Probabilistic Coexistence and Throughput of Cognitive Dual-Polarized Networks
© J.-M. Dricot et al. 2010
Received: 30 October 2009
Accepted: 25 April 2010
Published: 8 June 2010
Diversity techniques for cognitive radio networks are important since they enable the primary and secondary terminals to efficiently share the spectral resources in the same location simultaneously. In this paper, we investigate a simple, yet powerful, diversity scheme by exploiting the polarimetric dimension. More precisely, we evaluate a scenario where the cognitive terminals use cross-polarized communications with respect to the primary users. Our approach is network-centric, that is, the performance of the proposed dual-polarized system is investigated in terms of link throughput in the primary and the secondary networks. In order to carry out this analysis, we impose a probabilistic coexistence constraint derived from an information-theoretic approach, that is, we enforce a guaranteed capacity for a primary terminal for a high fraction of time. Improvements brought about by the use of our scheme are demonstrated analytically and through simulations. In particular, the main simulation parameters are extracted from a measurement campaign dedicated to the characterization of indoor-to-indoor and outdoor-to-indoor polarization behaviors. Our results suggest that the polarimetric dimension represents a remarkable opportunity, yet easily implementable, in the context of cognitive radio networks.
Cognitive radio networks and, more generally, dynamic spectrum access networks are becoming a reality. These systems consist of primary nodes, which have guaranteed priority access to spectrum resources, and secondary (or cognitive) nodes, which can reuse the medium in an opportunistic manner [1–4]. Cognitive nodes are allowed to dynamically operate the secondary spectrum, provided that they do not degrade the primary users' transmissions . From a practical viewpoint, this means that the secondary terminals must acquire a sufficient level of knowledge about the status of the primary network. This information can be gathered through the use of techniques such as energy detection , cyclostationary feature detection , and/or cooperative distributed detection . Due to the complexity and drawbacks of the detection phase, the FCC recently issued the statement that all devices "must include a geolocation capability and provisions to access over the Internet a database of protected radio services and the locations and channels that may be used by the unlicensed devices at each location" . Furthermore, the positions of the primary nodes and other meta-information can be shared in the same way. Though the locations of the nodes and their configurations can be obtained easily, the exploitation of such information remains an open problem. Considering that any diversity technique can be used by cognitive nodes, several approaches have been proposed to allow for the coexistence of primary and secondary networks . These include, for example, the use of orthogonal codes (code division multiple access, CDMA) , frequency multiplexing (frequency division multiple access, FDMA), directional antennas (spatial division medium access, SDMA) , orthogonal frequency-division multiple access (OFDMA) , and time division multiple access (TDMA) , among others.
In this paper, we investigate a simple, yet powerful, diversity scheme by exploiting the polarimetric dimension [15–17]. More specifically, a dual-polarized wireless channel enables the use of two distinct polarization modes, referred to as copolar (symbol: ) and cross-polar (symbol: ), respectively. Ideally, cross-polar transmissions (i.e., from a transmitting antenna on one channel to the receiving antenna on the corresponding orthogonal channel) should be impossible. In reality, this is not the case due to an imperfect antenna cross-polar isolation (XPI) and a depolarization mechanism that occurs as electromagnetic waves propagate (i.e., a signal sent on a given polarization "leaks" into the other). Both effects combine to yield a global phenomenon referred to as cross-polar discrimination (XPD) [18–20].
Since polarimetric diversity does not allow a perfect orthogonality between primary and secondary nodes' transmissions, its use is possible under the application of a so-called underlay paradigm [10, 22, 23]. This means that both cognitive and primary terminals carry out communications, provided that the capacity loss caused by cognitive users does not degrade communication quality for primary users. For this purpose, we can further characterize the underlaid paradigm by requiring that the primary system must be guaranteed a minimum (transmission) capacity during a large fraction of time. As will be shown, this can, in turn, be formulated as a probabilistic coexistence problem under the constraint of a limited outage probability in the primary network.
We argue that using the polarimetric dimension allows dynamic spectrum sharing to be efficiently implemented in cognitive systems. To this end, we propose a theoretical model of interference in dual-polarized networks and derive a closed-form expression for the link probability of outage. We theoretically prove that polarimetric diversity can increase transmission rates for the secondary terminals while, at the same time, can significantly reduce the primary exclusive region.
First, we validated the expected (theoretical) performance gains analytically. To the best of our knowledge, none of the past studies in literature has investigated the behavior of the XPD under a complete range of propagation conditions, such as indoor-to-indoor and outdoor-to-indoor. In particular, we conducted a vast experimental campaign to provide relevant insights on the proper models and statistical distributions which would accurately represent the XPD. Based on these measures, the achievable performance of these dual-polarized cognitive networks, considering both half-duplex and full-duplex communications, will be determined.
The medium access control (MAC) protocol considered is a variant of the slotted ALOHA protocol  such that in each time slot, the nodes transmit independently with a certain fixed probability . This approach is supported by the observations in [26, page 278] and [25, 27], where it is shown that the traffic generated by nodes using a slotted random access MAC protocol can be modeled by means of a Bernoulli distribution. In fact, in more sophisticated MAC schemes, the probability of transmission of a terminal's transmission can be modeled as a function of general parameters, such as, queuing statistics, the queue-dropping rate, and the channel outage probability incurred by fading . Since the impact of these parameters is not the focus of the this study, for more details we refer the interested reader to the existing studies in the literature [29–31].
The remainder of this paper is organized as follows. In Section 2, we demonstrate how the polarimetric dimension increases spectrum-utilization efficiency and supports the coexistence of primary and secondary users in a probabilistic sense, which requires guaranteed capacity for the primary network. After these theoretical developments, several insights are presented to move from the concept to practical implementation. First, Section 3 presents an experimental determination of the main parameters used to characterize cognitive dual-polarized networks in indoor-to-indoor and outdoor-to-indoor situations. These results are then used in Section 4 for analytical performance evaluation. Section 5 concludes the paper.
2. The Dual-Polarized Cognitive Network Architecture
2.1. Probabilistic Coexistence and Interference
where is the instantaneous received power (dimension: [W]) at distance , is the noise power spectral density of the noise (dimension: [W/Hz]), is the channel bandwidth, and is the cumulated interference power (dimension: [W]) at the receiver, that is, the sum of the received powers from all the undesired transmitters. We now provide the reader with a series of theoretical results, which stem from the following theorem.
where is the transmit power, is the average power of the background noise, is the SINR threshold, is the path loss exponent, is the reference cross-polar discrimination of the antenna at a reference distance , and is a function that characterizes the polarization loss over distance.
By plugging (14) and (15) into (13), one finally obtains expression (6) for the probability of successful transmission.
Theorem 1 gives interesting insights on the expected performance in a dual-polarized transmission subject to background and internode interference. First, the leftmost term of the expression at the right-hand side of (6) is relevant in a situation where the throughput is limited by the background (typically thermal) noise. In large and/or dense networks, the transmission is only limited by the interference and one can focus on the interference and polarization terms (i.e., the two other term of the expression, assuming is negligible). The first exponential term can be easily evaluated if .
The second and the third terms of expression (6) relate to the interference generated by the surrounding nodes transmitting in co- and cross-polarized channels. These terms depend on (i) the polarization characteristics of the interfering nodes, (ii) the traffic statistics, and (iii) the topology of the network. Note that the impact of the topology has been largely investigated in  and we will limit our study to the effect of polarization.
Finally, channel correlation is neglected here, as often in the literature, for the purpose of analytical tractability and because these correlations do not change the scaling behavior of link-level performance. For the sake of completeness, we note that in  an analysis of the impact of channel correlation is carried out. The authors conclude that, when the traffic is limited ( ), the assumption of uncorrelation holds. On the other hand, when the traffic is intense ( ), the link probability of success is higher in the correlated channel scenario than in the uncorrelated channel scenario.
2.2. Probabilistic Link Throughput
The probabilistic link throughput  (adimensional) of a primary terminal is defined as follows:
(ii)in the half-duplex communication case, it corresponds to the product of (a) , (b) the probability that the transmitter actually transmits (i.e., ), and (c) the probability that the receiver actually receives (i.e., ).
The probabilistic link throughput can be interpreted as the unconditioned reception probability which can be achieved with a simple automatic-repeat-request (ARQ) scheme with error-free feedback . For the slotted ALOHA transmission scheme under consideration, the probabilistic throughput in the half-duplex mode is then and in full-duplex case .
2.3. Properties and Opportunities of Polarization Diversity
Theorem 1 expresses a network-wide condition to support the codeployment of primary and cognitive terminals. In order to implement polarization diversity and make it work, proper considerations have to be carried out. In this section, we propose several lemmas, all derived from Theorem 1, that allow to design and operate dual-polarized systems.
In a dual-polarized system subject to probabilistic coexistence of primary and secondary networks, relocating a cognitive terminal from the copolar channel to the cross-polar channel increases its probability of transmission while keeping intact the transmission capacity of the primary network.
Let us consider a scenario with a single interferer located at distance and transmitting with power . For the ease of understanding, let us assume that if the terminal uses a polarized antenna, its probability of transmission will be denoted as , whereas if a classical (not dual-polarized) scenario is considered, then .
Lemma 2 indicates that polarization can be exploited as a diversity technique. Indeed, the achievable transmission rate will always be increased if the secondary network uses a polarization state that is orthogonal to that of the primary network and, furthermore, this remains true regardless of the values taken by the other system parameters (e.g., transmission power, acceptable outage rate , SINR value, etc.).
There exists a region of space, referred to as the primary exclusive region, where the cognitive terminals are not allowed to transmit and can be reduced by means of polarimetric diversity.
Therefore, the quantity at the right-hand side of (25) represents the minimum amount of XPD that the antenna of the cognitive terminal must possess. This value depends on the network configuration but also on the propagation environment (through the depolarization function ).
Lemma 4 indicates that, if the desired throughput remains limited, then the outage is guaranteed on the primary system without summoning up the diversity of polarization on the secondary terminal. Therefore, the cross-polar channel can be kept available for other terminals that may require higher data rates. This can be observed in Figure 2.
Besides being limited by probabilistic coexistence considerations, there exists an optimum probability of transmission by a terminal in the primary network, denoted as , that maximizes the throughput.
which is the probability of transmission that maximizes the throughput. The same derivation can be applied in the case of a full duplex system and leads to the solution . If the approximation is not used, then the optimal probability of transmission cannot be given in a closed-form expression but has to be numerically evaluated.
Obviously, the maximum value of will be the minimum between (i) the optimum probability of transmission in a slotted transmission system (in a general sense), given by (32), and (ii) the maximum rate that can be achieved under the constraint of a probabilistic coexistence in (20). Therefore, before selecting its transmission rate, a cognitive terminal must evaluate these two quantities, on the basis of the available information stored in the databases (positions of the nodes, acceptable outage, etc.), and use the smallest one.
From these figures, it can be observed that the probability of transmission of dual-polarized cognitive systems is mainly limited by the interference bound imposed to protect the primary system. In fact, the transmission rate of the terminals will nearly always be lower than the optimal transmission rate, except when the cognitive terminal is distant. In that specific case, the optimum probability of transmission (20) in the accessible region (in a probabilistic sense) saturates, that is, it reaches in the half-duplex case and in the full-duplex case. Note that these values correspond to the maximum achievable throughput observed in any half-duplex or full-duplex system. Indeed, the definitions of the probabilistic link throughput are and and the corresponding optimum terminal probabilities of transmission cannot exceed and , respectively.
In the scenarios where polarimetric diversity is exploited, this crossover distance is smaller ( ) than in the classical case ( ). Comparing the results in Figure 3(a) with those in Figure 3(b), another observation can be carried out. In the half-duplex case, for each distance , the optimal transmission probability lies inside the accessible region. In other words, has to be properly selected to maximize the throughput. In the full-duplex case, everywhere in the exclusive region. These observations will be confirmed by the results presented in Section 4.
Finally, it is confirmed that, in the accessible regions, one either has (i) with (i.e., ) or (ii) . Therefore, the approximation used in proof of Theorem 5 (i.e., ) holds and the value of derived in Theorem 5 can be considered as an accurate approximation of the true value.
2.4. Considerations for Practical System Implementation
In the previous subsections, we have shown that the capacity of a primary user can be guaranteed, while, at the same time, allowing efficient spectrum access, if the polarimetric dimension is exploited. Moreover, dual-polarized terminals will benefit from an increase of capacity by means of a higher transmission rate and reduced terminal-to-terminal interference. The efficiency of polarization diversity depends on the cross-polar discrimination of the antennas in use. More precisely, the value of the initial cross-polar discrimination (i.e., ) has to be as high as possible; yet, the XPD of well-designed antennas is typically on the order of dB [15, 39], which allows a significant discrimination between copolar and cross-polar channels. Depending on the achievable value of , the outage rate of a primary terminal, and the location of the terminals, the transmission rate of a cognitive terminal can be adapted taking into account the relations (20) and (32). Finally, the primary exclusive region can be determined by means of (22) and notified to the cognitive terminals which, in turn, can use it as a constraint.
3. Experimental Determination of the Indoor-to-Indoor and Outdoor-to-Indoor XPD
Several previous works have been undertaken in order to model the XPD for different kinds of environment. In , a theoretical analysis is conducted for the small-scale variation of XPD in an indoor-to-indoor scenario and it is concluded that it has a doubly, noncentral Fisher-Snedecor distribution. A mean-fitting (i.e., the pathloss) model of XPD as a function of the distance in an outdoor-to-outdoor scenario was studied in [16, 19]. The corresponding performance is analyzed in .
In this paper, we provide the reader with original measurements campaigns in both indoor-to-indoor and outdoor-to-indoor scenarios. Indeed, these correspond to real-life situations where various technologies, such as WiFi, sensor networks, personal area networks (indoor-to-indoor scenarios) or WiMax, public WiFi, and 3G systems (outdoor-to-indoor scenarios) are in use.
In the remainder of this section, we characterize the applicability of the three XPD models just introduced. In other words, we consider an experimental setup and, on the basis of an extensive measurement campaign, we determine which XPD model is to be preferred in each scenario of interest (indoor-to-indoor and outdoor-to-indoor).
The measurements were performed using a Vector Signal Generator (Rohde & Schwarz SMATE200A VSG) at the transmitter (Tx) side and a Signal Analyzer (Rohde & Schwarz FSG SA) at the receiver (Rx) side. The Tx chain was composed of the VSG and a directional antenna. The Rx antenna was a tri-polarized antenna, made of three colocated perpendicular antennas. Two of these antennas were selected to create a Vertical-Horizontal dual-polarized antenna. The three receiver antennas were selected one after another by means of a switch and were connected to the Signal Analyzer through a dB, low-noise amplifier. The Rx antennas were fixed on an automatic positioner to create a virtual planar array of antennas. A continuous wave (CW) signal at the frequency of GHz was transmitted and the corresponding frequency response was recorded at the receiver side. The antenna input power was 19 dBm.
3.2. Experimental Results and Their Interpretation
The analysis of the collected experimental results has shown that the values of the XPD, for a given distance, present a location-dependent variability. Therefore, in the following figures, where the XPD is shown as a function of the distance , the average value is shown along with the and being confidence intervals. Since the spatial variations were found to be Gaussian, these intervals account for and of the observed sets, respectively.
4. Numerical Performance Evaluation
4.1. Full Duplex Systems in an Outdoor-to-Indoor Scenario
Two different polarization strategies are investigated: (i) the primary and the cognitive networks do not use polarimetric diversity (this scenario is referred to as no polarization) and (ii) the systems reduce their interference by using two orthogonal polarization states (this scenario is referred to as full polarization).
In Figure 11, the performance of full duplex systems is presented. More specifically, in Figure 11(a), the throughput of the system is shown as a function of the terminal probability of transmission. It can be seen that the throughput of the dual-polarized system is significantly higher, particularly when the probability of transmission is high. In Figure 11(b), the corresponding link probability of success in the primary network is investigated. It can be seen that it confirms the conclusions of Lemma 2: for a given minimum value of the link probability of success, the achievable transmission rate is significantly higher in the dual-polarized mode with respect to the value observed with the classical approach. For instance, with , one has while, by using the dual-polarized approach, the maximum probability of transmission can be increased up to . In other words, virtually any transmission rate is achievable with a limited impact on the primary system.
4.2. Half-Duplex System in an Indoor-to-Indoor Scenario
In this paper, we have presented a novel theoretical framework to demonstrate the network-level performance increase that can be achieved in a polarimetric diversity-oriented system subject to Rayleigh fading and probabilistic coexistence of primary and secondary (cognitive) networks. The theoretical approach was supported by an extensive measurement campaign. It has been shown that different mathematical expressions must be used in order to suitably model the dependence of the XPD on the distance between transmitter and receiver. These models depend not only on the scenario of interest, but also on the initial antenna polarization. For instance, in an indoor-to-indoor scenario, we have observed that the horizontal polarization provides a significant diversity ( around 10 dB) while the vertical polarization leads to a more limited gain ( around 4 dB).
Our results suggest that dual-polarized networks are of interest, even if orthogonality (indicated by the XPD value) is limited. Indeed, with respect to the classical implementation of probabilistic coexistence of primary and secondary networks on the same (single polarization) channel, the use of polarization diversity allows to remarkably increase the per-link throughput and reduce the primary exclusive region. In some cases (i.e., at low transmission rates), it could even be possible to deploy a cognitive terminal closer to a primary receiver than the primary transmitter itself, that is, inside the primary exclusive region.
The support of the Belgian National Fund for Scientific Research (FRS-FNRS) is gratefully acknowledged.
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