CONCEPTION ET REALISATION D'UNE ANTENNE A GAIN ELEVE POUR LIAISON SATELLITE DANS LA BANDE 10,7GHz-12,75GHzLaurent Cirio(2) , Benoît Poussot (2), Jean-Marc Laheurte(2),
A. Diallo(3), C. Luxey(3), P. Le Thuc(3), R. Staraj(3)
(1) IREENA, Polytech.Nantes
*/34
Principes de base sur la diversité d’antennes et le MIMO
Le MIMO ou la fin des antennistes?
Reconfiguration d’antennes en diagramme et polarisation
Amélioration du couplage entre antennes
Plan de l’exposé
Démocratisation des réseaux locaux sans fil (WIFI, WLAN, HIPERLAN, WIMAX)
Développement de réseaux sans fil à plus grande échelle (MAN : Métropolitain Area Network)
Contexte (1)
Trajets multiples
Les systèmes MIMO augmentent le débit des communications en tirant profit de des multi-trajets sans nécessiter plus de bande passante
Contexte (2)
MIMO (Multiple Input Multiple Output) : utilisation de plusieurs antennes à l’émission et à la réception afin de créer de la diversité
*/34
Antennes de réception « intelligentes »
Réseaux d’antennes à formation de faisceaux ou à réjection d’interférences: adaptés à des milieux LOS (Line of sight) ou proches
Antenne directive peu pertinente en NLOS (indépendance statistique des signaux reçus)
Auto-ajustement contraintes en vitesse de commutation information sur l’angle d’arrivée trop coûteuse
*/34
• Diversité d’antennes: Récepteur multi-antennes (« Réseau ») + utilisation de techniques de combinaison des signaux reçus par chaque antenne défense contre les multi-trajets (années 60)
• MIMO: exploitation des multi trajets création de plusieurs canaux indépendants dans une même bande de fréquence avec des réseaux d’antennes en émission et en réception (1996: démonstrateur BLAST des Bell Labs)
Diversité d’antennes et antennes MIMO (dumb antennas)
*/34
x1
y1
h
• r: RSB moyen sur l’antenne de réception
• h: gain complexe normalisé associé au canal de propagation, incluant les caractéristiques des évanouissements et de l’antenne (diagramme, polarisation,.)
*/34
Recombinaison des signaux (EG, MRC…)
RX
Diversité simple
h=[h1, h2,…, hM]
Augmentation du RSB : meilleure robustesse de la liaison, possibilité de forts taux de modulation,…
La capacité du système augmente comme le logarithme de M
*/34
Données divisées en N sous-séquences
Envoyées en parallèle
Algorithme VBLAST…
Capacité de canal (bp/s/Hz)
Performances liées à la corrélation entre les trajets représentés par les coefficients hij de la matrice H
N liaisons indépendantes
Cas optimal (corrélation nulle entre trajets) N liaisons indépendantes
A puissance d’émission égale, la capacité augmente linéairement avec min(M,N). Pour N grand, C=Nlog2(1+r)
ú
û
ù
ê
ë
é
Capacité SISO, SIMO, MIMO
• SISO: augmentation lente: 3 dB de plus sur RSB augmente CSISO de 1bit/s/Hz
• Comparaison SIMO et MIMO avec N identique.
SIMO (1,3) et MIMO (2,2)
SIMO (1,5) et MIMO (3,3)
• Faibles RSB, CSIMO>CMIMO
• Forts RSB (30dB). CMIMO (3,3) ~ 2CSIMO (1,5)
• Si N, croisement entre les courbes CMIMO et CSIMO pour les faibles RSB
• CSIMO (1,3) et (1,5): pente identique N
• CMIMO(2,2) et (3,3) pente en fonction de N
canal de Rayleigh
r(t)=m(t).f(t)
Evanouissements rapides et lents
Amélioration de l’efficacité des antennes
Meilleure efficacité des antennes miniatures
Forte isolation entre antennes ou accès
A quoi sert l’antenniste?
Ericsson T65
Antennes reconfigurables (diversité de polarisation ou de diagramme)
Directions d’arrivée uniformément réparties en théorie, clusters de rayons en pratique
Minimum d’intelligence au niveau de l’antenne peut améliorer le RSB et la corrélation
*/34
Objectif : fournir plusieurs copies différentes (ou décorrélées) du signal transmis et les combiner judicieusement afin d’augmenter la capacité
Cette décorrélation est introduite en écartant les antennes à l’émission et à la réception (l/2 suffisant en milieu riche en multi-trajets)
Diversité d’espace
Diversité adaptative (typiquement : diversité d’espace + rayonnement reconfigurable)
La reconfiguration de diagramme augmente l’apport en diversité des antennes en tenant compte du canal de propagation
RSB & décorrélation des signaux reçus optimisés au cours du temps
Trajet 1
Trajet 2
MIMO adaptatif
*/34
MIMO adaptatif basé sur une cavité métallique cubique à 3 fentes commutables (IREENA)
Fentes court-circuitées modification du diagramme de rayonnement
3 configurations de rayonnement
5.2 GHz
Gφ
Gθ
*/34
Gφ
Gθ
simulation
mesure
*/34
Gφ
Gθ
simulation
mesure
*/34
32 configurations de rayonnement possibles
Application aux systèmes MIMO adaptatifs
d = 60mm
Antenne 1
Antenne 2
Corrélation d’enveloppe
(
)
(
)
(
)
= Coefficient de cross-polarisation (XPD)
Permet de quantifier l’apport en diversité des diagrammes de rayonnement
*/34
Corrélation d’enveloppe en fonction de l’angle d’incidence de la direction moyenne d’arrivée des signaux
Distribution gaussienne ( σ=20°)
configuration 1 & 1
configuration 1 & 2
configuration 3 & 2
MIMO adaptatif : choix de la configuration de rayonnement offrant le plus de diversité en fonction de l’évolution du canal
*/34
• Diversité en polarisation: V et H (3 SPDTs)
• Diversité en diagramme: 4 diagrammes (2 diodes par stub)
• Total de 8 canaux distincts
• Selection combining
SPDT
Pin diodes
The antenna operates in the 5.8 GHz frequency band. Active components are included in the feeding circuit, 3 SPDT using on 2 PIN diodes each for polarisation diversity and 2 PIN lead diodes for pattern diversity. This result in 4 optimized switched patterns for each polarization. Therefore, 8 branches are available for diversity.
Temps : 1 min
Total : 7 min 30 sec
are used as radiating elements. It can be excited by one of the two orthogonal crossed slots and feedlines etched on the top and bottom sides of an AR 1000 substrate (h2=0.787mm, er=10, tgd=0.0035, metallization=35mm). The DC-bias circuit and slot geometries are depicted on Fig.2. DC voltages are applied through l/4 high characteristic impedance lines (width: 150mm) connected to the RF lines. All geometrical characteristics of the antenna are given in the legends of Fig.1 and Fig.2.
The slot selection results either in an E-plane or H-plane coupling of the central patch with the adjacent parasitic patches. The selected slot also enforces one of the two linear orthogonal polarizations. The slot selection is performed with a SPDT, consisting of two beam-lead pin diodes metelics MBP-1030 (d1 and d4 on Fig.2), directly connected to the feeding microstrip line below the ground plane. By switching ON a diode while the other is OFF, the antenna can switch between horizontal or vertical polarization states with a single feeding port. The horizontal polarization is defined for a E-field oriented along the x-axis with a E-plane coupling between patches. The vertical polarization is defined for a E-field oriented along the y-axis with a H-plane coupling between patches.
Each of the slot pairs in the parasitic patches is loaded by a switchable stub (pure reactance) through a SPDT (d3-d6 and d2-d5). The stub lengths are adjusted by pin diodes (d7 and d8). The SPDT role is to select a slot parallel to the slot of the central patch to keep the same polarization in the three patches.
To keep the same diversity patterns for both polarisations, the gap between adjacent patches is adjusted so that similar E-plane and H-plane couplings are obtained. Similar diversity patterns for both polarizations are not a requirement to get good diversity performances, but it is an important feature to properly compare the different possible diversity combinations.
The values of the switchable stub lengths must provide a set of patterns with a proper antenna matching for each pattern.
All simulations include the effects of the DC-bias circuit and limited ground plane. The diodes states are modelled by electrical equivalent circuits extracted from the S-parameters provided by the manufacturer at 6 GHz (ON state: forward bias current IF=10 mA OFF state: reverse voltage VR=-10 volts).
The prototype is a three-element parasitic antenna array where aperture-coupled square patch antennas are used as radiating elements.
. The central patch antenna is printed on top of a 5880 Duroid substrate. It can be excited by one of the two orthogonal crossed slots and feedlines etched on the top and bottom sides of an AR 1000 substrate
The patches are printed on top layer of a 5880 Duroid substrate.
Cross slots and feeding network with bias circuit are printed respectively on top and bottom layer AR1000 substrate.
To have polarization diversity 3 SPDT based each on two Mettelics MBP-1030 beam lead diodes are include in the feeding network. In this way, it is possible to excite each slots independently and have an H-plane coupling effect or an E-plane coupling effect.
To control radiation pattern
b1
b2
b3
b4
b5
b6
b7
b8
Radiations pattern measurements show a good agreement for the main direction. The return loss bandwidth common to all patterns is 2%. A low cross polarisation level is observed for all configurations, lower than 15 dB. The measured gain is included in the range 5.5 to 7.5 dB.
Temps : 1 min
*/34
• Monopole vertical en émission
Banc de mesure
The diversity test bench includes a mobile transmitter composed of a vertically polarized monopole fixed
on a plastic arm connected to an RF generator. The plastic arm can move in rotation. The plastic arm fixed on a motorized trolley.
The receiver is the antenna under test. The antenna is connected to a spectrum analyzer through a low noise amplifier. The spectrum analyzer and the diodes state is controlled by the computer.
Temps : 1 min 30
*/34
• DG: Amélioration du RSB d’un capteur multi antennes par rapport à une antenne seule
• 4 branches (2 diagrammes + 2 polarisations): meilleur compromis complexité antenne / DG
• Orientation diagonale de l’antenne: DG=12.9 dB avec 4 branches
Mesures en diversité
Combinaison de
8 branches
Combinaison de
4 branches
If we combine more than 2 branches, The DG is between 12.3 and 12.8 dB for a number of branches between 4 and 8. In our case, The best trade-off between antenna complexity
and DG performance is obtained with 4 branches with polarization and pattern diversity.
We can see on the cumulative distribution function, the large power imbalance between the two polarization states.
*/34
2 PIFAs très proches (0.12λ0 ) opérant dans la bande UMTS
S21=-5 dB
The S-parameters are presented here.
We can see a good agreement between the simulated and measured curves even if there is a small frequency shift certainly due to the fact that the main plates of the PIFAs are difficult to maintain perfectly horizontal when not using supporting foam.
The return loss at each port are better than -6 dB in the whole UMTS band but the maximum mutual coupling reaches -5 dB at 1.9 GHz.
*/34
Amélioration min. : 15 dB
ligne 18x0.8mm2
The S parameters of this new structure are shown here. If the matching of this prototype and the previous one seems to be equivalent, a strong isolation enhancement of 15 dB is obtained on the whole bandwidth.
*/34
98 %
81 %
Sim.
94 %
Sans ligne
Avec ligne
For diversity and MIMO applications, the correlation between signals received by the involved antennas at the same side of a wireless link is an important figure of merit of the whole syst.
Usually, the envelope correlation is presented to evaluate the diversity capabilities of a multi-antenna system.
This parameter should be preferably computed from 3D radiation patterns but this method is actually laborious and may suffer from errors if no sufficient pattern cuts are taken into account in the computation.
Assuming that the antennas will operate in a uniform multi-path environment, an alternative consists in computing this parameter from its scattering parameters definition.
This formula is derived from the field radiation pattern of the antenna system when the port number i is excited and all the other ports are loaded with 50Ω.
It offers a simplest procedure as compared to the radiation pattern approach but it should be emphasized that this equation is strictly valid for the three following assumptions:
lossless antenna case that is high efficiency antennas.
Antenna-system positioned in a uniform multi-path environment which is not the case in real environments. However, the evaluation of some prototypes in several real environments has already shown that there is no major difference between a real and a uniform environment.
Load termination of the non-measured antennas is 50Ω. In reality, the system does not always present this loading situation but this evaluation set-up and procedure is commonly accepted.
All these limitations are clearly showing that in real systems, the envelope correlation calculated with the S parameters does not give the exact value of this parameter but is nevertheless a good approximation of the diversity behavior of the antennas. Moreover, antennas with an envelope correlation coefficient less than 0.5 provide significant diversity performances.
Firstly, we can see that the minimum of the envelope correlation of the antenna-system without any line is not located in the middle of the UMTS bandwidth. This frequency shift is attributed not only to the frequency shift encountered on the magnitude of the reflection coefficients of this structure but also due to the phase of these coefficients.
However, on the whole bandwidth, we can see that the envelope correlation is lower than 0.5 for the system without any neutralization line, and even better for the structure with the neutralization line.
The structure with the neutralization technique is also providing a broader bandwidth in terms of envelope correlation.
However it should be noticed that the first prototype has a low isolation between the elements and in this sense could not be considered as a strictly lossless case. Consequently, Eq. 2 should not have been used in the case of the blue line; it’s only giving a lower limit that will be never reached in practice.
These results are showing that our neutralized antenna-system is not only exhibiting radiators with high total efficiency via an increase of the isolation between the PIFAs but also provide a very low envelope correlation on the whole UMTS bandwidth.
This structure seems to have a very strong potential for diversity and MIMO applications.
*/34
Performances MIMO caractérisées avec 3 dipoles d’émission XYZ
Mesure de capacité en chambre réverbérante
The simulated and measured return losses of each element are shown here. They are all in a very good agreement.
A good matching is revealed on these curves.
*/34
Evaluation des performances MIMO du système à 2 antennes en chambre réverbérante
SNR=10dB
Capacité augmente de 8.3 à 9.1bits/s/Hz (de 12 à 13 pour 4 éléments)
The simulated and measured total efficiencies are presented here. They are in a very good agreement.
The maximum values are better than 89% for the PIFAs 1/2 and better than 93% for the PIFAs 3/4.
As expected, the antennas 3 and 4 are more efficient than the elements 1 and 2 because their isolation is better.
*/34
Conclusion
Diversité d’antennes et MIMO améliorent significativement la capacité des liaisons riches en multitrajet (Merci aux traiteurs de signaux!) mais augmentent la consommation et la complexité des systèmes
L’antenniste peut apporter sa contribution:
• en miniaturisant les terminaux (co-localisation)
• en limitant les couplages entre antennes
• en optimisant l’efficacité
• en reconfigurant sans pertes l’antenne (ok si peu de multitrajets)
L’antenniste doit repenser ses (réseaux d’)antennes en terme de corrélation, gain en diversité, RSB, capacité de canal et taux d’erreur binaire etc… et les évaluer (les comparer) dans le système complet avec des scenarii variables.
The simulated and measured total efficiencies are presented here. They are in a very good agreement.
The maximum values are better than 89% for the PIFAs 1/2 and better than 93% for the PIFAs 3/4.
As expected, the antennas 3 and 4 are more efficient than the elements 1 and 2 because their isolation is better.
*/34
802.11.G capacité annoncée 11 Mbits/s à 50m
802.11.N MIMO+OFDM 100 Mbits/s à 90 m (3 x3 antennes)
BP réel << BP annoncée
WIMAX 802.16: transmission utilise le beamforming, quelques kms de portée
Technologies wideband: égalisation, OFDM, DS-CDMA
The simulated and measured total efficiencies are presented here. They are in a very good agreement.
The maximum values are better than 89% for the PIFAs 1/2 and better than 93% for the PIFAs 3/4.
As expected, the antennas 3 and 4 are more efficient than the elements 1 and 2 because their isolation is better.
*/34
Accès 2 adapté
*/34
Une corrélation assez importante (r<0.7) peut être tolérée sans trop sacrifier de gain en diversité
Gain en diversité en fonction de la corrélation pour un récepteur à 2 branches
1.5dB
*/34
Amélioration du SNR moyen en fonction du nombre de canaux pour différentes méthodes de combinaison
*/34
Fonction de répartition combinée par « selection combining » pour différents nombres de canaux
*/34
ú
ú
ú
ú
û
ù
ê
ê
ê
ê
ë
é
de Nantes-Atlantique
de Nantes-Atlantique