Multiple access technology refers to the technology of connecting multiple users in different locations to a common transmission medium to achieve communication between users. It allows multiple users or devices to share resources at the same time, frequency band or physical channel without interfering with each other. Multiple access technology is widely used in mobile communication networks, satellite communication networks and computer area networks, and is a communication method with great development potential.
Common multiple access technologies are divided into frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA) and space division multiple access (SDMA). The basic principles of these multiple access technologies can be simply distinguished from their names, which are realized by different frequencies, different times, different sequence codes and different spatial orientations.
Frequency division multiple access (FDMA), its basic principle is to divide the total frequency band of the communication system into several equally spaced channels and allocate these channels to different users. Each user occupies a specific frequency band, and the frequency bands do not overlap, thus avoiding mutual interference. The advantages of this technology are mature technology, simple equipment, and suitable for radio frequency bands with limited bandwidth. But it also has the disadvantages of low frequency band utilization, small capacity, complex handover and easy call drop. Early cellular network mobile communication systems mainly used frequency division multiple access technology.
The principle of time division multiple access (TDMA) is to divide time into periodic frames, and each frame is divided into several time slots. Each user occupies a specific time slot for communication, thereby achieving the purpose of multiple users sharing the same channel.
Time division multiple access (TDMA) has strong anti-interference ability, high frequency band utilization, and large system capacity. The base station complexity is reduced, the intermodulation interference is small, and the handover is simple. However, its disadvantages are also obvious: precise clock synchronization is required, and the equipment requirements are high. Time division multiple access (TDMA) is widely used in modern mobile communication systems, such as GSM, TD-SCDMA, etc.
The basic principle of code division multiple access (CDMA): different user signals are distinguished by different coding sequences. Each user is assigned a specific code type and transmitted using a common channel. The receiving end uses a code type that is exactly the same as the transmitting end for demodulation to restore the original signal. Its advantages are large communication capacity and soft capacity characteristics, that is, when the number of users increases, the communication quality will decline, but there will be no blocking. Strong anti-interference ability and good confidentiality. However, there are disadvantages such as prominent multi-access interference problems and the need for complex power control and soft switching technologies. Code Division Multiple Access (CDMA) is widely used in third-generation mobile communication systems, such as CDMA2000, WCDMA, etc.
Space Division Multiple Access (SDMA) is to form different channels through space division. It uses adaptive array antenna technology to form different beams in different user directions. Different beams can use the same frequency and the same multiple access method, or different frequencies and different multiple access methods. It can significantly increase system capacity and reduce the impact of multipath fading.
However, the technology is complex and has high requirements for equipment. This technology is mainly used in satellite communications and smart antenna systems.Multiple access technology is widely used in mobile communications, satellite communications, the Internet of Things and other fields, bringing great convenience to our lives. Whether it is daily mobile phones or vehicle-mounted communication equipment, they cannot do without the support of multiple access technology. Multiple access technology enables a large number of devices to access the network at the same time, realizing the interconnection between devices. This has promoted the development of smart homes, smart cities, industrial automation and other fields.