than that o the transmitted signal. Frequency selective ading causes the transmitted signal to attenuate certain requencies more than others. Radio requency propagation parameters, in practice, are either measured in the requency domain or in the time domain. Multipath delay spread is one o the key parameters used to dene a multipath channel model as it limits the use o dierent systems deploying dier- ent rates in their operations. I the delay spread is small compared to the inverse o the signal bandwidth, then there is little time spreading in the received signal. However, when the delay spread is relatively large compared to the symbol time there is sig- nicant time spreading o the received signal. This can lead to substantial signal distortion in non-spreading systems where as it provides time diversity in spread systems (RAKE combin- ing in CDMA). The large delay spreads are present in both vehicular and pedestrian mobility situations due to the small height o the antennas, and the act that the mobile unit is typically using omni-directional antennas. The root-mean-square (RMS) delay spread is the most important single measure or the delay time extent o a multipath radio channel. RMS delay spread or channels range anywhere between several hundreds o nanoseconds to several tens o microseconds. The impulse response o mobile radio channel exhibit delay spread and doppler spreading. The rst eect re- sults in time dispersion and requency selective ading whereas the second results in requency dispersion and time selective ading. The delay in the signal that comes rom a distant Base Station (BS) translates to a delayed impulse response which increases the delay spread o the equivalent channel causing channel distortion and dispersion leading to inter-symbol in- tererence (ISI). ISI, in turn, can cause transmission errors. As the distance range or the data rate o the system increases, ISI becomes more severe due to requency selective ading, requir- ing powerul channel estimation and equalization algorithms. Orthogonal Frequency Division Multiplexing (OFDM) systems The behavior o a communication channel is not always well dened. Wireless communication relies on a propagation channel to carry inormation rom one point to another. Hence, it is mandatory to characterize wireless communication chan- nels or optimizing the perormance o a communication sys- tem. This technical brie attempts to clariy the requirement o mul- tipath measurement or a wireless communica- tion system. The radio propagation eects can be charac- terized by ree space path loss, long term ading (including shadowing), short term ading (multipath ading), doppler spread and co-channel and adjacent channel inter er- ence. In a mobile wireless communication system, a transmitted signal ollows di- erent paths beore arriving at the receiver causing fuctuations in the received signal’s amplitude, phase and angle o arrival resulting in multipath ading. Behavior o a wireless communication channel depends on location, time and requency . As a result, channel models are realized to mimic real world propagation scenarios. Since the perormance computed by the channel models are close to the physical reality, these models can be used to evaluate perormance o a system in non- real time. The quality o the communications link depends on a commu- nication medium, and there is a signicant probability that the channel will experience a deep ade due to destructive interer- ence (signals adding and subtracting rom each other). Loss- less operation over broadband channels implies ability to op- erate even with aded channels where signicant multipath is present. Any signal with a large bandwidth is susceptible to the potentially destructive eects o ading such as time dispersion and requency s elective ading. Time dispe rsion stretches s ig- nal in time so that the duration o the received signal is greater Clarifying RF BERKELEY V ARITRONICS S YSTEMS ® By Ranjani Pai WiMAX Software Engineer Berkeley Varitronics Systems, Inc.