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Next generation wireless systems and networks phần 3

Các nghiên cứu của các kỹ thuật để giảm thiểu MI đã trở thành một đề tài nghiên cứu rất phổ biến trong 20 năm qua, được thúc đẩy bởi một nhu cầu lớn về điện thoại di động tế bào và các thông tin liên lạc không dây. | 88 FUNDAMENTALS OF WIRELESS COMMUNICATIONS transmitter say C located at the other end of the network. As the transmitter C is outside A s detection range A will not know the existence of C as well as the busy status of the receiver B. In this case terminal C is called the hidden terminal for A. Obviously the communication between A and B fails because B is already in a busy receiving state. The busy tone can be used in terminal B to overcome the problem. If all transmitters delay by a random delay before transmitting the traffic spreads out and the capacity of the channel improves. Kleinrock and Tobagi call this channel a nonslotted nonpersistent channel and calculate the capacity of the channel as Gt e aGT Gt 1 2a e aGT 2.62 where aT is again the one-way propagation delay of the channel. For the slotted nonpersistent channel they assert that the capacity can be calculated as S aGĩ e aGT a 1 e aGT 2.63 For both channels when the propagation delay is zero that is limit a 0 then the capacity of the channel is Gt 1 Gt 2.64 The nonpersistent channel can therefore approach a capacity of one as the offered load increases. This is the ideal approach. The optimum values of the initial delay and the retransmission delay are functions of the offered load. Therefore at high offered load the central control of the system must send information to all transmitters to notify the channel status. We have already seen this control capability on the control channels in cellular and PCS systems. Spreading code protocols The random multiple access techniques can also work jointly with conventional FDMA TDMA and CDMA to form different hybrid versions of multiple access techniques. A popular combination is the joint application of pure ALOHA or slotted ALOHA with CDMA in which every user will be assigned one or two signature codes for sending their packets 749 . With the joint application of ALOHA and CDMA a packet radio network can support much more users simultaneously and the .