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We consider the impact of antenna mutual coupling on receive antenna selection systems. Prior work on selection with mutual coupling has not considered the effects of the inactive (i.e., unselected) antenna terminations and spatial noise correlation. In this work, we show that the presence of inactive antennas can profoundly alter system performance when the antennas are strongly coupled. We also propose a new antenna selection technique that seeks to exploit coupling to improve performance. Simulations suggest that the new technique can significantly outperform traditional selection when coupling is present.

Multipath fading is known to deteriorate the performance of a wireless communication system. Spatial diversity techniques employing multiple-antenna systems at the receiver have been shown to combat fading and promise significant performance improvement over single-antenna systems [

However, these techniques demand deployment of large arrays, and, thus, the benefits come at the cost of an increased system and hardware complexity. At the receive side, each antenna is associated with a low-noise amplifier (LNA), a demodulator and an A/D converter, together constituting a receive radio frequency (RF) chain, which is expensive. In addition to that, employing more antennas requires sophisticated digital signal processing, and, for small-sized or hand-held receivers, it puts a burden on providing an extended battery life. All these factors pose a challenge to the wide-scale deployment of multiple-antenna systems.

Antenna selection is one such scheme which tries to bridge the gap between the complexity and diversity benefits. It utilizes only a subset of available antenna signals, followed by down-conversion and digital signal processing. This reduces the requirement of a large number of RF chains and brings down the system and hardware complexity. An analysis of generalized diversity schemes with maximal-ratio combining (MRC) technique is reported in [

Although, MIMO techniques have been demonstrated to achieve the much needed high-throughput and reliable wireless communication, practical constraints on the physical dimensions of a transceiver limit the deployment of large arrays. Thus, in order to build low-cost and compact multiple-antenna devices, packing antennas closer together becomes indispensable.

As the antenna elements in an array are brought closer, the fading path gains become correlated, and the antennas begin to couple with one another. For closely spaced antennas, the current flowing in one element alters the voltage across the other, commonly known as mutual coupling. Traditionally, antenna selection applied to MIMO systems chooses a set active antennas to be employed for transmission or reception based on some performance metric and ignores the inactive antennas. However, in the case of closely spaced antennas, mutual coupling can have a profound impact on the performance, while also opening new avenues for transceiver design with respect to the presence of inactive elements in the vicinity of the active subset.

It is well known that employing parasitic antennas in compact arrays can leverage significant performance improvements. In this paper, we investigate the design of compact receive antenna selection systems from a communication-theoretic perspective that put the inactive elements at use. The underlying motivation is to come up with selection architectures that offer improved diversity performance, compared to a system without selection with an equal number of RF chains and array size. An illustration of this comparison is given in Figure

Configurations: (a) antenna selection (AS), (b) reduced full complexity (RFC), (c) full complexity (FC).

A study of receive antenna selection in the presence of mutual coupling has been reported in [

Besides, all studies, thus far, have assumed that the noise at the receiver is additive white Gaussian in nature. It has been recently shown in [

As we present a more realistic model for receive antenna selection applied to compact arrays, we seek to address some of the above-mentioned issues. It will be shown in this paper that for compact arrays, selection is in fact preferred over compact full-complexity and reduced full-complexity systems. We shall demonstrate that, by appropriate termination of the inactive elements, the performance of antenna selection can be further improved. We call this strategy

The organization of this paper is as follows. In Section

Traditional antenna selection schemes assume that the inactive elements are left open circuited. While the termination of inactive elements does not matter when the array elements are uncoupled, it is imperative to study the role played by the inactive elements in a compact selection system.

One such study that explores this aspect is [

For long, inactive (

Vaughan [

Harrington [

Hence, we seek to explore ways in which the coupling between the array elements can be exploited to improve the performance of selection systems. The key idea here is to use a different kind of termination that can reflect the power off of the inactive elements in order to make more power available to the active subset. In arriving at the analytically optimal designs, we shall begin with modeling the transceiver accurately.

We consider the receiver circuit model illustrated in Figure

A circuit model of a receiver with antenna selection and mutual coupling.

The

In a flat-fading environment, the open-circuit voltage in (

For perfectly conducting antennas,

The array is followed by a noiseless switching network that selects

Circuit model of the antenna array and switching network.

In traditional antenna selection, the inactive antennas are considered to be open circuit. Here we also consider the possibility that terminating the inactive antennas with some

Most of the voltages and currents in Figure

According to [

When a source impedance

The switching network is connected to the amplifiers by a

The relationship between the voltages and currents at the input and output of the matching network are described by equations similar to (

The rest of the RF chain (

The input to the linear combiner given by the voltage

The amplifier parameters are denoted by

The receiver employs a linear combiner to the input

For a given

We also consider a simpler, suboptimum matching strategy that applies to each antenna the two-port matching network that achieves the minimum noise figure for that antenna in isolation. This is called

Observe that the optimal matching network discussed above depends on (

The derivation of optimal nondiagonal

To that end, consider a circular array of

The set of orthonormal eigen vectors for each block corresponding to the eigen-values

Since the subblocks of

A circuit model of a circular array receiver with antenna selection and mutual coupling.

The post-MRC output SNR for amplifier-noise-dominant scenarios with optimal parasitic network given by (

The fading-independent, subset-dependent optimal

Optimal Parasitic Network: Circular Array

Active Subset | Parameter | Value |
---|---|---|

Any | ||

Others | ||

The optimal parasitic network designs presented here for circular arrays with equal number of active and inactive elements can be extended to a broader set of arrays and configurations although the optimal solution may need to be computed numerically. In Section

In this section, we present numerical results for the two types of antenna arrays considered above. These arrays consist of half-wavelength dipole antennas spread over a length

Configurations: (a) uniform linear array, (b) uniform circular array.

We compare three combinations (a), (b), and (c), as shown in Figure

The closed form expression for the fading path-gain covariance given in [

For an

The amplifier selected for this study is a low-cost SiGe LNA [

It is worthwhile to note that according to the model used in earlier studies, [

Before presenting the results, it is useful to normalize (

To that end, consider a sufficiently large antenna spacing such that

We divide the results into two categories: uniform circular arrays and uniform linear arrays.

We begin by presenting the results for traditional antenna selection scheme for a ULA, where the inactive elements are left open circuited. The active elements are terminated into optimal or suboptimal matching networks derived in Section

Traditional antenna selection (ULA):

The antenna selection results with the coupling and fading correlation notated by AS (self) and AS (optimal) denote the choice of active element matching network. The curves show that antenna selection in compact arrays provides a 3-4 dB improvement over conventional RFC systems, at a

For sake of completeness, we also include results with the model assumed in the earlier studies on antenna selection. The curves reproduced with these models are shown in Figure

Next we present results for a UCA with

Parasitic antenna selection (UCA):

The traditional antenna selection with optimal matching (denoted by Conj/Open) provides a 3-4 dB improvement over its self-matching counterpart (denoted by self/open). However, the optimal matching with optimal parasitic network (denoted by Conj/

The optimal parasitic networks considered so far have all been subset-dependent (i.e., depend on which set of antennas is active), where as the suboptimal ones are not. However, both types of networks are independent of the fading conditions. In the next section, we present a simpler parasitic scheme that adapts to the varying fading conditions.

In this section, we provide a suboptimal design for the parasitic network that not only depends on the active subset but also the channel conditions. However, the implementation complexity is simplified by replacing multiport parasitic networks with two ports.

We propose a novel but simple strategy, in which each of the inactive element is terminated into an impedance

We evaluate the performance of parasitic antenna selection system for

Figure

Parasitic switching (ULA):

Similar results are observed for a UCA

Parasitic switching (UCA):

A special case of this study surfaces when we consider selection combining (SC), that is,

Parasitic selection combining (ULA):

One of the nifty things about this scheme is that as more antennas are packed in a fixed length array, the diversity gains relative to traditional selection become large. These gains however, diminish with increasing interelement spacings, as expected. It turns out that PSC

Apparently, the loss incurred by employing one less RF chain is recovered by the availability of an additional parasitic element at the cost of an increased overhead as far as the number of parasitic switching states is concerned. It is important to point out that the active element is selectable, unlike ESPAR systems which employ a single active element with a set of parasitic elements arranged in a circular fashion around it.

The performance benefits for any selection system come at the cost of an additional hardware-switching circuitry. Although, in selection systems with i.i.d. fading and noise, it suffices to pick the antenna subset with the

One drawback of antenna selection is the insertion loss arising due to the presence of switches in the direct RF signal paths. This loss typically ranges from a fraction of a dB to several dB [

We study parasitic switching

The results are shown in Figure

Parasitic switching w/o antenna selection (ULA):

In the previous sections, we have seen how coupling can be exploited to improve the performance of antenna selection systems for compact arrays with appropriate matching networks. However, it is important to mention some of the challenges posed by the techniques presented here.

The optimal matching network shown here has certain limitations. While it provides the optimal performance for any given channel condition, multiport matching networks are nonrobust and known to shut down the RF bandwidth of the system [

We have already touched on the switching aspects in antenna selection systems. However, how often active subsets have to be chosen could be a significant overhead on the implementation costs. But in a slow-fading or quasistatic channels like those of indoor/wireless LAN scenarios, the switching frequency is considerably reduced.

Another aspect is that of the associated parasitic switching states. Figure

Histogram of parasitic switching combinations (ULA): (5,2).

As can be seen, the interelement spacing plays a vital role here. In [

Besides, in the above mentioned studies, it has usually been considered that the open-circuit elements are transparent, which is a valid assumption for circular arrays with distances of the order of

We presented a detailed transceiver model for antenna selection applied to compact receive arrays which accounts for coupling among the elements and models the impact of inactive-element termination on system performance. We considered various design approaches for antenna selection strategies primarily for two types of uniform arrays-linear and circular. Apart from considering optimal designs for matching networks and parasitic networks (which may not be practically feasible from an implementation and/or cost perspective), we provided some simpler strategies that achieve near-optimal performance. We also considered designs where we only apply parasitic switching over a predefined set of elements in order to minimize losses arising due to RF switches.

Specifically, we showed that uniform linear arrays could in fact benefit from selection over using all of the available antennas, thereby, reducing the number of RF chains and saving power and cost. The performance improvement, though, depends on the spacing (i.e., amount of coupling in the system). For circular arrays, we showed that the optimal matching networks can easily be implemented by use of Butler matrices since the actual matching reduces to that of two-port networks (or self-matching). Based on the space and cost constraints, different array geometries can be designed and employed differently as shown in this paper.

Simulations results reveal how important it is to model the transceiver and account for noise correlation in the RF chain. Besides, it is vital to model the inactive elements as scatterers for close spacings and that wrong assumptions could result in misleading conclusions.

We conclude that different inactive-element terminations can impact performance in profoundly different ways for different array configurations. It turned out that such a system even with a single RF chain can deliver a huge performance improvement, thereby, making the hardware much simpler and power efficient. The overheads involved with this system like exhaustive search and possible insertion loss were provided alternatives with, which renders this approach as a lucrative option to apply in practical systems.

In a nutshell, antenna selection offers a distinct advantage in portable or handheld devices due to significant coupling among the antennas. Thus, packing more antennas in a constrained space and employing appropriate selection scheme can profoundly improve the system performance.

Observe that we can rewrite (

The equivalent fading path-gains can thus be written as

For

From

For a circular array with a full receive scattering, the fading path-gain covariance matrix