Indoor IR/optical wireless communications channels have been investigated since the late 1970s,1 and subsequent research has characterized this channel up to a frequency of 400MHz.2–4 Optical wireless links can operate in two basic modes, diffuse and line of sight. Advantages of the former are robustness to shadowing and better coverage, although lower received power requires high-sensitivity photodetectors. Line-of-sight channels can provide sufficient power for industrial-grade photodetectors, despite being vulnerable to shadowing. Here, we report on the feasibility of 1Gbit/s transmission rates over a directed non-line-of-sight5 optical link.
Figure 1 illustrates our experimental setup. A network analyzer modulates the 808nm-wavelength light from the transmitting laser between 10MHz and 1GHz, and obtains amplitude and phase characteristics from the receiving avalanche photodiode (APD). The laser points toward the ceiling, where light is reflected from a diffuse spot and captured on the photodiode. Amplifiers are incorporated to provide sufficient gain to obtain measurements above the noise floor. We attached a focusing lens (2in diameter) to the APD to achieve higher optical power on the device's active area (0.2mm2). We set the transmitted power level at 50mW and placed the receiving optical elements on a mechanical assembly for pointing. We selected a total of 1601 frequencies to perform our measurements.
Figure 1. Experimental setup for indoor IR/optical wireless channel characterization. APD: Avalanche photodiode.
To obtain the channel characteristics exclusively from the transmitting and receiving elements, we performed calibration measurements by placing the laser and APD back-to-back with a known reduced power level in the linear modulation range. We used the resulting response as reference for calibration and conducted measurements at three different receiver locations. (The transmitter was kept fixed at one location.) In a 7.8×4×3m3 room, the three receiver locations were (2.54, 1.9, 0.5m), (4.14, 1.08, 0.5m), and (6.08, 1.46, 0.5m), with the transmitter residing at (2.21, 1.98, 0.5m). Figure 2 shows the resulting magnitude and phase responses.
Figure 2. (a) Magnitude and (b) phase responses of the indoor IR/optical wireless channel at the three receiving locations (pos).
The obvious result from Figure 2 is that the magnitude response is almost flat over the entire bandwidth of interest, while the phase response is linear. By varying the receiver location, the magnitude response retains its overall shape and shifts by a constant amount, corresponding to the path loss. The flatness of the channel up to 1GHz provides important verification of its ability to support data rates up to and exceeding 1Gbit/s. The approximately ideal nature of the frequency responses implies that broadband data can be received with minimal or no intersymbol interference, and simple modulation schemes, such as on/off keying, can be used to obtain high transmission rates.
We applied windowing and smoothing functions to the measured frequency responses before employing inverse Fourier transformation to obtain the corresponding impulse responses (see Figure 3). Windowing limits the impulse response to less than 1ns. At each location, the response is a positive impulse with a height corresponding to the path loss, and the peak occurring at a time point corresponding to the propagation distance. We can derive the delay spread from these impulse responses, the inverse of which is proportional to the channel's coherence bandwidth. Upon data transmission within this band, all frequencies experience similar degrees of fading and intersymbol-interference suppression by equalization is not required. We calculated the delay spreads at the three receiving locations to be within 0.4ns of each other. This closely corresponds to the 0.5ns rise time of the APD, implying that the resolution is limited by the device rather than the channel. As a first approximation, this means that the wireless optical link can support data rates well beyond 1Gbit/s without requiring complex equalization and modulation schemes.
Figure 3. Impulse responses at the three receiving locations.
To our knowledge, this is the first set of measurements for indoor optical wireless channels that demonstrates the feasibility of the highest bit rates thus far achieved with no line of sight and Lambertian reflection. The impact of our research could be transformational. Current wireless-networking techniques are based on radio frequency (RF) ranges. RF standards specify up to 600Mbit/s data transmission. An optical network with gigabit-per-second transmission capacity can be a comparatively inexpensive and energy-efficient complement or alternative to RF wireless technology. The inherent security in the physical layer of optical networks represents an added value. The drawback of connecting different rooms can be circumvented by employing a networked approach, whether through gigabit Ethernet or power lines. Optimization and fabrication of high-bandwidth and high-sensitivity miniaturized opto-electronic devices, particularly with the purpose of communications, is a prerequisite to make this technology a commercial success. This represents the direction of our future research efforts.
Mohsen Kavehrad, Jarir Fadlullah
Pennsylvania State University
State College, PA