to improve its performance. For example, FD-CDMA (Frequency Division Code Division Multiple Access) combines FD-CDM with Code Division Multiple Access (CDMA) to provide high spectral efficiency and good resistance to interference.

In conclusion, FD-CDM is a multiple access technique that uses spreading codes to modulate signals in the frequency domain. FD-CDM provides high spectral efficiency, good resistance to multipath fading, low PAPR, and can be combined with other multiple access techniques to further improve its performance. FD-CDM is used in various wireless communication systems, including wireless LANs, 4G and 5G cellular networks, and satellite communication systems.

One example of a wireless communication system that uses FD-CDM is the IEEE 802.11a/g/n wireless LAN standard. This standard uses Orthogonal Frequency Division Multiplexing (OFDM) in combination with FD-CDM to provide high data rates and good resistance to multipath fading. OFDM is a modulation technique that uses a large number of subcarriers to transmit data simultaneously. Each subcarrier is modulated using QAM (Quadrature Amplitude Modulation), and the resulting signal is spread using a unique code. The use of FD-CDM in conjunction with OFDM allows IEEE 802.11a/g/n to achieve high data rates and good resistance to multipath fading.

Another example of a wireless communication system that uses FD-CDM is the Long Term Evolution (LTE) cellular network standard. LTE uses FD-CDM in combination with Orthogonal Frequency Division Multiple Access (OFDMA) to provide high data rates and good spectral efficiency. OFDMA is a multiple access technique that uses OFDM to divide the frequency band into several subcarriers, each of which can be assigned to a different user. Each user's data signal is spread using a unique code, which allows it to be separated from the signals of other users. The use of FD-CDM in conjunction with OFDMA allows LTE to achieve high data rates and good spectral efficiency.

In conclusion, FD-CDM is a powerful multiple access technique that has many advantages over other multiple access techniques such as TDM and FDM. FD-CDM provides high spectral efficiency, good resistance to multipath fading, and low PAPR. FD-CDM can be used in various wireless communication systems, including wireless LANs, cellular networks, and satellite communication systems, to provide high data rates and reliable communication.