Reconfigurable parasitic antennas for compact mobile terminals in multiuser wireless systems
© Barousis et al; licensee Springer. 2012
Received: 15 October 2011
Accepted: 3 February 2012
Published: 3 February 2012
This article considers the exploitation of parasitic antenna arrays in multi-user (MU) wireless communication systems by using their adaptive beamforming capabilities in order to improve the average system throughput. The use of parasitic arrays and especially the electrically steerable passive array radiator (ESPAR) antennas enables the design of terminals with a single RF front-end and reduced antenna dimensions, i.e., lightweight and compact mobile terminals. Although the beamforming capabilities of active element arrays at the receiver have been well investigated in the past, this article highlights the potentials of pattern reconfigurable parasitic arrays based on the beamspace representation of the ESPAR antenna. The advantages of using ESPAR at the receiving terminal are examined both in opportunistic beamforming and in MIMO broadcast channel MU systems, optimizing correspondingly the SNR or the SINR of the forward link.
The use of multi-element antenna arrays has proven to be an effective means of turning multipath propagation to an advantage in wireless communication systems, by exploiting the diverse propagation characteristics of multipath components to increase the robustness of communication through diversity techniques, or the capacity of wireless links through spatial multiplexing of multiple symbol-streams. Recently, Knopp and Humblet  have used the same properties of multi-element array systems in multi-user (MU) environments, focusing on the reverse channel of cellular communication systems, where a large number of users, each equipped with a single antenna, access a single base station (BS) through a time-varying wireless channel. In their study, they prove that the average throughput of the system is maximized when the BS grants access to the user with the highest channel gain. The same results apply to the forward link . The main idea in opportunistic beamforming scenarios is the use of a different radiation pattern at the BS at each TDMA time slot, in order to induce a time-varying environment, even in the case of slow fading conditions. This idea could be implemented with the use of a multiple antenna array at the BS, which would produce a random radiation pattern on each TDMA time slot . If the BS had full Channel State Information (CSI) for all users at all times, then it could optimize the radiation pattern in order to maximize the signal to noise ratio (SNR) at each user. However, since full CSI knowledge would require excessive use of the channel resources for exchange of CSI information through feedback from the users to the BS, in practice the BS only requests for SNR level information from the users, which is inadequate for optimal beamforming. With opportunistic beamforming, due to the large number of users within a single cell, a random radiation pattern created on each user time slot would be close to the optimum radiation pattern for at least one user, with high probability. That user with the highest SNR would therefore be granted access on that time slot.
The use of pattern-reconfigurable antennas for improved capacity is not a new idea and actual implementations have been presented in [4–6]. The idea in these studies consists in enabling the dynamic reconfiguration of the antenna radiation patterns to provide some level of dynamic control over the channel itself. The antenna property, namely its instantaneous 'state', is thus an additional degree of freedom that can be optimized at each time slot by the algorithm implementing the smart antenna capability. As a result, this concept applies to both beamforming and MIMO schemes. It is also worth mentioning that here 'pattern reconfiguration' refers to the control of both polarization and spatial power spectrum of the radiated field since both these parameters affect channel property. In , Du and Gong present an operational antenna for 2 × 2 MIMO but do not assess its impact on the capacity. In , the mutual coupling between the two antenna elements for 2 × 2 MIMO is dynamically controlled, which in turn affects their radiation patterns (indeed, it can be shown that the coupling between antennas is directly related to their radiation patterns). For SNR of 10 dB and 20 dB, 10% and 8% capacity improvements are obtained with respect to a non reconfigurable system. In , the effect of both antenna diversity and gain in 2 × 2 MIMO are evaluated at SNR of 10 dB, 20 dB, and 30 dB, leading to capacity improvements of 70%, 40%, and 26%, respectively. However, the capacity gains achieved strongly depend on the test scenario. The approach in these studies essentially consists of designing some reconfigurable antennas with a certain level of pattern diversity, and subsequently evaluate the impact of this capability on the capacity. As explained in detail in the remainder of this article, here a more advanced strategy is proposed by exploiting the particular nature of parasitic array antennas, and in particular the decomposition of their instantaneous reconfigurable patterns onto a basis of orthogonal functions.
In [7–9], it was clearly shown that parasitic array antennas preserve the capability to also perform MIMO transmission. Therefore, the design of single RF front-end MIMO terminals is feasible, , and efficiently addresses the long experienced limitations imposed by the physical size of the terminals. The existence of a single active port motivates the representation of the MIMO functionality at the beamspace domain, where diverse symbols are mapped to different basis patterns. Indeed, the degrees of freedom (DoFs) of the electrically steerable passive array radiator (ESPAR) antennas have been explored by providing the expansion of the far field pattern in a complete set of orthonormal basis functions, or basis patterns. The operation was initially described in  and then a generalized and analytic methodology was presented in [12, 13]. This alternative analysis takes advantage of the beamforming capabilities provided by the parasitic elements that are connected to tunable loads, and determines the DoFs at the beamspace domain. Thus, single port antennas with beamforming capabilities can be used to emulate MIMO transmission. The significantly reduced antenna dimensions, as well as the single RF chain required to support diversity and multiplexing capabilities, are the enabling characteristics of parasitic antennas for lightweight and compact mobile terminals. The use of electronically steerable parasitic antennas is not the only way to get compact, lightweight and low cost MIMO transceivers. Recently, a novel MIMO scheme based on analog combining has been explored in depth [14–18]. This MIMO architecture solves the implementation complexity by shifting spatial signal processing from the baseband to the radio-frequency (RF) front-end and is known as RF-MIMO. The basic idea of the RF-MIMO transceiver is to perform adaptive signal combining in the RF domain. After combining, a single stream of data must be acquired and processed, thereby reducing cost and power consumption as compared to the conventional MIMO scheme with multiple active streams. An experimental evaluation of the RF-MIMO concept can be found in . Although this scheme has been shown to provide full diversity and array gain, its multiplexing gain is limited to one, as a result of processing a single data stream. In contrast, ESPAR based MIMO provide multiplexing gain thanks to the novel aerial modulation technique. However, RF-MIMO architecture has been shown to support OFDM schemes, while ESPAR based MIMO support to the moment single carrier transmission. Other similarities and differences between RF-MIMO and ESPAR based MIMO concern the beamforming process and are reviewed in .
The major contribution of this study is the use of recent developments in reconfigurable parasitic arrays and in the beamspace representation of their patterns, in order to optimize the performance of the forward link in opportunistic beamforming and MIMO broadcast channel MU systems. The presentation of our findings is organized in the following sections. In Section 2, we present a review of reconfigurable parasitic antenna technologies with emphasis on their feasibility and adaptive capabilities, which enable the analysis of this article. Section 3 presents the advantages of using parasitic arrays on mobile terminals in opportunistic beamforming multiuser scenarios, while Section 4 presents the respective gains achieved in MIMO broadcast channel MU scenarios. Section 5 concludes the results of this research activity. One paragraph describing paper contents and contribution. (Actually in opportunistic beamforming MU systems and in MU-MIMO broadcast channels).
2 Reconfigurable parasitic antennas for lightweight terminals
Multiple antenna arrays have been for long considered for increasing the wireless link performance in applications where the size and cost of their implementation is not restrictive. Indeed, multi-element arrays have been widely used in BSs, but their implementation in mobile terminals is restricted by the available real estate for the antennas and the need for separate RF chain for each antenna element (except if the array is used to achieve SISO beamforming only). Although the size issue can be quite efficiently tackled by the use of 'orthogonal resonant modes', see e.g., , the burden of the multiple RF chains remains.
Recently, a novel parasitic array architecture has been developed [21–23], which can significantly decrease the size and cost of arrays, thus making their integration in handheld terminals feasible. These arrays consist of only a single active element and a number of parasitic elements placed in close proximity. Due to strong mutual couplings, the feeding of the active element is responsible for the currents induced to all parasitics. The dynamic control of the parasitic array radiation patterns is performed directly in baseband, through the dynamic control of passive reactive loads connected directly to all parasitics and thus altering mutual coupling and antenna radiation characteristics . Importantly for practical designs, the complete description of the parasitic array performance (return loss, efficiencies, patterns, etc.,) in all possible dynamic states, can be computed based on a single electromagnetic full-wave simulation followed by simple post-processing . It is of course of prime importance to precisely implement the reactive loads, as detailed in . So far, their control has been achieved using varactors or p-i-n diodes. However, as in other applications the use of MicroElectroMechanical Systems (MEMS) would result in better performance in terms of insertion loss, linearity, while having virtually zero DC power consumption. In this study, the simulation results presented in the following sections we have not assumed a specific implementation technique but we have restricted our interest to the values of the loads and the corresponding radiation characteristics of the antenna.
Traditionally parasitic array implementations focused on SISO beamforming, since the use of a single RF port constrained them from being used in MIMO systems. In this sense, they present a similar functionality as conventional arrays achieving beamforming through analog RF phase shifters. However, recently such parasitic array systems have been effectively used in MIMO systems simultaneously transmitting multiple bit streams over the air, through the decomposition of their instantaneous reconfigurable patterns onto a basis of orthogonal functions. As will be shown, the resulting radiation pattern is the linear combination of the baseband symbols and the basis patterns and can be viewed as creating multiple symbol streams at the beamspace domain. To emphasize its principle of operation, the resulting single RF MIMO system is known as beamspace MIMO (BS-MIMO). It should be noted that this MIMO approach takes advantage of the coupling between the adjacent ESPAR elements. Indeed, the strong coupling enables the beamforming capability, which in turn is required to emulate MIMO transmission over the air [12, 13]. In fact this idea has already been quite extensively exploited on the transmitter side, from the initial concept presented in  and the detailed design of the actual reconfigurable parasitic antenna and experimental demonstration in . These studies demonstrated the tremendous advantages of using ESPAR antennas at the transceiver, since it was shown both theoretically and experimentally that a single ESPAR with a particular feeding scheme allows to multiplex data while using a single antenna and RF chain [9, 10].
The MIMO functionality is presented at the beamspace domain. At the transmitter, symbols are not driven to diverse active antenna elements as in conventional case, but they modulate the orthogonal radiation patterns of the basis. The presented decomposition implies that the number of DoFs, i.e., the beamspace dimensionality, is equal to the number of ESPAR elements. However in [12, 13] it was shown that the electromagnetic coupling between the ESPAR elements, which is heavily dependent on the antenna dimensions, strongly affects the subset of significant DoFs, N eff ≤ M, called effective DoFs (EDoFs).
3 SNR optimization in opportunistic beamforming systems
The idea of opportunistic beamforming has shown that in MU environments, fading is actually a desired property of the wireless channel. Opportunistic beamforming will therefore improve the performance of wireless channels having a strong line-of-sight (LoS) component (i.e., Rician channels), by transforming them into severely faded channels. In this section, we enhance this idea by introducing the use of multi-element arrays on the receiver side, in order to maximize the received signal's SNR. We consider two different cases of static channels: Rayleigh and Rician. In the former case, it has already been shown that opportunistic beamforming has no enhancing effects of the average system throughput. Therefore, for Rayleigh channels we only consider the optimal beamforming scenario on the receiver side. In the case of Rician channels we examine the enhancement of average network throughput when in addition to the opportunistic beamforming at the BS, switching is performed at the receiver among different radiation patterns having significant antenna gains.
3.1 System model
where s(u)and n(u)are the transmitted signal and the Gaussian noise for user u, respectively.
3.2 Receiver beamforming in Rayleigh channels
3.3 Receiver beamforming in Rician channels
where is the vector of the azimuth samples of the transmit radiation pattern, while the column vectors φT,iof matrix contain the samples of the basis pattern. Similarly, we define vector pu,Rfor the receive radiation patterns. The LoS component has a phase shift ϕ u caused by propagation over the path connecting the j th angle of departure and i th angle of arrival. The amplitude of this component is naturally affected by the complex gains of the transmit and receive radiation patterns towards the same directions. In our approach, ϕ u is considered as a uniformly distributed random variable in the range of [0, 2π).
4 SINR optimization in MIMO broadcast channels
In this section, we consider the case where the BS is capable of granting access to U users simultaneously, by means of MU-MIMO broadcast channel. In this case, we are interested in the maximization of the signal to interference plus noise ratio at the user terminals, as considered also in [29, 30]. The former publication is a generalization of the opportunistic beamforming technique, where a set of orthogonal radiation patterns is considered at the BS, each being assigned to a user with the maximum SINR. The orthogonal patterns are randomly assigned in each time slot, using an orthonormal pre-coding matrix, whose columns can be regarded as weighting vectors corresponding to orthogonal radiation patterns in the beam-space domain. The latter publication expands this concept by designing the orthogonal patterns according to the previous knowledge of the channels, rather than randomly. In this section, we propose an interference cancelation technique based on the use of parasitic arrays on the user terminals, exploiting the Neff, uDoFs of such antennas.
4.1 An interference cancellation technique on user terminals
We consider a broadcast channel of a multiuser environment where the BS and the mobile terminals are equipped with parasitic antennas of Neff,Tand Neff, uDoFs, respectively, where in the general case, Neff,T≠ Neff, u. The BS transmits simultaneously to U ≤ Neff,T. users on each time slot, and each user is assigned to a different basis transmit radiation pattern. Therefore, the BS functions as a MIMO transmitter with parasitic arrays as described for example in . The users produce a linear combination of the Neff, uorthogonal patterns of their antenna in order to create the optimal receive pattern. This cancels out the interference caused by the (U - 1) simultaneous transmissions of the BS to the rest of the users, and at the same time maximizes the desired signal power. The ability of fulfilling these requirements depends on the number of DoFs Neff,Tand Neff, u, available on the BS and the user, respectively, as well as on the number of simultaneous users, U ≤ Neff,T.
where and have dimensions (Neff, u× Neff, u), with rank one and U - 1 respectively. The scalar term γ corresponds to the average SNR at each user. H l is formed by the h(u,i), i ≠ u column vectors of the interference channels of user u. Note that rank (D I ) = U - 1, i.e., is equal to the number of interfering signals.
where is a (U - 1 × U - 1) matrix, while the null matrix 0 is a (Neff, u- U + 1 × Neff, u- U + 1) matrix. The vectors of the required null space are the right eigenvectors of U l which correspond to the zero eigenvalues.
User u can cancel out interfering signals when there exists at least one zero eigenvalue, or equivalently when there exists at least one null row in matrix Λ I . This is the case when Neff, u≥ U, meaning that the user may cancel out interference whenever the DoFs of the receiving antenna are greater than or equal to the total number of users. We identify the following two cases:
When Neff, u= U, the null space has a single eigenvector (the first column of U 1 from the right), which can be used for interference cancelation.
When Neff, u> U the null space has Neff, u- U + 1 eigenvectors.
where and . It is therefore evident that the power of the desired signal is maximized when c = r/k, where k = ||r|| F . The use of the Frobenius norm to normalize r is required in order to ensure that .
We note that when Neff, u= U ≤ Neff,T, (19) shows that the user can cancel out interference, but it is not able to maximize the desired signal strength, since the null space has a single eigenvector. Furthermore, if N eff, u < U, then (19) does not hold, meaning that not all interfering signals can be canceled out. However, the user can still null out (N eff, u - 1) stronger interfering signals, maximizing the SINR value for the available antenna capabilities.
4.2 Evaluation of the proposed Scheme
The equation above gives approximate results, since the probability that a user will have the optimal channel for more than one transmit radiation pattern, is not taken into consideration. However, we do include the cases for which , where the user will be able to null out only the (N eff, u - 1) most significant interferences. In the results that follow, we consider parasitic arrays with N eff, u = 3 or 5 DoFs, while at the BS N eff, T = U.
Conventional MU systems use either opportunistic beamforming or MU-MIMO in the broad-cast channel and assume single antennas at the mobile station. We propose to take advantage of the developments in reconfigurable parasitic arrays in order to increase the performance of forward channels. The main idea is to integrate such antenna systems into mobile terminals, expand their beamspace domain into a basis of orthonormal radiation patterns and use this basis in the analysis for optimal beamforming and SINR optimization. The results show that in the case of opportunistic beamforming scenarios in Rayleigh channels only beamforming gains are achieved, as expected. In Rician channel environments the performance gains are significant and directly related to the K-factor of the channel. In MU-MIMO scenarios, the use of reconfigurable parasitic arrays at the receiver side significantly increases SINR and consequently the performance of the forward channel. Depending on the effective DoFs of the parasitic arrays and the total number of users, the receiver can cancel out all interfering signals, maximize channel gains, or cancel out the most significant interferers. We therefore conclude that the use of parasitic arrays in multiuser scenarios shows considerable advantages, even in the case where the number of DoFs is limited due to implementation constraints of mobile terminals.
This work was partially funded by the European Union and national resources under the National Strategic Reference Framework (NSRF) and the THALES research project: INTENTION".
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