Đối với một kênh đa cụ thể, chặt chẽ băng thông của nó là luôn luôn cố định. Nếu băng thông tín hiệu B là loại bia mạch lạc hơn so với băng thông của kênh đa Bco, một fading chọn lọc tần số xảy ra, nếu không chỉ mờ dần không fading lựa chọn tần số hoặc căn hộ sẽ xảy ra. | 36 FUNDAMENTALS OF WIRELESS COMMUNICATIONS or T. Therefore we obtain Td -1 Bco which states that a multipath channel always specifies a particular coherent bandwidth beyond which the signal may suffer frequency-selective fading. The frequency-selectivity of the channel is relative to the signal bandwidth. For a particular multipath channel its coherent bandwidth is always fixed. If the signal bandwidth Bs is lager than the coherent bandwidth of the multipath channel Bco a frequency-selective fading takes place otherwise only a non-frequency-selective fading or flat fading will occur. The study of the techniques to mitigate MI has become a very popular research topic in the last 20 years driven by a great demand for mobile cellular and wireless communications. Many papers 44-48 have been published as a result of great effort made by both academia and industry. Due to limited space we will not dwell more on the theory of radio communication channels in this book. For more reading on this subject the readers may refer to the following publications in the open literature 69-118 . Spread Spectrum Techniques Spread spectrum SS techniques originated from the development of modern radar systems 119131 at the end of the second world war in the 1950s. The earlier radar systems employed continuous waves CW that were sent as a series of short bursts into the air. The delayed and attenuated echoes received from those short CW bursts were used to measure the distances and directions of the objects in the air as well as in the sea. Constrained by the maximal available peak power in the CW radio transmission the earlier radar systems could not detect the incoming objects further than 100 kilometers depending on the conditions in their operational environments. In order to improve the maximal detection range pulse-compression techniques were introduced to the radar systems so that the detection range could be greatly extended beyond that range without increasing the peak .
