Channel Models and Signaling Schemeselhamt/index_files/Presentation.pdf · Outline 2 1. Overview and Introduction to the IEEE 802.15.4a Standard 2. Channel Models • Generic Channel

Channel Models and Signaling Schemes

Department of Electrical & Computer EngineeringThe University of British Columbia

Elham Torabi

Supervisor: Dr. Robert Schober

Outline 2

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 3

• 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 alternativephysical layer for sensor networks and similar devices that work with the IEEE802.15.4 MAC layer.

• Technical characteristics summary

– Topology

– Bit Rate

– Range

– Coexistence and Interference Resistance

– Power Consumption

– Quality of Service

– Antenna

– Complexity

– Location Awareness


1. Overview and Introduction to the IEEE 802.15.4a Standard 4

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


2. Channel Models 5

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) andnone-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 amodel for body area networks (BANs)

Main Goals are modeling the

• Attenuation

• Delay Dispersion


2. Generic Channel Model 6

• 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.


2. Generic Channel Model: Pathloss - Preliminary Comments 7

• 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 thedistance between transmitter and receiver, fc is the center frequency.

Note that E · = Elsf Essf ·, where ‘lsf ’ and ‘ssf ’ indicate large-scalefading 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 df

,

where H(f, d

)is the transfer function from antenna connector to antenna

connector.



• 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 thereference distance, and n is the pathloss exponent.


2. Generic Channel Model: Pathloss - Recommended Model 9

• 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)

(ffc

)−2(k+1)

(dd0

)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 dependentreceive antenna efficiency.


2. Generic Channel Model: Shadowing 10

• Large-scale fading or shadowing is defined as the variation of the local meanaround the pathloss, and has log-normal distribution about the mean. Thepathloss, 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 sensestationary (WSS), therefore, for the simulation procedure according to the se-lection criteria document, shadowing shall not be taken into account.


2. Generic Channel Model: Power Delay Profile (PDP) 11

• A statistical model for indoor multipath propagation is introduced, known asSV (Saleh-Valenzuela) model.

• The physical realization: received signal rays arrive in clusters.

• The cluster arrival times are modeled as a Poisson arrival process with somefixed 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 clusterk = 0, 1, 2, ... (aka Excess Delay)



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

)Lexp

(−L)

L!



• 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 areuniformly distributed in the interval [0, 2π].

• The Power Delay Profile (PDP) of the channel is defined by taking the spatialaverage 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 powerof the different paths, is found to be

E|ak,l|2

= Ωl

1

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



where Ωl is the integrated energy of the lth cluster,and γl is the intra-clusterdecay 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.


2. Generic Channel Model: Small-scale Fading 15

• 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 thusgiven 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 thesecan have a delay dependence

µm (τ ) = m0 − kmτ

σm (τ ) = m0 − kmτ

m0 and km are Nakagami-m factor mean and m0 and km are Nakagami-mfactor variance.


2. Generic Channel Model: Auxilary Parameters 16

• 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


2. UWB Model Parametrization and Simulation Results for 2-10 GHz 17

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



Impulse responses for 100 realizations/channels.



Mean excess delay.



RMS delay spread.



Average power delay profile (PDP).



Number of significant paths within 10 dB of peak.



Number of significant paths capturing > 85% energy.


3. Signaling Schemes: Time-Hopping UWB 24

• Time-Hopping UWB (TH-UWB) is a spread-spectrum technique used in im-pulse radio (IR) signaling and can be employed to support multiple-accessscenarios.

• Pulse position modulation (PPM) and pulse amplitude modulation (PAM) arecommonly used modulation schemes.

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



• 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.



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].



• When K transmitters are active in the multiple-access system, the compositereceived 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 fromtransmitter k and the receiver clock. n (t) is White Gaussian noise.


3. Signaling Schemes: RAKE Receivers 28

System model.

• The zero-mean i.i.d. data symbols dn are passed through a unit energypulse shaping filter wtr (t).

• After pulse shaping, the signal undergoes the effects of a channel with L pathswhose 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 ofthe channel.



• 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 isa filter matched to the received waveform, this is implemented in a RAKEreceiver structure with M arms , which can be represented as a filter withresponse

f (t) =M−1∑m=0

cmwtr (−t − φm) ,

where cm is RAKE tap coefficient, and φm is RAKE delay.



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


3. Signaling Schemes: UWB Transmitted-Reference (UWB-TR) 31

• The transmitted signal of a UWB transmitted-reference (UWB-TR) systemwith antipodal modulation is given by

str (t) =

∞∑i=−∞

[wtr (t − iTf) + bi/Nswtr (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 bitenergy in the receiver.

• The data bits are bi/Ns ∈ +1,−1 with equal probability.



• 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) + αlbi/Nswrx (t − iTf − Td − τl)

]+n (t) ,

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

• There are a few types of receivers for TR signals; Generalized likelihood ra-tio test (GLRT) receiver, simple transmitted reference (STR) receiver, andaveraged transmitted reference (ATR) receiver.



GLRT receiver BEP:

Pbit = Q

[

2

Ns

(N0

Ef

)+

L

N 2s

(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 2s

(N0

Ef

)2]−1

2

,

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


3. Signaling Schemes: UWB Differential Transmitted-Reference (UWB-DTR) 34

• In UWB Differential Transmitted-Reference (UWB-DTR) system no referencesare transmitted, but instead, the data signal in the previous frame is used asreference.

• The modulation and demodulation techniques of this DTR system are similarto 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 asdefined for the TR system.



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



• The received signal of this differential system is

r (t) =∞∑

i=−∞

L∑l=1

αlmi−1bi/Nswrx (t − iTf − τl) + n (t)

DTR receiver BEP:

Pbit = Q

[

2Ns − 1

N 2s

(N0

Ep

)+

WTmds

2Ns

(N0

Ep

)2]−1

2

,

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



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


3. Signaling Schemes: Comparison 38

• 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 theDTR modulated UWB signals (also known as differential detector), is muchless sensitive to jitter on the receiver PRF clock than the RAKE receiver.



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



• Another challenge for the RAKE receiver is to generate an impulse that closelymatches 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 asthe channel is slow relative to the PRF, which will typically be the case.



• The required delay needed by the RAKE receiver for each arm is unknown atthe beginning of a communications session and it must be found using sometype of search procedure.

• The DPSK receiver does not have this search requirement, since it continu-ously correlates the received signal with a delayed replica. As a result, theDPSK receiver architecture could be used to rapidly acquire the transmittedpackets without a long training sequence or search algorithm.



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


43


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Channel Models and Signaling Schemeselhamt/index_files/Presentation.pdf · Outline 2 1. Overview and Introduction to the IEEE 802.15.4a Standard 2. Channel Models • Generic Channel