Power Angle Profile Measurements and Capacity Evaluation of a SIMO System at 60 GHz Nektarios Moraitis, Philip Constantinou
Demosthenes Vouyioukas
Mobile Radiocommunications Laboratory National Technical University of Athens Athens, Greece e-mail:
[email protected]
Dept. of Information and Communication Systems Engineering, University of the Aegean Samos, Greece e-mail:
[email protected]
Abstract—This paper presents an experimental power angle profile measurement procedure, as well as theoretical capacity results of a single-input-multiple-output (SIMO) system which operates in the millimeter wave band and especially at 60 GHz. Power angle profiles (PAPs) were measured in different indoor locations while detailed angle-of-arrival (AoA) shape factors describing the spatial properties of the millimeter wave channel were derived. The measurement results were related to the sitespecific environments and showed correlation between the propagation environments and the multipath channel structure. The 10% outage capacity was found theoretically between 5.7 and 6.2 b/s/Hz for a 1 × 4 SIMO system, and in the range of 4.3 and 4.6 b/s/Hz for a 1 × 2 SIMO system. Keywords-millimeter wave band; outage capacity; power angle profile; single-input-multiple-output.
I.
INTRODUCTION
The millimeter wave band and especially the band around 60 GHz, is of special interest for high data rate Wireless Local Area Networks (WLANs) foreseeing rates up to 155 Mb/s [1], [2]. Furthermore, in the same frequency band the Wireless Personal Area Networks (millimeter-wave WPANs) are emerging rapidly and will allow very high data rate, over 2 Gb/s applications such as high speed internet access, streaming content download (video on demand, HDTV, home theater, etc.), real time streaming and wireless data bus for cable replacement [3]. The exploitation of the large amount of spectral space in the millimeter wave band in combination with multiple element antennas at the transmitter or receiver is expected to offer high transmission data rates in order to provide enhanced broadband services. In this paper, we focused in two specific goals. The first goal was to measure the channel multipath structure, separating the multipath components based on their angle-of-arrival (AoA). Using a measurement system that provides an angular resolution better than 3o, we recorded the power angle profiles (PAPs) and extracted the spatial channel characteristics thus comparing them with the existing results. Measurements were conducted in line-of-sight (LoS) and in non-LoS (NLoS) locations so as to relate the spatial properties of the channel with different radio link conditions. The second goal was to theoretically evaluate the channel capacity of a single-input-multiple-output (SIMO) system [4], utilizing uniform linear array antennas (ULAs) at the receiver terminal. Rather trying to evaluate multiple-input-multiple-
output (MIMO) systems using a large number of antennas at both ends, this study is based on a SIMO case. This approach, though simpler, will nevertheless enable us to characterize the directional radio channel at the user terminal with a constant link to physical considerations. By introducing the measured PAPs (received signal at a specific angle of arrival) into a physical channel model we evaluated the capacity of a SIMO system with different elements at the receiver antenna. The remainder of this paper is organized as follows. Section II presents the experimental setup for the power angle profile measurements, the measurement procedure as well as the results in reference with angle of arrival. In Section III, we present the capacity evaluation procedure, which based on the angular measurements. Finally, Section IV is devoted to the conclusions summarizing this work. II.
POWER ANGLE PROFILE MEASUREMENTS
A. Measurement Environment The measurements took place in a typical office environment, as illustrated in Fig. 1. The two fixed transmitter positions and the 4 chosen receiver locations as well as the main furniture equipment (desks, workbenches and book closets) are indicated. As it is evident, dark shaded colors represent the furniture (wooden or metal closets and workbenches) that are above the direct path between the transmitter (height 1.6 m) and the receiver (height 1.6 m) and which could potentially block the signal propagation (apart from the partitions). Lighter colors depict the furniture surfaces (e.g., desks) that do not obstruct the direct signal propagation. The closets are of 2 m height, the workbenches of 1.4 m height and the desks (including the computers) of 1.15 m height. Surface A is an external wall with windows in consecutive order, which are separated by concrete pillars. Each window is made of a 5 mm glass with aluminum frame. The windows have metallic window shades in front, which were pulled down during the measurements. Surface B is an internal thick wall made of brick and fronted with plaster and paint on both sides. The total wall thickness is 23 cm. The floor is made of concrete and covered with marble and a thin antistatic plastic layer. The ceiling is made of concrete and its total height is 3.4 m. Approximately 60 cm below the ceiling a metal frame structure suspends, holding the fluorescent light tubes. The Partition is made of a 5 mm glass with aluminum studs every 1.5 m. The internal doors consist of 5 mm glass with aluminum frame.
Figure 1. Measurement environment and superimposed the derived Power Angle Profiles.
During the measurements, the internal doors were closed. The wooden closets are 42 cm thick and made of 1.5 cm wooden chipboard covered with melamine and 5 mm glass as a front cover. Similarly the metal closets are 36 cm thick and consist of 3 mm galvanized steel with 5 mm glass as a front cover. However, we should mention, that the actual total true material thickness, which the signal penetrates, is 2 cm for the wooden and 8 mm for the metal closet. The indicated whiteboard is made of wooden chipboard (1.5 cm) covered with melamine. B. Measurement System and Procedure The measurements were conducted by transmitting a continuous wave (CW) signal at 60 GHz, from a fixed base station to a fixed receiver, recording the signal envelope as a function of time. Details for the measurement setup can be found in [2]. The transmitter was fixed at 1.6 m above the floor, at two different locations (Tx1 and Tx2), as shown in Fig. 1, whereas the output power was +10 dBm. Positions LoS1 and NLoS-1 were measured from the first transmitter location (Tx1), whereas the positions LoS-2 and NLoS-2 were measured from the second location (Tx2). The receiver hardware was located on a trolley, which was stationed at the measurement position. After amplification, the received signal was down-converted to 300 MHz IF and inserted to a commercial receiver. The input to the automatic gain control (AGC) of the receiver was then sampled at 2 kHz and the data values were stored to a portable PC. The recording time was 10 s. The noise figure of the receiver was 7 dB and the limit input sensitivity -90 dBm. The total losses (from cables, connectors etc.) were 6.5 dB. For this measurement procedure, a biconical antenna with 0 dBi gain was used as the transmitter antenna, having an omnidirectional pattern in azimuth and an elevation of 38o. A vertical polarized horn antenna with 38 dBi gain was
used at the receiver. The half power beamwidth of the horn antenna was 2.5o in azimuth and 2.2o in elevation. When a highly directional antenna is used, the system provides high spatial resolution in order to resolve multipath components with different angle of arrivals (AoAs). During the measurements, a mechanically steered directional antenna was used to resolve multipath components. An automated stepmotor system was used to precisely position the receiver antenna along a linear track and then rotate the antenna in the azimuthal direction. The direction of the linear track was always heading along the direct path between each transmitter and receiver. The receiver antenna was mounted at 1.6 m above the floor. At each position, the receiver antenna was rotated clockwise in an azimuth level from 0 to 360o with a step of 3o and envelope amplitude was recorded at each of the 120 angular steps. Hence, each recorded power angle profile is a function of space, angle and time b( xi , φ j , tk ) with i = 1…4, j
= 1…120, and k = 1…20000. From the measurement results, a local average is derived, which is calculated at 4 different positions alongside the linear track, being 5λ apart, according to the equation:
b (φ ) =
1 4
⎛ 1 20000 ⎞ b( xi , φ j , tk ) ⎟ ⎜ ⎜ 20000 ⎟ x =1 ⎝ k =1 ⎠ 4
∑
∑
(1)
The local average helps to remove any residual small-scale or time-varying fading that may occur at individual positions [5]. The precisions of the track and spin positions are better than 1 mm and 1o, respectively. Finally, the measurements were conducted with no personnel activity in the area. Therefore, the channel may be regarded as physical stationary.
C. Measurement Results For each one of the four measured positions the average PAP was extracted according to (1). The final PAPs are indicated in Fig. 1 and are superimposed on the ground plan. The measured PAPs are imported to the site map, in order to identify the origin of the multipath components. Fig. 1 depicts in solid lines the direct path (either LoS or NLoS) between the transmitter and the receiver. Dashed lines depict the main reflections from the furniture equipment. As we observe in Fig. 1, the shape of each PAP is affected significantly by the presence of the furniture, since the main reflections derive from the wooden closets. Then, at each receiver position the AoA shape factors were calculated characterizing the directional distribution of multipath power [5]. The extracted AoA shape factors are the angular spread Λ, the angular constriction γ, the maximum fading angle φmax , and the maximum AoA direction, given by the following expressions [5]:
Λ = 1−
γ =
φmax =
F1
2
F0
2
(2)
F0 F2 − F12 F0
2
− F1
(3)
2
{
1 Phase F0 F2 − F12 2
}
(4)
2π
∫
b (φ ) exp( jnφ )dφ
Angular constriction γ ranges from 0 to 1, with 1 indicating multipath energy equally distributed in two directions. In our case γ was found 0.39 and 0.42 in LoS cases, and 0.27 and 0.31 in NLoS positions. The values are lower than those reported in [5], a fact indicating that the multipath power is not equally distributed in both directions, which becomes evident if one observes Fig. 1. The differences with the existing literature are again attributed to the presence of the furniture in the propagation channel, with materials that enhance the reflections of the propagated signal. Finally, the K-factor was calculated for each measured location, according to the procedure found in [6], from 20000 time samples of received amplitude, taking the mean received angular amplitude from the 120 angular steps and then the four mean values alongside the linear track. The experimental results, presented in Table I, are consistent with the values found in [2]. III.
where:
Fn =
multipath reflections around the receiver. Differences were also observed in NLoS cases, where lower values of Λ were obtained than in [5]. These are attributed to the environment and the large attenuation introduced by the partition and the whiteboard (6 and 11.6 dB [2]), which resulted in the reduction of very strong reflections greater than ± 45o in respect to the direct component, as it is clearly shown in Fig. 1. Hence, in our case, Λ exhibits lower values in NLoS than in LoS cases, whereas in [5], LoS and NLoS average room values of Λ were comparable.
(5)
0
and Fn (n =1 or 2) is the n-th Fourier complex coefficient of b (φ ) , which is the measured average PAP. The angular spread characterizes the angular distribution of the multipath power, the angular constriction indicates how multipath components are distributed along two directions, and the maximum fading angle shows the direction of the receiver motion required to achieve maximum fading rate. Fig. 2 depicts the orientation of the antenna elements. Finally, maximum AoA provides the direction of the multipath component with the maximum power. All the aforementioned results together with the total received power for each one of the measured positions are summarized in Table I. The derived results of Λ, in LoS room cases, are higher compared with the average room values in [5], indicating that the multipath comes from multiple directions (ΛÆ1). This is attributed to the heavy cluttered environment where a lot of furniture is present in the propagation channel. Furthermore, the metal closets, as well as the wooden closets and desks, which have very reflective surfaces (covered with melamine), significantly increase the
CAPACITY EVALUATION
In order to calculate the capacity of a SISO system with one antenna element at both terminals, it is necessary to calculate the channel response, h(t), between the transmit and the receive antenna. The time-varying channel impulse response of the measured power angle profiles considering a narrowband system can be described by [8], [9]:
h (t , φ ) =
L −1
∑ b (t )e θ δ (φ − φ ) l
j
l
(6)
l
l =0
where bl(t) is the time-varying received power of each arrival at a specific angle φl . The e jθl term represents a statistically independent random phase associated with each arrival, where θl is uniform [0, 2π). In case of a SIMO system, with one transmit and MR receive antenna elements, the channel vector h has to be calculated [7]. In this study, we assume ULA antennas at the receiver terminal where the elements are positioned along a single axis with Δl spacing and vertical polarization. We presume that at each position the elements are fixed vertical to the direct path between the transmitter and receiver, as shown in Fig. 2. For the specified antenna geometry the array response vector at the receiver is given by: 2π 2π ⎡ −j Δl sin φR −j Δl ( M R −1)sin φR ⎤ ⎥ a R (φR ) = ⎢1, e λ , ... , e λ ⎢⎣ ⎥⎦
Τ
(7)
TABLE I. Tx 1
2
ANGLE OF ARRIVAL RESULTS
Rx
dTx-Rx [m]
Pr [dBm]
K [dB]
Λ
γ
φmax [o]
max{ΑοΑ} [o]
LoS-1
6.1
-32.4
12.4
0.85
0.39
329.7
2
NLoS-1
7.3
-52.7
5.4
0.49
0.31
15.2
6
LoS-2
5.8
-30.6
11.7
0.87
0.42
35.1
1
NLoS-2
5.2
-49.4
4.6
0.55
0.27
347.3
0
where λ is the wavelength and φR is the azimuth angle, with respect to the antenna broadside of the impinging at the Rx array propagation path arriving at the receive antennas.
channel multiplied by the receiver response vector. Therefore, if we know the amplitude b for the specific AoA ( φ ), the so called Power Angle Profile of a SISO channel, then we can calculate the channel impulse response, h of a SIMO channel using (7) and (8). The PAP of a SISO channel can be yielded by PAP measurements between fixed transmit and receive terminals, as in our case. Finally, after calculating the channel vector h, we can easily estimate the channel capacity as a function of the averaged signal-to-noise ratio (SNR), assuming a channel unknown to the transmitter, according to the expression [7]: MR ⎧⎪ ⎛ ⎞⎫ 2 ⎪ CSIMO = Ε ⎨log 2 ⎜1 + ρ hi (t ) ⎟ ⎬ ⎜ ⎟⎪ ⎪⎩ i =1 ⎝ ⎠⎭
∑
Figure 2. Indicative placement of the elements at each received point.
Vector h ∈ M R ×1 can be obtained by the matrix representation, which in case of a ULA configuration is given by the following relationship:
where ρ is the averaged SNR at each Rx branch and Ε {} ⋅ denotes time average. A condition for applying (10) is combination per maximal ratio combining (MRC) rule from the receiver antenna elements. The channel vector h is normalized MR
⎡ b (φ1 , t ) ⎤ ⎢ ⎥ ⎢ b (φ2 , t ) ⎥ ⎢ . ⎥ h SIMO (t ) = ⎡⎣a(φR,1 ) … a(φR, L ) ⎤⎦ ⋅ ⎢ ⎥ ⎢ . ⎥ ⎢ . ⎥ ⎢ ⎥ ⎣⎢b (φL , t ) ⎦⎥
so that
(8)
array response vector for the l-th path, and b (φ , t ) contains the amplitude of each propagation path spatially averaged according to the equation: 1 4
4
∑ b( x , φ , t ) i
∑h
i
2
= M R . The capacity is referred as the error free
i =1
where L is the number of propagation paths calculated by the PAP at each position (hence L = 120 in our case), a(φR ,l ) is the
b(φ , t ) =
(10)
(9)
x =1
Hence, from (8) if we know the azimuth angle of arrival (AoA) of each one of the L propagation paths, the antenna elements and their spacing, we can calculate h. The SIMO channel representation given by (8), can be regarded as a SISO
spectral efficiency, or the data rate per unit bandwidth that can be sustained reliably over the channel. We consider that the receiver antenna comprised of either two (MR = 2) or four (MR = 4) elements, apart from the basic SISO configuration. At a given location and SNR (10 dB), we select different element spacing from λ/2 up to 10λ. It was found that the capacity increases gradually from λ/2 to 4λ and it was uncorrelated with respect to Δl for values between 4λ and 10λ. Hence, for the rest of the calculation procedure, the selection of the element spacing was equal to 4λ. The cumulative distribution functions (CDFs) of the channel capacity for 10 dB SNR, which derived from the aforementioned procedure, are illustrated in Fig. 3. There is an obvious increase in the capacity when the number of the elements is increased reaching in average from 4.5 b/s/Hz up to 6.0 b/s/Hz (10% outage capacity). Furthermore, the capacity in LoS positions is 0.2-0.5 b/s/Hz greater than in NLoS positions. The 10% outage capacity was found between 5.7 and 6.2 b/s/Hz for a 1 × 4 SIMO system, and in the range of 4.3 and 4.6 b/s/Hz for a 1 × 2 SIMO system.
system, and in the range of 4.3 and 4.6 b/s/Hz for a 1 × 2 SIMO system. V.
FUTURE WORK
Further work is under process statistically characterizing the radio channels and moreover modeling the angle of arrival characteristics for LoS and NLoS conditions, thus evaluating the performance for indoor environment at the millimeter wave band. REFERENCES [1]
[2]
Figure 3. Potential capacity of a SIMO channel at 60 GHz utilizing uniform linear array antennas.
IV.
CONCLUSION
A measurement indoor campaign was performed in the frequency of 60 GHz so as to characterize the indoor multipath channel in space and define valuable site-specific prediction techniques. The derived AoA shape factors and multipath parameters were related to the site-specific environments and analytical processing showed a correlation between the propagation environment and the multipath channel structure. Our results were found to differ from the existing literature due to the heavy cluttered propagation environment, providing valuable results due to the uniqueness of the millimeter-band channel. Using the measurements multipath results of AoA and the derived PAPs, the capacity evaluation of a SIMO system was evaluated with either two or four receive elements. The channel capacity was calculated for a SIMO system assuming ULA elements, showing significant improvement as the number of the elements increases. The 10% outage capacity was found between 5.7 and 6.2 b/s/Hz for a 1 × 4 SIMO
[3]
[4]
[5]
[6]
[7] [8]
[9]
H. Yang, M. H. A. J. Herben, and P. F. M. Smulders, “Impact of antenna pattern and reflective environment on 60 GHz indoor radio channel characteristics,” IEEE Trans. Antennas Propag. Lett., vol. 4, pp. 300303, 2005. N. Moraitis, and P. Constantinou, “Indoor channel measurements and characterization at 60 GHz for wireless local area network applications,” IEEE Trans. Antennas Propag., vol. 52, no. 12, pp. 3180-3189, Dec. 2004. IEEE 802.15 WPAN Millimeter Wave Alternative PHY Task Group 3c (TG3c) [Online]. Available: http://www.ieee802.org/15/pub/TG3c.html, 2007. M. Shafi, D. Gesberd, S. Da-shan, P. J. Smith, and W. H. Tranter, “Guest editorial: MIMO systems and applications. 1,” IEEE J. Select. Areas Commun., vol. 21, no. 3, pp. 277-280, Apr. 2003. H. Hu, V. Kukshya, and T. S. Rappaport, “Spatial and temporal characteristics of 60-GHz indoor channels,” IEEE J. Select. Areas Commun., vol. 20, no. 3, pp. 620-630, Apr. 2002. L. J. Greenstein, D. G. Michelson, and V. Erceg, “Moment-Method Estimation of the Ricean K-Factor,” IEEE Commun. Letters, vol. 3, no. 6, pp. 175-176, June 1999. A. Paulraj, R. Nabar, and D. Gore, Introduction to space-time wireless communications, Cambridge University Press, 2003. Y. Shoji, H. Sawada, C. Choi, and H. Ogawa, “A modified SV-model suitable for line-of-sight desktop usage of millimete-wave WPAN systems,” IEEE Trans Antennas Propag., vol. 57, no. 10, pp. 2940-2948, Oct. 2009. Q. H. Spencer, B. D. Jeffs, M. A. Jensen, and A. L. Swindlehurst, “Modeling the statistical time and angle of arrival characteristics of an indoor multipath channel,” IEEE J. Select. Areas Commun., vol. 18, no. 3, pp. 347-360, Mar. 2000.
