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than that o the transmitted signal. Frequency selective ad
causes the transmitted signal to attenuate certain requencmore than others.
Radio requency propagation parameters, in pract
are either measured in the requency domain or in
time domain. Multipath delay spread is one o the
parameters used to dene a multipath channel mode
it limits the use o dierent systems deploying di
ent rates in their operations. I the delay spread is sm
compared to the inverse o the signal bandwidth, th
there is little time spreading in the received signHowever, when the delay spread is relativ
large compared to the symbol time there is s
nicant time spreading o the received sign
This can lead to substantial signal distort
in non-spreading systems where as it provid
time diversity in spread systems (RAKE comb
ing in CDMA). The large delay spreads are presen
both vehicular and pedestrian mobility situations d
to the small height o the antennas, and the act tthe mobile unit is typically using omni-directional antennas.
The root-mean-square (RMS) delay spread is the m
important single measure or the delay time extent o a multip
radio channel. RMS delay spread or channels range anywh
between several hundreds o nanoseconds to several tens
microseconds. The impulse response o mobile radio chan
exhibit delay spread and doppler spreading. The rst eect
sults in time dispersion and requency selective ading where
the second results in requency dispersion and time select
ading. The delay in the signal that comes rom a distant Ba
Station (BS) translates to a delayed impulse response wh
increases the delay spread o the equivalent channel caus
channel distortion and dispersion leading to inter-symbol
tererence (ISI). ISI, in turn, can cause transmission errors.
the distance range or the data rate o the system increases,
becomes more severe due to requency selective ading, req
ing powerul channel estimation and equalization algorithm
Orthogonal Frequency Division Multiplexing (OFDM) syste
The behavior o a communication channel is not always
well dened. Wireless communication relies on a propagationchannel 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 propagationeects 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 interer-
ence. In a mobile wireless communication
system, a transmitted signal ollows di-
erent paths beore arriving at the receivercausing 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 selective ading. Time dispersion stretches sig-
nal in time so that the duration o the received signal is greater
C l a r i f y i n g R F
BERKE V ARITROS YSTE
By Ranjani PaiWiMAX Software EngineerBerkeley Varitronics Systems, Inc.
7/30/2019 Multipath Measurement in Wireless Communication Systems
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combat requency selective ading by splitting the data stream into
many parallel streams thus increasing the symbol duration (being
inversely proportional to the data rate) o each stream such that
the delay spread is only a small raction o the symbol duration.
OFDM eliminates ISI completely by inserting a cyclic prex (CP),
whose length is chosen to be larger than the maximum excess
delay o the channel, making OFDM symbol length greater than the
delay spread o the channel, or else the dispersion that occurred
degrades system perormance considerably.
The main parameters o the channel are the user’s velocity
and the channel delay spread. These parameters aect the corre-
lation unctions, which are used or channel estimation. It is thus
indispensable to have a better understanding o these phenom-
enon. Ability to tackle adverse eects due to this phenomenon is
required to design a reliable and high perormance systems that
can adapt themselves to the changing nature o the transmission
medium.Time dispersion o the channel due to delay spread can
be estimated using the channel impulse response (CIR). Delay
spread is the time duration o the autocorrelated channel’s time-
varying impulse response. CIR can be eciently estimated in the
requency domain (as in OFDM) or in time domain (as in CDMA)
using known reerence symbols.
Multipath measurement or mobile WiMAX under vehicu-
lar speed o 60 Km/hr is shown in the gure-1. RMS de
spread can be calculated rom the delay prole o the chan
under consideration. From this gure it can be observed t
the multipath exist at CP length o TFFT/8 where TFFT is
FFT size. This helps network operators analyze and to cho
the best suited CP length or various scenario or network
timization and while improving the quality o transmission
Multipath ading eects can also be combated us
diversity and combining techniques. OFDM system explo
requency diversity using requency slicing, whereas CD
system exploits time diversity through Rake receiver. Per
mance o dierent communication systems are compa
in terms o the delay spread that can be tolerated. But de
spread is not an indicator o perormance unless the para
eter is well dened and is related to a known channel mod
Measuring the perormance o a system in various chan
conditions could help device manuactures/OEMs’ in depling an ecient system. An accurate description and emulat
o wireless channel characteristics allows thorough testing
wireless devices such as access points, base stations, mo
stations, and other wireless devices.
Multipath delay spread can be a major transmiss
problem, which must be characterized beore design o mo
lation, equalization and diversity can be nalized.
Clar i fy ing RF
BERKELE V ARITRONS YSTEM
Providing wireless solutions for over 35 years.
Reerences:
1. Tevfk Yucek and Huseyin Arslan : Delay Spread and Time Dispersion Estimation or Adaptive OFDM Systems.
2. A. F. Molisch, Wideband Wireless Digital Communications
3. Karen Halord and Mark Webster : Multipath measurement in WLANs
4. IEEE 802.16.1pp-00/01 :Multipath Measurements and Modelling or Fixed Broadband Wireless Systems in a Residential Environment .
Berkeley Varitronics Systems (www.bvsyste
com) has been providing advanced wire
solutions and products to the domestic
international wireless telecommunications
dustry or over 35 years. Since 1995, BVS
introduced over 50 unique wireless test dev
or a variety o applications including the po
lar Cellular, iDEN, PCS, CDMA, RFID, WiM
802.11b/a/n/g & Bluetooth specifcations.
Berkeley Varitronics Systems,255 Liberty Street, Metuchen NJ 08
TOLL FREE 1-888-737-4+1 732-548-3
www.bvsystems.
Figure 1 Multipath measurement using BVS’ YellowFin -WiMAX analyzer