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Indoor Message System Evaluated
April 1, 2009 By: Borje Forssell GPS World
Public and commercial services such as search-and-rescue, firefighting, and location-based services (LBS) require accurate indoor position determination. Users face the fundamental problem of knowing location at the room level, requiring accuracies on the order of a few meters. Because of the need for floor determination, vertical accuracy can be more important than horizontal.
The United States and Japan have passed emergency-call legislation requiring third-generation cell phones to have functions determining their positions. Rapidly increasing numbers of cell phones with built-in GPS capabilities stimulate the LBS market, which is expected to be the dominating market for GNSS receivers in the future.
Significant problems remain, however. Standard GPS receivers do not work well indoors, giving position errors up to several tens of meters — if a position fix is possible at all. Assisted GPS (AGPS) improves the situation, but makes equipment more complicated and expensive, while reliability and accuracy may still not suffice. Local solutions based on RFID, Wi-Fi, ultra-wideband, pseudolites, and others have been and are being tried, but with no definitive solution in sight.
The IMES Concept
The Japan Aerospace Exploration Agency, GNSS Technologies, and Lighthouse Technology and Consulting have proposed and developed an indoor messaging system (IMES) to solve the availability issue. Composed of transmitters, GPS receivers with modified firmware embedded in cell phones, and systems of servers, IMES aims to provide seamless positioning anywhere in a covered area. Hitachi Ltd., Systems Development Laboratory is also working on such a system, as shown here.
IMES uses satellite signals outdoors in the usual way, while using signals from IMES transmitters indoors, where satellite signal quality is strongly reduced. IMES signal structure is similar to that of GPS satellite signals, except for the contents of the navigation message. Thus, the same receiver can be used for both.
An IMES transmitter sends an RF signal similar to that of GPS and the Japanese Quasi-Zenith Satellite System (QZSS), giving its 3-D position, the position of the center of its cell coverage zone, or linking the receiver to a database corresponding to an identifier.
Instead of the ephemeris data, clock corrections, ionospheric parameters, and so on contained in the GPS message, the IMES navigation message periodically dispatches position broadcasts and additional information in a similar format.
The concept for seamless transition between indoor and outdoor use is built on three further requirements:
- low power consumption,
- low transmitter cost, and
- no harmful interference to other users of GPS or QZSS.
IMES assumes that positioning accuracies on the order of 10 meters will satisfy users, who just want to know where they are in rooms of moderate size, shopping areas, parking garages, and so on. Stable and reliable position information means more than high accuracy.
Signals and Codes
IMES uses the same L1 center frequency as GPS and QZSS, and the same BPSK modulation. IMES dedicated spread-spectrum codes are from the same family of Gold codes as GPS and QZSS (numbers 173–182 from the C/A-code assignment table). It is assumed that signals similar to L1C can be used in the future. These dedicated codes are re-used repeatedly but without using the same code as neighboring transmitters.
The IMES receiver uses the codes only for de-spreading the spread-spectrum modulation and as a step to decoding the navigation message, including transmitter identification. No pseudorange or time determination is necessary, because the desired position is read directly out of the navigation message. This gives a much simpler equipment architecture than when using pseudolites.
Transmitter Power
The transmitted power varies depending on the location and the distance to neighboring transmitters. Japanese radio regulations for license-freesignals require that the signal level at 3 meters from the transmitter stay below 35 µV/meter, limiting the radiated power density from IMES transmitters.
The radiated power level determines the coverage zone of a particular IMES transmitter: larger coverage implies more power. Because of the requirement for compatibility with outdoor GPS signals, IMES power levels received indoors are expected to be found in the interval between 2126 dBm and 2130 dBm at a distance of about 5 meters from the transmitter antenna.
The inventors of IMES have conducted experiments with these power limits in mind, using transmitter signal levels from 264 to 276 dBm. The former is the maximum level allowed for license-free signals. The working range of IMES is assumed to be 2–10 meters from the transmitting antenna, and 264 dBm at the transmitter means about 2120 dBm at the receiving antenna at a distance of 10 meters. Accordingly, 276 dBm transmitted means about 2132 dBm at the receiving antenna.
This latter level is close to what can be expected for a GPS satellite signal outdoors and would not create harmful interference in satellite receivers. However, the IMES concept foresees the possibility of having transmitters in many rooms in large buildings, shopping areas, and so on, which may imply that ordinary GPS receivers outdoors in such an area receive emissions from a considerable number of transmitters.
The Interference Problem
It is of crucial importance that IMES signals do not reduce performance for ordinary GPS receivers used outside the intended IMES coverage area. Because IMES uses the same L1 frequency and the same type of Gold codes as GPS satellites, the interference will be of the usual Gold-code cross-correlation type. If the received power from two different transmitters is the same, the cross correlation levels of Gold codes are between 221.8 and 223.6 dB (depending on Doppler difference) below the autocorrelation peak. The actual level of cross-correlation interference in a GPS receiver operating in an IMES environment is determined by the power received from each IMES transmitter in question and the number of these transmitters.
IMES signals are attenuated by walls, floors, roofs, and other obstacles between the transmitter and the GPS receiver. Such attenuation varies between just a few dB (window or open door) to several tens of dB (floors/ceilings of reinforced concrete). Measurements in our university building in Trondheim have given attenuation values such as 20 dB through a gypsum wall, 35 dB through a ceiling of reinforced concrete, and 66 dB through four such ceilings.
Each IMES signal received increases the cross-correlation interference level in the GPS receiver. From an interference point of view, these signals should be regarded as noise, and the resulting total variance of these noise-like signals is the sum of the variances of all the interfering signals, that is, the sum of the cross-correlations. Thus, 10 IMES transmitters in the immediate neighborhood of a GPS receiver imply a signal-to-interference ratio up to a maximum of 211 to 214 dB (referred to the autocorrelation peak), depending on the transmitted power and the attenuation situation. Obviously, in some situations, ordinary reception, and above all, acquisition of GPS L1 C/A-code signals can be blocked or at least severely hampered. In addition, a portable IMES transmitter would make a good short-range GPS jammer. Using new GPS signals such as L2C with better cross-correlation margins would reduce the problem.
Conclusions
IMES represents a new idea in the field of position determination in confined areas. However, the advantages should not be gained at costs of ordinary GPS/QZSS users. For this reason, transmitted power levels should be set lower than those indicated here and as proposed by the system developers, to reduce the probability of harmful interference. Because of the increasing availability of high-sensitivity receivers, it is possible to obtain the wanted performance with lower power levels.
The IMES group seems to be aware of this, conducting various experiments and analyses for optimum setting and design. We have to wait for final results, but received power levels around 2140 dBm (10 dB below GPS satellite signals) might still do the job. In all cases,
IMES transmission levels must be adapted to the local situation. We must keep in mind, however, that IMES reception itself is vulnerable to external interference (including signals from GPS satellites), so transmitted power cannot be arbitrarily low.
References
Satoshi, K. et al.: “The concept of the Indoor Messaging System.” The European Navigation Conference ENC-GNSS, Toulouse, France, April 2008.
Manandhar, D. et al.: “IMES for mobile users. Social implementation and experiments based on existing cellular phones for seamless positioning.” Proceedings of the International Symposium on GPS/GNSS 2008, Tokyo, Japan, November 2008.
Spilker Jr., J.J.: Ch. 3 in “Global Positioning System: Theory and Applications.” Progress in Astronautics and Aeronautics, Vol. 163, American Institute of Aeronautics and Astronautics, 1996.
Forssell, B.: “GPS/GNSS indoors: Possibilities and Limitations.” Presentation at the GPS/GNSS Seminar of the Swedish National Survey, Gävle, Sweden, March 2005.
BÖRJE FORSSELL is professor of navigation in the Department of Telecommunications at the Norwegian University of Science and Technology in Trondheim, Norway.
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