LDSMA (Low Density Spreading Multiple Access)
Low Density Spreading Multiple Access (LDSMA) is a spread spectrum multiple access technique designed for use in wireless communication systems. It was developed in the late 1990s as a response to the growing demand for wireless data communication and the limitations of existing multiple access techniques.
LDSMA is based on a combination of direct-sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) techniques. DSSS spreads the signal across a wide frequency band, while FHSS hops the signal from one frequency band to another at a high rate. By combining these techniques, LDSMA is able to achieve a high degree of resistance to interference and jamming.
In LDSMA, each user is assigned a unique pseudorandom spreading code, which is used to spread the user's signal across the frequency band. The spreading code is generated by a code generator at the transmitter and a corresponding code generator at the receiver. The code generator produces a sequence of chips, which are used to modulate the user's data signal. The chip rate is typically much higher than the data rate, which results in a spreading gain that helps to improve the signal-to-noise ratio (SNR) at the receiver.
The spreading code used in LDSMA is designed to have a low density, which means that it has a low autocorrelation and cross-correlation with other codes in the system. This helps to reduce the probability of interference and collisions between users. The low density property also makes LDSMA well-suited for use in systems with a large number of users.
In addition to the spreading code, each user in LDSMA is also assigned a unique frequency hopping sequence. The frequency hopping sequence determines the sequence of frequency bands that the user's signal will hop between. The frequency hopping sequence is also generated by a code generator at the transmitter and a corresponding code generator at the receiver. The frequency hopping sequence is designed to be unpredictable and to change at a high rate, which helps to further improve resistance to interference and jamming.
LDSMA uses a random access scheme, which means that users are allowed to transmit at any time without prior coordination with other users. However, before transmitting, the user must first sense the channel to determine if it is free. If the channel is busy, the user will defer transmission until a later time. If the channel is free, the user will transmit its data signal using its assigned spreading code and frequency hopping sequence.
At the receiver, the signal is despread using the same spreading code and frequency hopping sequence as the transmitter. The despread signal is then demodulated to recover the user's data signal. Because the spreading code and frequency hopping sequence are unique to each user, the receiver is able to extract the user's data signal even in the presence of interference and jamming.
One advantage of LDSMA is its ability to support a large number of users. Because each user is assigned a unique spreading code and frequency hopping sequence, the system can support a large number of users without the risk of collisions or interference. This makes LDSMA well-suited for use in systems with a large number of users, such as wireless LANs and cellular networks.
Another advantage of LDSMA is its resistance to interference and jamming. By using a combination of DSSS and FHSS techniques, LDSMA is able to achieve a high degree of resistance to interference and jamming. This makes LDSMA well-suited for use in environments with high levels of interference or in military applications where resistance to jamming is critical.
In conclusion, Low Density Spreading Multiple Access (LDSMA) is a spread spectrum multiple access technique that combines direct-sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) techniques to achieve a high degree of resistance to interference and jam ming. LDSMA uses a unique pseudorandom spreading code and frequency hopping sequence for each user, which helps to reduce the probability of interference and collisions between users. The low density property of the spreading code makes LDSMA well-suited for use in systems with a large number of users. LDSMA also uses a random access scheme, which allows users to transmit at any time without prior coordination with other users. However, users must first sense the channel to determine if it is free before transmitting.