TSINGHUA SCIENCE AND TECHNOLOGY ISSNll1007-0214ll07/17llpp294-298 Volume 15, Number 3, June 2010

Performance Evaluation of a Four-Element Antenna Array with Selection Circuits for Adaptive MIMO Systems*

LI Zhengyi ( ), WANG Xuan ( ), DU Zhengwei ( )**, GONG Ke ( )

State Key Laboratory on Microwave and Digital Communications, Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China

Abstract: Reconfigurable antenna arrays increase the flexibility of adaptive MIMO systems. At present, most

designs have adopted antenna arrays with reconfigurable elements. However, antenna selection is also an

effective method, which has not been fully investigated. In this paper, the potential benefits of a four-element

antenna array with selection circuits in the UMTS band (1920-2170 MHz) are explored. The array has eight

pin-diodes embedded in the feeding network to select any sub-set of elements. For evaluation, an adaptive

MIMO system was set up and a measurement campaign was taken in an indoor multi-path environment.

The measurements were performed over a 300 MHz bandwidth centered at 2.05 GHz, covering the UMTS

band. The results show that different channel conditions prefer different antenna array configurations.

Therefore, in varying channel conditions the antenna array can support antenna selection algorithms to se-

lect the best sub-set of elements to increase channel capacity.

Key words: adaptive MIMO systems; antenna selection; pin-diode; reconfigurable antenna array

Introduction

Adaptive MIMO systems, which are MIMO systems incorporating link adaptation techniques, have been recently proposed[1]. In these systems, some parame-ters such as the modulation and coding rate are dy-namically adapted to the time-varying channel condi-tions. Then, a new concept was introduced to increase system flexibility, based on reconfigurable antenna arrays, which have reconfigurable configurations and radiation/polarization properties[2]. At present, there are

two techniques in this field: one is antenna arrays with reconfigurable elements and the other is antenna selec-tion. Several designs have been proposed with the first technique[2-4], but few with antenna selection.

Compared to terminal antennas, the RF chains, in-cluding the low-noise amplifiers and downconverters, are usually quite expensive, and some antenna selec-tion algorithms have been studied to provide optimal performance when the number of RF chains is less than the number of antenna elements[5]. Furthermore, for some particular channels, using the optimal sub-set of transmit antenna elements can lead to an increase in capacity. A low rank matrix channel would be a good example[6]. Therefore, antenna selection is an effective method to achieve reconfigurable antenna arrays for adaptive MIMO systems. Its theory is to select the best sub-set of elements from among all the antenna ele-ments based on some criterion such as the sig-nal-to-noise ratio (SNR) or capacity[5], so it is a trade-off between cost, complexity, and multi-antenna

Received: 2008-12-29; revised: 2009-11-30

* Supported by the National Basic Research Program of China (No.2007CB310605), the National High-Tech Research and Develop-ment (863) Program of China (No. 2006AA01Z265), the Special-ized Research Fund for the Doctoral Program of Higher Education (No. 20060003100), and the Tsinghua-QUALCOMM Associated Research Plan

** To whom correspondence should be addressed. E-mail: zwdu@tsinghua.edu.cn; Tel: 86-10-62784107

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resources. To achieve antenna selection, the radiation pattern of

each selected element needs to be steady whether other elements are selected or not. However, this is not the case for an antenna array with conventional switching circuits, in which the selected element is connected to a matched source and the un-selected element is choked by an open-circuit. Karaboikis et al.[7] showed an example of this where the main beams of antenna elements are in opposite directions. To overcome this, Li et al.[8] suggested terminating the un-selected ele-ment to a matched lumped resistor. In that study, a printed dual-monopole array with selection circuits is achieved. Moreover, a four-element antenna array with selection circuits is proposed[9]. The antenna array can select any sub-set of elements with eight pin-diodes embedded in the feeding network.

In this paper, the performance of the four-element antenna array with selection circuits is evaluated. At first, the channel capacity is fully discussed to charac-terize the performance of adaptive MIMO systems. Then, an adaptive MIMO system is set up, and a meas-urement campaign is taken in an indoor multipath environment.

1 Antenna Array Characteristics

The four-element antenna array with selection circuits is illustrated in Fig. 1. The antenna array is printed on the front side of an FR4 substrate board with dimen-sions 95 mm × 60 mm × 0.8 mm and relative permit-tivity of 4.4. The four elements can be further divided into two pairs. Each pair has two monopoles, which have the symmetric configuration with the same di-mensions. Elements 1 and 2 (pair 1), which are located at the upper part, are two back-to-back monopoles. Elements 3 and 4 (pair 2), which are positioned at the lower part, are simple inverted-L monopoles. The main rectangular ground plane with two holes in the lower part is printed on the back side of the substrate board, and treated as the circuit part on the lid of a folder-type mobile phone. In order to increase the isolation of pair 1 and adjust the resonant frequency, a T-shaped and dual inverted-L-shaped ground branches are introduced. The ground stubs for elements 3 and 4 are used to ob-tain better matching characteristics within the UMTS band. The detailed geometry parameters were given by Li et al.[9]

(a) General view

(b) Schematic of the selection circuit for element 1

(c) Photograph of the prototype

Fig. 1 Configuration of the four-element antenna ar-ray with selection circuits

With eight pin-diodes embedded in the feeding net-work, the antenna array can select any sub-set of ele-ments with the un-selected ones terminated to matched lumped resistors. In the design, the dc block capacitors

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and the RF choke inductors separate the dc signal and the RF signal in the feeding network efficiently so that unwanted interference can be prevented. By control-ling the dc bias of the pin-diodes D1-D8 properly, the antenna array can change its configuration: the selec-tion of elements. For example, the schematic of ele-ment 1 with pin-diodes D1 and D2 is shown in Fig. 1b. If D1 is forward-biased and D2 is reverse-biased, ele-ment 1 is selected, while if D1 is reverse-biased and D2 is forward-biased, element 1 is terminated to a matched lumped resistor (Rm1). For elements 2, 3, and 4, the conditions are similar.

The pin-diode model number is Philips BAP64-03 and some surface mounted components are utilized in the current design. The connector in Fig. 1a is for the purpose of connection to a remote control unit for easy testing. After simulations, a prototype antenna array was fabricated and its photograph is shown in Fig. 1c. Whenever any sub-set of elements are selected, the measured impedance bandwidth (Sii < 10 dB) of each element within this sub-set will cover the UMTS band. Meanwhile, across the band, the two largest Sij (i j) parameters, S21 and S34, are still lower than 11.5 dB and 16.5 dB, respectively. Furthermore, the measured radiation pattern of each selected element changes very little whether other elements are selected or not. Pat-tern stability is essential for antenna selection.

2 Channel Capacity

In MIMO systems, channel capacity is used to evaluate performance. When the transmitter does not know the channel conditions and the power is equally divided to each transmit antenna element, the capacity is given by

R

*2

T

log det NCN

I HH (1)

where NR is the number of receive antenna elements, NT is the number of transmit antenna elements,

RNI is the NR×NR identity matrix, is the average received SNR, R T[ ] N N

ijhH C is the normalized channel

matrix, and *{ } denotes the conjugate transpose[10]. Equation (1) is in terms of a narrowband MIMO

system. When the system operates at several sub-carriers, the capacity should be averaged among the frequency domain, which is computed as

1

1 m

ii

C Cm

(2)

where m is the total number of sub-carriers[4,11]. The (i, j)-th entry of the channel matrix is acquired

using a vector network analyzer (VNA) to measure the transfer scattering parameter S21, while the other an-tenna elements are terminated to 50 loads[11]. In ad-dition, in order to remove the effect of path loss, the channel matrix needs to be normalized. Given that

R T[ ] N NijgG C is the measured channel matrix, the

normalization factor f is defined as 2

R T

|| ||=fN N

G (3)

where || || is the Frobenius norm of a matrix. The average received SNR in Eq. (1) is calculated as

R T2

T1 1

R

| |N N

iji j

P g

N v (4)

where PT is the power of each transmit antenna ele-ment, and v is the noise power added at each receive antenna element[12].

In adaptive MIMO systems, the definition of the channel capacity is complicated because of the recon-figurability of the antenna array. Boerman and Bern-hard[12] proposed a new method in which the channel capacities are calculated at the same level of noise power v instead of SNR. This method is appropriate since different configurations of antenna array may lead to different received signal powers, which means different average received SNRs. Therefore, the channel capacity at each configuration is calculated as follows.

The measured channel matrix is normalized by its own normalization factor, as in Eq. (3).

The average received SNR is computed using Eq. (4).

By substituting the above results into Eqs. (1) and (2), the channel capacity is acquired.

3 Measurements and Results

In order to evaluate the performance of the four-ele-ment antenna array with selection circuits, a measure-ment campaign was carried out in an indoor multipath environment, Room 919 in Main Building at Tsinghua University. As shown in Fig. 2, the room has a dimen-sion of 7 m×15 m with two sub-rooms separated by an inner door. There are several tables and partition boards in the room. The tables are 0.8 m high and the

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partition boards is 1.2 m high. The antenna array was equipped at the transmitter and a traditional two-ele-ment antenna array[13] was located at the receiver.

In the measurement, the transmitter was fixed at one position (Tx), while the receiver was placed at five different locations (Rx1-Rx5) as shown in Fig. 2. The heights of the transmitter and the receiver were both set at 1 m, so Tx-Rx1 and Tx-Rx5 (inner door was open) are line-of-sight (LOS) scenarios while Tx-Rx2, Tx-Rx3, and Tx-Rx4 are non-line-of-sight (NLOS) scenarios. At each pair of locations, the transmitter can change its configuration: the selection of elements. For brevity, only some typical configurations are presented here. “Up (U)” means only elements 1 and 2 are se-lected; “Down (D)” means only elements 3 and 4 are selected; “Left (L)” means only elements 1 and 3 are selected; and “Right (R)” means only elements 2 and 4 are selected. Therefore, a 2×2 adaptive MIMO system is set up.

Fig. 2 Plan of Room 919 in Main Building at Tsinghua University

The complex gains of the channel matrix were measured sequentially by a VNA (Agilent E5062A). The measurements were performed over a 300 MHz bandwidth spaced 1 MHz apart (301 points) and cen-tered at 2.05 GHz, which covers the UMTS band. The transmitted power was 10 dBm and the dynamic range was nearly 120 dB with an intermediate frequency of 100 Hz. During the measurements, there was no movement in the room (inner door was open and outer door was shut), so the channel is assumed to be quasi-stationary. Furthermore, for each pair of loca-tions and for each configuration, three measurements were taken in order to verify the stationary and to av-erage the results.

Fig. 3 Channel capacities against different antenna array configurations at five pairs of locations with an assumed noise power of 5 nW

According to the methodology in Section 2, the channel capacities are calculated with an assumed noise power of 5 nW, and plotted against different con-figurations in Fig. 3. Referring to the figure, the capac-ity in Tx-Rx5 is the highest among the five pairs of locations. That is because Tx-Rx5 is LOS scenario and the distance between the transmitter and the receiver is the shortest. Meanwhile, the results show that different channel conditions prefer different antenna array con-figurations. For example, in Tx-Rx2 (NLOS), the highest capacity is achieved at 3.50 bps/Hz with con-figuration “R”, acquiring 16%, 10%, and 32% im-provements compared to configurations “U”, “D”, and “L”. In comparison, in Tx-Rx5 (LOS), the highest ca-pacity is achieved at 9.16 bps/Hz with configuration “D”, acquiring 30%, 21%, and 14% improvements compared to configurations “U”, “L”, and “R”. The reason is that different antenna array configurations have different radiation properties. As a result, differ-ent antenna array configurations can obtain different average received SNRs and different channel capaci-ties. The channel capacities at five pairs of locations with the best configuration are plotted in Fig. 4, la-beled with “Best”. As a reference, the channel capaci-ties with configuration “U” are also plotted in Fig. 4. The results show that configuration “Best” outper-forms “U” greatly (16% improvement in Tx-Rx2, 14% in Tx-Rx3, 25% in Tx-Rx4, and 30% in Tx-Rx5) ex-cept that in Tx-Rx1 configuration “U” is the best con-figuration. Therefore, in varying channel conditions the antenna array can support antenna selection algorithms to select the best sub-set of elements to increase chan-nel capacity.

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Fig. 4 Channel capacities (with an assumed noise power of 5 nW) at five pairs of locations with the best configura-tion (labeled with “Best”) and configuration “U”

4 Conclusions

Antenna selection is an effective method to achieve reconfigurable antenna arrays for adaptive MIMO sys-tems. In this paper, the performance of a four-element antenna array with selection circuits in the UMTS band is evaluated. With eight pin-diodes embedded in the feeding network, the antenna array can select any sub-set of elements from among the four elements. Then an adaptive MIMO system was set up and a measurement campaign was taken in an indoor multi-path environment, including LOS and NLOS scenarios. The results show that different channel conditions pre-fer different antenna array configurations. Therefore, when the channel conditions vary, the antenna array can support antenna selection algorithms to select the best sub-set of elements to increase channel capacity.

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