关于认知无线电的,不要软件翻译的,自己用N个软件试过了,能看出来的。最好有一些相关专业的知识,不然一些术语可能不会准确。如果好的话还会追加分数。
COGNITIVE radios are ideally suited for use in dynamic spectrum access networks in which there may be large variations in channel conditions from one session to the next. Such variations are common in networks that operate in a fixed frequency band, but the variations are more severe if the frequency band is changed for consecutive sessions. Each radio in a dynamic spectrum access network must be aware of its communication environment, and it must provide the information that other radios need in order to communicate with it efficiently. This information should besimple, easy to derive, and easy to send to neighboring radios. We focus on the information needed to adapt the error-control coding, modulation, and transmitter power for half-duplex packet transmissions.
A new session begins when one radio, referred to as the source, has a collection of packets to send to another radio, the destination. At the start of a new session, which may be in a different frequency band than the previous session, the protocol must adjust the transmitter power to provide reliable communications with minimal energy consumption and minimal interference to other radios. As the session progresses, the protocol must adjust the transmissions to compensate for changes in channel conditions. Our results demonstrate the extent to which the adaptive transmission protocol can rely only on code-rate adaptation to offset increased propagation loss. We show that when the cognitive radios must compensate for very large variations in the channel conditions, it is necessary to adapt the modulation also. Power increases are employed by our protocol only if the channel deteriorates so much during a session that changes in coding and modulation cannot provide enough compensation, which occurs very rarely.
The throughput performance of our protocol is compared with theoretical limits that are derived from considering ideal protocols and applying Shannon capacity results. We demonstrate that our protocol performs nearly as well as an ideal protocol that is given perfect channel-state information. The modulation formats that are available to the adaptive
transmission system span a range from bandwidth-efficient modulation to power-efficient modulation, which permits the adaptation to compensate for large changes in propagation loss. In order to keep the complexity low, bit-interleaved coded modulation is employed to obtain each combination of coding and modulation that is used by the adaptive transmission protocol. In this approach, it is simple to change the code and modulation independently, because the error-control codes are not tailored to specific modulation formats.