tailieunhanh - MC và các hệ thống phổ Bá P3

Hybrid Multiple Access Schemes The simultaneous transmission of multiple data streams over the same medium can be achieved with different multiplexing schemes. Most communications systems, such as GSM, DECT, IEEE , and HIPERLAN/2, use multiplexing based on either time division, frequency division or a combination of both. Space division multiplexing is applied to further increase the user capacity of the system. The simplest scheme of space division multiplexing is antenna sectorization at the base station where often antennas with 120◦ /90◦ beams are used | 3 Hybrid Multiple Access Schemes Introduction The simultaneous transmission of multiple data streams over the same medium can be achieved with different multiplexing schemes. Most communications systems such as GSM DECT IEEE and HIPERLAN 2 use multiplexing based on either time division frequency division or a combination of both. Space division multiplexing is applied to further increase the user capacity of the system. The simplest scheme of space division multiplexing is antenna sectorization at the base station where often antennas with 120 90 beams are used. Recently multiplexing schemes using code division have gained significant interest and have become part of wireless standards such as UMTS IS-95 and WLAN. Moreover code division multiplexing is also a promising candidate for the fourth generation of mobile radio systems. Time Division Multiplexing The separation of different data streams with time division multiplexing is carried out by assigning each stream exclusively a certain period of time . time slot for transmission. After each time slot the next data stream transmits in the following time slot. The number of slots assigned to each user can be supervised by the medium access controller MAC . A MAC frame determines a group of time slots in which all data streams transmit once. The duration of the different time slots can vary according to the requirements of the different data streams. If the different data steams belong to different users the access scheme is called time division multiple access TDMA . Time division multiplexing can be used with both time division duplex TDD and frequency division duplex FDD . However it is often used in communication systems with TDD duplex transmission where up- and downlink are separated by the assignment of different time slots. It is adopted in several wireless LAN and WLL systems including IEEE and HIPERLAN 2 as well as IEEE and HIPERMAN. Frequency Division Multiplexing With .