Pilot-Aided and Blind Equalization
in FBMC Modulation for PMR Networks
Séminaire SCEE CentraleSupélec,
Campus de Rennes
12 Février 2015
Vincent Savaux
Outline
• PMR Networks
• FBMC Modulation Scheme
• OFDM/OQAM• OFDM/OQAM
• Pilot-Aided Estimation/Equalization
• Blind Equalization
Foreword …
What is the PROFIL project ?
PROFIL: Evolution de la PROfessional Mobile Radio large bande basée sur lamodulation FILter Bank MultiCarrier
Evolution of the Wideband Pro fessional Mobile Radio Based on the Fil ter Bank MultiCarrier Modulation
• ANR Project with Cassidian, CEA-Leti, TeamCast and CentraleSupélec
• CentraleSupélec’s part: Blind equalization in FBMC for PMR Networks
PMR NetworksPMR : Professional Mobile Radio
• Emergency Networks for police, firefighters, ambulances …
• Transmission of 2G-like data
TV GSM WiFi
Currently:
frequency380 - 395 MHz
radio
Standard TETRA -TETRAPOL
TV GSM WiFi
PMR Networks
• Specificities
UL
DL
P to P
� mobility� Narrowband channels
PMR Networks
Goal: transmission of broadband signal in the 380 – 395 MHz band
OFDM
Low ratePMR
Broadband PMR
frequency
Applications
FBMC Modulation Scheme
x0,n
SFB
x1,n
xm,n
Transmissionchannel
H
w
… +
+
IFF PPN
……
……
P/S
AFB
FFT
PPN
……
……
S/P
…
y0,n
y1,n
ym,n
- OFDM/OQAM Modem
xm,n
xM-1,n
H
…
+T … … T… …
…
ym,n
yM-1,n
FBMC Modulation Scheme
Bellanger’s filter frequency response [1]:
The subcarriers are orthogonal only in the real field
FBMC Modulation Scheme
m
m-1
m+1
n-1 n n+1 time
m+1
Surrounding interference part
xm,n
frequency
The channel induces complex interferences from neighboring subcarriers and symbols
FBMC Modulation Scheme
Comparison with OFDM:
Advantages Drawbacks
• Modem more complex
• Intrinsic interferences
• No cyclic prefix higher spectral efficiency
• Low out-of-band interferencesbetter coexistence
• Specific processes at receiverbetter coexistence properties
Pilot-Aided Equalization
Reminder : Channel equalization in OFDM/OQAM
Transmitted signal x Received signal y
Equalizer F
Equalized signal a
Pilot-Aided EqualizationFirst solution: pilots are multiplexed in the data stream (1/2)
t
pilot, help pilot
data
Loss of spectralefficiency
Transmitted data knownat the receiver
f
efficiency
BUT
Simple Equalizationprocess
Pilot-Aided Equalization
Pilot positions
First solution: pilots are multiplexed in the data stream (2/2)
2 steps : 1) The channel H is estimated on pilot tones2) The signal is equalized with a one-tap per-carrier channel inversion
F = 1/Ĥ
Unknownchannel
Pilot-Aided Equalization
Different pilot allocation methods in OFDM/OQAM [2], [3], [4] (1/4)
p 0m
n
Simple implementation
POP
Pair of pilots (POP) [3]
Simple implementation
Good spectral efficiency (same as OFDM)
Weak performance [c1]
noisevariance
interferencevariance
channelvariance
Pilot-Aided Equalization
p a
Different pilot allocation methods in OFDM/OQAM [2], [3], [4] (2/4)
m
nAP
Auxiliary pilot (AP) [4]
Good spectral efficiency
Cancellation of the interference(elegantly)
High complexity at transmitter
Pilot-Aided Equalization
p ba
p-
Different pilot allocation methods in OFDM/OQAM [2], [3], [4] (3/4)
m
nIAMIAM-R [2]: p- = ±1, p = ±1, p+ = -p- and a = 0,b = 0.
p+
Interference approximation
method (IAM) [2]
Pilot-Aided Equalization
p ba
p-
Different pilot allocation methods in OFDM/OQAM [2], [3], [4] (4/4)
m
nIAM
Reduction of the noise and interference energyp+
Interference approximation
method (IAM) [2]
Reduction of the noise and interference energy
Simple implementation
Loss of spectral efficiency
Good performance [c1]
Pilot-Aided Equalization
LMMSE in OFDM/OQAM [c2], [c3]
? ? ?
?
OFDM-like
Pilot-Aided Equalization
Some results: Typical urban (TU) channel, 3 km.h-1, no channel coding
Pilot-Aided EqualizationApplication to PMR band: some changes in the LTE PHY parameters [c4] (1/3)
From LTE parameters …
… to proposed LTE-like parameters
More flexibility
Pilot-Aided Equalization
time
freq
uenc
y
Nt
Nf
…
…
1 RBin LTE
…
…
1 RB in proposed
scheme
Application to PMR band: some changes in the LTE PHY parameters [c4] (2/3)
… …
2τ0
…
…
… … …
…
…
TOFDM
15 KHz 5 KHz
addedsubcarrier
a. LTE pilot distribution b. Proposed rectangular distribution
Pilot-Aided EqualizationApplication to PMR band: some changes in the LTE PHY parameters [c4] (3/3)
1 RB
Channel estimationalong the whole band
pilot
time
freq
uenc
y
frequency
PMR signals
BB BB
PMR signal
added carriers
frequency
More flexibility
Good channel estimationover each sub-band
Pilot-Aided Equalization
Some results (1/2): achieved bit rate
- 4-QAM symbols, 3 MHz bandwidth- reference in OFDM in LTE: 4.8 Mbits.s-1
- Proposed scheme:
Similar to LTE
Enable high bit rate applications, as video streaming
Pilot-Aided Equalization
Some results (1/2): achieved BER
- 4-QAM, no channel coding
Blind Equalization
Second solution: blind equalization
Iterative processus, without pilot
Transmitted signal
Receivedsignal Recovered
signal
H F ��
x y
w
z a+AFB
Channel Blind equalizer
- Goal: achieve H.F =1
- Constraint : limited knowledge of the signal feature
H F ��+
AFB
Blind Equalization
Second solution: blind equalization
H.F H.F H.F
Itération 0 Initial state Transient Steady state
Gain in terms of bit rate
Requires a convergence delay
state
Blind Equalization
Basics (1/2), [5], [6], [7]: AFB
FFT
PPN
……
……
y0,n
y1,n
ym,n
yM-1,n
INP
UT
Equal.am,nFm,n
- Goal: Solve the optimizationproblem
yM-1,n
- How? Use the stochastic gradient
with
- µ is the step-size parameter- J a given cost function
2 degrees of freedom
Blind Equalization
Basics (2/2), [5], [6], [7]:
Cost function: Constant modulus algorithm [5], [6]
with p: another degree of freedom
Transient state = 1000 OFDM/OQAMsymbols !
Blind Equalization
Some results: blind equalization in mobile environment (1/2)
MSE = (1/M).Σ(|x|-|a|)²
SNR = 20 dB
Fast increase
Blind Equalization
Some results: blind equalization in mobile environment (2/2)
Blind equalization does not track the channel variations
Blind Equalization
Any solutions ? … Yes !
Several ways are investigated:
Can be reconsidered [c5]
Can be adapted to OFDM/OQAM [c6], [c7]
New blind receiver designAFB
FFT
PPN
……
……
y0,n
y1,n
ym,n
yM-1,n
INP
UT
Equal.am,nFm,n
Bibliography[1] M. Bellanger , “Specification and Design of a Prototype Filter for Filter Bank Based Multicarrier Transmission,” in proc. Of ICASSP, vol. 4, Salt Lake City, UT, May 2001, pp. 2417 – 2420.[2] E. Kofidis, D. Katselis, A.Rontogiannis, and S. Theodoridis , “Preamble-based channel estimation in OFDM/OQAM systems: A review,” Signal Processing, Elsevier, vol. 93, pp. 2038 – 2054, January 2013. [3] C. Lélé, J.-P. Javaudin, R. Legouable, A. Skrzyp czak, and P. Siohan , “Channel estimation methods for preamble-based OFDM/OQAM modulations,” European Transactions on Telecommunications, vol. 19, no. 7, pp. 741 –750, November 2008. Transactions on Telecommunications, vol. 19, no. 7, pp. 741 –750, November 2008. [4] J.-P. Javaudin, D. Lacroix, and A. Rouxel , “Pilot-Aided Channel Estimation for OFDM/OQAM,” in VTC’03 - Spring, vol. 3, Jeju, Korea, April 2003, pp. 1581 – 1585. [5] D. N. Godard , “Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems,” IEEE Transactions on Communications, vol. com-28, no. 11, pp. 1867 – 1875, November 1980.[6] B. Farhang-Boroujeny , “Multicarrier Modulation With Blind Detection Capability Using Cosine Modulated Filter Banks,” IEEE Transactions on Communications, vol. 51, no. 12, pp. 2057 – 2070, December 2003.[7] A. Goupil and J. Palicot , “New Algorithms for Blind Equalization: The Constant Norm Algorithm Family,” IEEE Transactions on Signal Processing, vol. 55, no. 4, pp. 1436 – 1444, April 2007.
BibliographyContributions:
[c1] V. Savaux, F. Bader , “Mean Square Error Analysis and LMMSE Application for Preamble-Based Channel Estimation in OFDM/OQAM Systems,”[c2] V. Savaux, F. Bader, Y. Louët “A Joint MMSE Channel and Noise VarianceEstimation for OFDM/OQAM Modulation,” [c3] L. Caro, V. Savaux, D. Boiteau, M. Djoko-Kouam , Y. Louët “Preamble-Based LMMSE Channel Estimation in OFDM/OQAM Modulation,” in proc of VTC’15 Spring, Glasgow, May 2015Glasgow, May 2015[c4] V. Savaux, F. Bader “Pilot Adaptation for Broadband LTE-Like FBMC System in PMR Band,” in proc of VTC’15 Spring, Glasgow, May 2015[c5] V. Savaux, F. Bader, J. Palicot “Frequency Blind Equalization Using Constant Norm Algorithm for OFDM Systems,”[c6] V. Savaux, F. Bader “Sub-Optimal Initialization for Blind Equalization with Fast Convergence in OFDM/OQAM Modulation,”[c7] V. Savaux, F. Bader “Enhancing the Constant Modulus Algorithm for Blind Equalization in OFDM/OQAM Modulation,”
Thanks for your attention !
Merci de votre attention !
Des questions ?