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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