Ngoài ra, trong khi trong hệ thống macrocellular uplink và downlink hiện khoảng cùng một hiệu suất can thiệp, trong các hệ thống microcellular sự can thiệpCác vị trí chính xác của các tế bào được đưa ra trong Phụ lục D. | Location registration IMSI attachment IMSI detachment Access occurs on a contention basis with the slotted ALOHA protocol as the access algorithm. Basically an access attempt occurs as follows. The terminal chooses a time slot transmits its message and waits for an acknowledgment for a fixed time. If no acknowledgment arrives a random time is waited and a new trial is carried out. This procedure is repeated up to a maximum number of times as specified on the BCCH. An acknowledgment consists of sending the following information The number of the time slot in which the access was made A 5-bit codeword transmitted in the access procedure a loop back of the codeword The time slot number of the SDCCH Note from Figure that in the access burst of bits are used for guard time purposes. This is because before the initial access no information on the terminal timing is known. Therefore the guard time ensures that in the initial access the information bits remain within a single time slot upon arrival at the base station when transmitted from any part of the cell. By determining the arrival time the base station calculates the timing advance information that is sent to the terminal to be used in the subsequent transmissions. The 252- s guard time X due to the bits corresponds to a propagation distance of approximately 75 km which therefore establishes that a maximum cell radius is of km. The raw access message is in fact embodied by only eight bits. These eight bits are split into two fields one containing three bits and the other containing five bits. The three-bit field identifies the type of access call origination paging acknowledgment etc. . The five-bit field contains a randomly generated code used to distinguish the messages of two or more terminals transmitting in the same time slot contending for the time slot . These eight bits are CRC encoded which adds six parity bits to the eight bits. The resulting 14 bits together with 4
