Channel Models and Signaling Schemes Department of Electrical & - - PowerPoint PPT Presentation

• Channel Models and Signaling Schemes

Department of Electrical & Computer Engineering The University of British Columbia Elham Torabi Supervisor: Dr. Robert Schober

Outline

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• 1. Overview and Introduction to the IEEE 802.15.4a Standard
• 2. Channel Models
• Generic Channel Model
• UWB Model Parametrization and Simulation Results for 2-10 GHz
• 3. Signaling Schemes
• Time-Hopping UWB (TH-UWB)
• RAKE Receivers
• UWB Transmitted-Reference (UWB-TR)
• UWB Differential Transmitted-Reference (UWB-DTR)
• Comparison

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 1. Overview and Introduction to the IEEE 802.15.4a Standard

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• The IEEE 802.15 low-rate alternative PHY task group 4a (TG4a) for WPANs,

named subgroup IEEE 802.15.4a, has the mandate to develop an alternative physical layer for sensor networks and similar devices that work with the IEEE 802.15.4 MAC layer.

• Technical characteristics summary

– Topology – Bit Rate – Range – Coexistence and Interference Resistance – Power Consumption – Quality of Service – Antenna – Complexity – Location Awareness

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 1. Overview and Introduction to the IEEE 802.15.4a Standard

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• The principle interest of this subgroup is in providing communications for

WPAN applications such as

• 1. Sensors networks
• 2. High precision positioning
• 3. Security/authentication
• 4. Smart home systems
• 5. Networks of wearable mobile devices
• 6. Real time location services

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Channel Models

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Different Proposed Channel Models in the IEEE 802.15.4a standard

• UWB channel models covering the frequency range from 2 to 10 GHz, con-

sidering indoor residential, indoor office, industrial, outdoor, and open out- door environments (usually with a distinction between line-of-sight (LOS) and none-LOS (NLOS) properties)

• UWB channel model for the frequency range from 100 to 1000 MHz, consid-

ering a model for indoor office-type environments

• UWB channel model for the frequency range from 2 to 6 GHz, considering a

model for body area networks (BANs) Main Goals are modeling the

• Attenuation
• Delay Dispersion

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model

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• Used for the 2-10 GHz frequency range.
• model treats only channel, while antenna effects should be modeled separately.
• block fading is assumed, i.e., channel stays constant over data burst duration.
• modified Saleh-Valenzuela (SV) model is adapted.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Pathloss - Preliminary Comments

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• The pathloss for a narrowband system is conventionally defined as

PL(d) = E {PRX(d, fc)} PTX , where PTX and PRX are transmit and receive power, respectively, d is the distance between transmitter and receiver, fc is the center frequency. Note that E {·} = Elsf {Essf {·}}, where ‘lsf ’ and ‘ssf ’ indicate large-scale fading and small-scale fading, respectively.

• The pathloss related to wideband pathloss is defined as

PL (f, d) = E f+∆f/2

f−∆f/2

• H
• ˜

f, d

• 2

d˜ f

• ,

where H

• ˜

f, d

• is the transfer function from antenna connector to antenna

connector.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Pathloss - Preliminary Comments

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• To simplify computations, we assume

PL (f, d) = PL (f) · PL (d)

• The frequency dependence of the pathloss is given as
• PL (f) ∝ f−k,

where k is the frequency dependence coefficient of the pathloss.

• The distance dependence of the pathloss in dB is described by

PL (d) = PL0 + 10n log10 d d0

• ,

where the reference distance d0 is set to 1 m, PL0 is the pathloss at the reference distance, and n is the pathloss exponent.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Pathloss - Recommended Model

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• According to the proposed model the pathloss is found to be

PL (f, d) = Pr (d, f) PTX−amp (f) = 1 2PL0 · ηTX−ant (f) · ηRX−ant (f)

• f

fc

−2(k+1)

• d

d0

n , where PTX−amp (f) is the output spectrum of the transmit amplifier, Pr (d, f) is the received frequency-dependent power, ηTX−ant (f) is the frequency depen- dent transmit antenna efficiency, and ηRX−ant (f) is the frequency dependent receive antenna efficiency.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Shadowing

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• Large-scale fading or shadowing is defined as the variation of the local mean

around the pathloss, and has log-normal distribution about the mean. The pathloss, averaged over the small-scale fading in dB, can be written as PL (d) = PL0 + 10n log10 d d0

• + S,

where S is a Gaussian-distributed random variable with zero mean and stan- dard deviation σS.

• If shadowing effects come into play, the overall channel is no longer wide sense

stationary (WSS), therefore, for the simulation procedure according to the se- lection criteria document, shadowing shall not be taken into account.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Power Delay Profile (PDP)

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• A statistical model for indoor multipath propagation is introduced, known as

SV (Saleh-Valenzuela) model.

• The physical realization: received signal rays arrive in clusters.
• The cluster arrival times are modeled as a Poisson arrival process with some

fixed rate Λl.

• Subsequent rays arrive according to a Poisson process within each cluster,

with another fixed rate.

• Tl : arrival time of the lth cluster l = 0, 1, 2, ...
• τk,l : arrival time of the kth ray measured from the beginning of the lth cluster

k = 0, 1, 2, ... (aka Excess Delay)

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Power Delay Profile (PDP)

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• According to this model, the distribution of the cluster arrival times are given

by a Poisson process p(Tl | Tl-1) = Λl exp [−Λl (Tl − Tl-1)] ,

• Ray arrival times are modeled with mixtures of two Poisson processes

p(τk,l | τ(k-1),l) = βλ1 exp

• −λ1
• τk,l − τ(k-1),l
• +(β − 1) λ2 exp
• −λ2
• τk,l − τ(k-1),l
• where β is the mixture probability, λ1 and λ2 are the ray arrival rates.
• The number of clusters L is assumed to be Poisson-distributed

PL (L) = ¯ L L exp

• −¯

L

• L!

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Pathloss - Preliminary Comments

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• The complex, low-pass impulse response of the channel

h (t) =

L

• l=0

K

• k=0

ak,l exp (jφk,l) δ (t − Tl − τk,l) , where ak,l is the gain of the kth ray of the lth cluster and the phases φk,l are uniformly distributed in the interval [0, 2π].

• The Power Delay Profile (PDP) of the channel is defined by taking the spatial

average of |h (t)|2 over a local area, in general P (t) ≈ K |h (t)|2.

• For the SV model, and for the LOS case, the PDP, which is the mean power
• f the different paths, is found to be

E

• |ak,l|2

= Ωl 1 γl [(1 − β) λ1 + βλ2 + 1] exp (−τk,l/γl) ,

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Power Delay Profile (PDP)

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where Ωl is the integrated energy of the lth cluster,and γl is the intra-cluster decay time constant. γl ∝ kγTl + γ0, where kγ describes the increase of the decay constant with delay. kγ and γ0 are intra-cluster decay time constant parameters. 10 log (Ωl) = 10 log (exp (−Tl/Γ)) + Mcluster, where Mcluster is a normally distributed variable with standard deviation σcluster around it and Γ is the inter-cluster decay constant.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Small-scale Fading

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• The distribution of the small-scale amplitudes ak,l, is Nakagami

PX (x) = 2 Γ (m) m Ω m x2m−1 exp

• −m

Ωx2 m ≥ 1/2 is the Nakagami m-factor, Γ (m) is the gamma function, and the parameter Ω corresponds to the mean power, and its delay dependence is thus given by the power delay profile.

• The m-parameter is modeled as a lognormally distributed random variable,

whose logarithm has a mean µm and standard deviation σm. Both of these can have a delay dependence µm (τ) = m0 − kmτ σm (τ) = ˆ m0 − ˆ kmτ m0 and km are Nakagami-m factor mean and ˆ m0 and ˆ km are Nakagami-m factor variance.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. Generic Channel Model: Auxilary Parameters

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• Mean Excess Delay: First moment of the PDP

¯ τ = ∞

−∞ P (τ) τdτ

∞

−∞ P (τ) dτ

• RMS Delay Spread: Square root of the second central moment of the PDP

στ =

• ¯

τ 2 − (¯ τ)2 ¯ τ 2 = ∞

−∞ P (τ) τ 2dτ

∞

−∞ P (τ) dτ

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Residential Environments: The model was extracted based on measurements that cover a range from 7-20 m, up to 10 GHz.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Impulse responses for 100 realizations/channels.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Mean excess delay.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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RMS delay spread.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Average power delay profile (PDP).

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Number of significant paths within 10 dB of peak.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 2. UWB Model Parametrization and Simulation Results for 2-10 GHz

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Number of significant paths capturing > 85% energy.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Time-Hopping UWB

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• Time-Hopping UWB (TH-UWB) is a spread-spectrum technique used in im-

pulse radio (IR) signaling and can be employed to support multiple-access scenarios.

• Pulse position modulation (PPM) and pulse amplitude modulation (PAM) are

commonly used modulation schemes.

• A typical time-hopping format of the kth impulse radio transmitter output

signal is s(k)

tr

• t(k)

=

∞

• j=−∞

wtr

• t(k) − jTf − c(k)

j Tc − d(k) j

• , k = 0, 1, 2, . . . , K −1,

where K is the number of transmitters, t(k) is the transmitter clock time, and wtr (t) represents the transmitted monocycle waveform.

• Tf is the frame time or pulse repetition time.
• Each link uses a distinct pulse-shift pattern
• c(k)

j

• called a TH sequence,

which are pseudorandom with period Np. Tc is the chip time.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Time-Hopping UWB

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• The sequence
• d(k)

j

∞

j=−∞ is the data sequence, which is a sample sequence

from a wide-sense stationary random process d(k) (t) A typical idealized monocycle.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Time-Hopping UWB

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Signal generated by the PPM-TH-UWB, in the case of {dj} = [1 0], Tf = 3e − 9, Ns = 5, Tc = 1e − 9, and TH Sequence= [1 0 1 0 2].

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Time-Hopping UWB

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• When K transmitters are active in the multiple-access system, the composite

received signal r (t) at the receiver antenna is modeled as r (t) =

K

• k=1

Aks(k)

rec (t − τk) + n (t) ,

where Ak models the attenuation over the propagation path of the signal s(k)

rec (t − τk) received from the kth transmitter, the random variable τk rep-

resents the time asynchronism between the clock of the signal received from transmitter k and the receiver clock. n (t) is White Gaussian noise.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: RAKE Receivers

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System model.

• The zero-mean i.i.d. data symbols {dn} are passed through a unit energy

pulse shaping filter wtr (t).

• After pulse shaping, the signal undergoes the effects of a channel with L paths

whose response given by h (t) =

L−1

• l=0

αlδ (t − τl) , where αl and τl are the attenuation and delay introduced by the lth path of the channel.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: RAKE Receivers

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• The received signal can then be expressed as

r (t) =

∞

• j=−∞

di

L−1

• l=0

αlwtr (t − τl − iTf) + w (t) ,

• In the case of no ISI and when the noise is AWGN, the optimal receiver is

a filter matched to the received waveform, this is implemented in a RAKE receiver structure with M arms , which can be represented as a filter with response f (t) =

M−1

• m=0

cmwtr (−t − φm) , where cm is RAKE tap coefficient, and φm is RAKE delay.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: RAKE Receivers

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• The sampled output of the RAKE receiver is then

yn = [r (t) ∗ f (t)] |t=nTc =

∞

• i=−∞

di

L−1

• l=0

M−1

• m=0

αlcmRwtr (nTc − iTc + φm − τl)+ ˜ w (n where Rwtr (t) ∞

−∞ wtr (τ) wtr (τ + t) dτ is the time-autocorrelation of the

pulse shape, and ˜ w (t) is filtered noise.

• The optimal combiner for the AWGN multipath channel is maximum ratio

combining (MRC), where M = L fingers, cm = αm, and φm = τm, and when the received signals on each finger are orthogonal (as is the case when there is no ISI), MRC attains the matched filter bound.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Transmitted-Reference (UWB-TR)

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• The transmitted signal of a UWB transmitted-reference (UWB-TR) system

with antipodal modulation is given by str (t) =

∞

• i=−∞
• wtr (t − iTf) + b⌊i/Ns⌋wtr (t − iTf − Td)
• ,

where wtr (t) is a transmitted monocycle waveform, and Tf is its pulse rep- etition or frame time.

• Each bit is transmitted in Ns successive frames to achieve an adequate bit

energy in the receiver.

• The data bits are b⌊i/Ns⌋ ∈ {+1, −1} with equal probability.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Transmitted-Reference (UWB-TR)

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• The received TR signal in a stationary channel over a bit time is modeled by

r (t) =

Ns−1

• i=0

L

• l=1
• αlwrx (t − iTf − τl) + αlb⌊i/Ns⌋wrx (t − iTf − Td − τl)
• +n (t) ,

where L is the number of specular propagation paths, lth path’s propagation delay and amplitude are being denoted by τl and αl, and n (t) is white Gaussian noise.

• There are a few types of receivers for TR signals; Generalized likelihood ra-

tio test (GLRT) receiver, simple transmitted reference (STR) receiver, and averaged transmitted reference (ATR) receiver.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Transmitted-Reference (UWB-TR)

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GLRT receiver BEP: Pbit = Q  

• 2

Ns N0 Ef

• + L

N 2

s

N0 Ef 2−1

2

 , STR receiver BEP: Pbit = Q  

• 2

Ns N0 Ef

• + 2WTmds

Ns N0 Ef 2−1

2

 , ATR receiver BEP: Pbit = Q  

• 2

Ns N0 Ef

• + 2WTmds

N 2

s

N0 Ef 2−1

2

 , where W is the one-sided noise bandwidth of the receiver, Ef is the received energy per frame at two pulses per frame, and Tmds = τl + Tw.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Differential Transmitted-Reference (UWB-DTR)

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• In UWB Differential Transmitted-Reference (UWB-DTR) system no references

are transmitted, but instead, the data signal in the previous frame is used as reference.

• The modulation and demodulation techniques of this DTR system are similar

to those used in differential phase shift keying (DPSK).

• The differentially modulated UWB transmitted signal is

str (t) =

∞

• i=−∞

miwtr (t − iTf) , where mi = mi−1b ⌊i/Ns⌋, and all the other parameters are the same as defined for the TR system.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Differential Transmitted-Reference (UWB-DTR)

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Block diagram of the modulator and demodulator in DTR UWB system.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Differential Transmitted-Reference (UWB-DTR)

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• The received signal of this differential system is

r (t) =

∞

• i=−∞

L

• l=1

αlmi−1b⌊i/Ns⌋wrx (t − iTf − τl) + n (t) DTR receiver BEP: Pbit = Q  

• 2Ns − 1

N 2

s

N0 Ep

• + WTmds

2Ns N0 Ep 2−1

2

 , where W is the one-sided noise bandwidth of the receiver, Ep is the received energy per pulse at one pulse per frame, and Tmds = τl + Tw.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: UWB Differential Transmitted-Reference (UWB-DTR)

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BEP of GLRT, ATR, DTR and STR receiver structures in a dense resolvable multipath environment.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Comparison

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• One of the challenges of a UWB system implementation is providing very sta-

ble reference clocks for the transmitter and receiver pulse repetition frequency (PRF) generators.

• The differential phase shift keying (DPSK) receiver, which is used to detect the

DTR modulated UWB signals (also known as differential detector), is much less sensitive to jitter on the receiver PRF clock than the RAKE receiver.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Comparison

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Sensitivity of RAKE and DPSK receivers to PRF clock time offset.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Comparison

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• Another challenge for the RAKE receiver is to generate an impulse that closely

matches the received impulse at the input to the receiver.

• Since the DPSK receiver is correlating with the delayed replica of itself (al-

though noisy), the distortions will automatically be accounted for as long as the channel is slow relative to the PRF, which will typically be the case.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Comparison

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• The required delay needed by the RAKE receiver for each arm is unknown at

the beginning of a communications session and it must be found using some type of search procedure.

• The DPSK receiver does not have this search requirement, since it continu-
• usly correlates the received signal with a delayed replica. As a result, the

DPSK receiver architecture could be used to rapidly acquire the transmitted packets without a long training sequence or search algorithm.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

• 3. Signaling Schemes: Comparison

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For a low-power low data-rate UWB system, the DPSK receiver architecture could be a simple and low-cost alternative to the traditional RAKE receiver.

Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication

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• Elham Torabi: Low-Power Low-Rate Ultra-Wideband Communication