This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
The propagation of radio waves into and within buildings at 900 and 1800 MHz frequencies have been undertaken using many faculty buildings at NEU in the Turkish Republic of Northern Cyprus (TRNC). Furthermore, the propagation for the cases into and within elevator has also been investigated.
The main emphases of this article are that the radio transmission modeling into buildings which are dependent on the measured penetration loss values and can be used in adjusting the propagation models developed for the outside areas, and also in modeling of radio transmission within buildings.
In planning and constructing a mobile communication network, we must take into consideration the features of the electric waves so to define the frequency band, frequency allocation, radio coverage, communication probability calculation, electromagnetic interference between systems and final parameters of radio devices. The keystone for system design is the efficient use of frequency spectrum and EMC (Electronic Magnetic Compatibility).
It is well-known that the radio wave can be transmitted from the transmitting antenna to the receiving antenna in multiple modes: forward wave or free space wave, earth wave or surface wave, troposphere reflecting wave, and ionosphere wave.
As far as electronic propagation is concerned, the simple method between transmitters and receivers is free space propagation. Free space refers to isotropy (identical in axes characters) and uniformity (even texture) in such zone. Other names for free space are forward wave or stadia wave. Forward wave transmits along straight lines, so that it can be used for communication between satellite and exterior space. In addition, this definition is also used for stadia propagation in land (between two microwave towers).
Hence, it is necessary to review these technologies in order to gain a good overview of all types of mobile radio networks, and not just the ubiquitous mobile phone technology market.
In the present article we will investigate some radio networks and technologies used in order to make implementation for them. This will include older system types because many operators still use them; indeed these networks are continually being modified and in some cases still rolled out. Often it is because of the time taken to agree and introduce new systems for worldwide applications.
This article identifies all these problems particularly the path loss propagation models. Also, it shows how our model is efficient, and explains how the practical experiments were done.
II. Path Loss Measurements
Into Building Experiments
The tests were undertaken using a fixed base station transmitter and a mobile receiver. The signal transmitted from the base station was received using a purpose-built data logging system which was moved around the building. The base station consisted of a CW transmitter feeding a collinear antenna, raised clear of local obstructions. It produced an effective radiated power (ERP) of 29 dBm at 900 MHz and 24 dBm at 1800 MHz. A vertically polarized omni-directional antenna was also used at the mobile, with a ground plane 1.65 m above the floor.
Each sample of data collected in a particular room in the building was normalized by the average signal strength within that room. The normalized data for each room were then collated to form a data file consisting of fast fading only. The distribution of this component describes the small-scale signal variations. The local mean was estimated by averaging the signal strength over 200 samples symmetrically adjacent to every point (i.e., the process known as moving average). The large-scale signal distribution was determined by testing the departure (in decibels) of the average signal strength of each room from the average signal strength for the whole building. The inside average signal strength was determined for each room of the buildings measured. Outside signal strength was measured at street level around the perimeter of the building, along the closest available path to the buildings outside walls.
Fifteen tests were conducted at NEU in Turkish Republic of Northern Cyprus (TRNC) in order to assess the effect of transmission condition on signals propagating into buildings at 900 and 1800 MHz in different buildings of the university to examine the values of penetration loss at ground-floor level at 1800 MHz only. The two distinct regions where penetration loss measurements, at ground-floor level, took place can be characterized as a highly built-up area i.e., the Faculty of Medicine (FOM) and a medium built-up area i.e., the Faculty of Engineering (FOE). Two different locations for the transmitter were selected for each area. For the first set of trials (i.e., at FOM), the transmitter was located on the roof of the FOM (TX1), which is approximately 5 floors in high. For the second set of measurements in the FOE which is 4 floors in high including Innovation and Information Technologies Center (IITC), as shown in Figure 1, the transmitter was set up on the roof of the FOE (TX2), which is approximately 20 m in high. Fifteen experiments were conducted for each transmitter location.
The 10 buildings selected in the university campus precinct for the 20 penetration loss tests are described for 1800 MHz in Table 1, and their relative positions of buildings appear in Figure 1, 1800. The description of FOE is as follows: four floors, 20 m in high, floor area of 600 m2, steel-framed construction with offices, large laboratories, lecture and research halls, 2Ã-3 glass windows, overlooking to the FOM, and large glass entrance. The offices on all floors of all buildings were crowded with typical office furniture and teaching or research laboratories contained experimental equipment according to the specialized demand of each field of study, and NEU Techno Park which is the complex that will host the joint venture between NEU and IBM (the IITC). The aim of IITC is to carry out research, developments and innovations. The IITC is the only one found in the region and this includes the following regions collectively. Eastern Europe, the Middle East, the Central Asia and the Northern Africa. The new supercomputer with its advance platform is ranked 76th in the world, 13th within certain nations and is ranked 10th amongst the other universities in the world.
b) Penetration Loss at Ground-Floor Level
The mean signal levels outside and inside buildings at ground floor level and the mean value of penetration loss for the two experiments carried out in the university are presented in Tables 2 and 3.
In the experiments conducted in the university, the average values of penetration loss at ground-floor level were found to be significantly different (20.39 dB and 17.4 dB) for the two transmitter locations. This difference (i.e., approximately 2.99 dB) was due to the Internet receiver dish mounted on the roof of the FOE. The two sets of field's trials yielded an average value of penetration loss equal to 18.8957 dB.
There were important changes in the relative position of the transmitter concerning the measured buildings were changed.
However, it is necessary to remember that the validity of most outdoor propagation models, such as those of Okumura , Hata  and Ibrahim [4, and 5], have been developed for large cells, whereas for personal communication the suitable cell diameter is often taken less than 700 m.
Therefore, those models cannot be fully trusted when used for the indoor environment without further investigations. In addition, predicting first the signal outside the building of interest and then, from that result, determining the signals inside the building leads to an inevitable reduction in accuracy. Therefore, prediction of the path loss for radio transmissions into buildings may be more accurate if it has been undertaken directly and not merely as an extension of outdoor propagation models. A similar approach was adopted by Barry and Williamson  to analyze measurements undertaken in New Zealand at 851 MHz. Toledo and Turkmani  have obtained direct modeling of propagation into buildings at 900, 1800, and 2300 MHz. The latter have performed the prediction using information based on what might be termed incomplete data. Such information may have an element of uncertainty and a risk of being incorrect. Using appropriate statistical techniques, it was possible to generalize from a given set of data to a more broadly applicable statement, and for that purpose specific and rigorous techniques have been applied in order to estimate the degree of uncertainty. Details of the statistical techniques applied can be found in  and a full description in . Propagation into (and within) buildings involves a more complex multipath structure than that of the outdoor land-mobile radio channel, which is dependent on path length, effective base station antenna height, and the environment local to the mobile. In addition to these variables, indoor propagation is also affected by other empirically observed variables such as building structure and layout of the rooms. After collating all the survey measurements in the university precinct buildings and investigating the relationships between a large number of variables, the best of all results, for the into building case, was obtained when three variables were present in the regression the logarithm of the distance, , the logarithm of the floor area, , and the number of building sides seen by the transmitter on each floor of the building housing the receiver, . The resulting models for the path loss are given by
for 900 and 1800 MHz, respectively. The root mean square errors RMSE = 2.07 .
c) A look at Elevator's Hardwares
The isolated platform load weigher is recommended for installations with isolated platform elevator cars that require anti-nuisance, lobby dispatching, load bypass and/or overload. Pretorquing is available for IMC PERFORMA, IMC-SCR and IMC-MG controls. The load weigher consists of an inductive proximity switch and an amplifier. The amplifier output is connected to the machine room via two conventional wires (special wiring is not required). The output circuit is virtually impervious to damage from transients or accidental connection to voltages up to 120 VAC. A controller-mounted input buffer board and software are required in order to process the signal from the load weigh system. The proximity switch and amplifier shall be mounted either under the car (preferred position), or on top of the car. When mounted under the car, a voltage signal is generated that is inversely proportional to the distance between the bottom of the car floor and the isolated platform frame. When mounted on top of the car, a voltage signal is generated that is proportional to the distance between the crosshead and the top of the cab.
Electrical requirements: Input 120 VAC, single phase 50Hz/60Hz, Output 10mA @ 18VDC.
A crosshead deflection load weigher is required for installations with non-isolated platform elevator cars. The load weigher consists of load sensor(s), amplifier(s) and a buffer board. The buffer output is also connected to the machine room via two conventional wires (special wiring is not required). The sensor(s) is mounted to the crosshead to measure deflection as the elevator is loaded. The voltage signal generated is directly proportional to the deflection of the crosshead. The amplifier(s) and buffer board are mounted on the cartop.
Different communication networks can be used to allow an Embedded Monitoring Interface (EMI) to communicate with the CMS station. The most popular means of communication are phone lines using modems, hardwiring using line drivers or Ethernet with built in device servers installed in the controls. Device servers require a 10 Base-T connection to a computer network supporting TCP/IP protocol.
It is generally believed that AC inverter drives are the ideal technology providing maximum power savings, reduced motors cost and lower maintenance costs. AC inverter drives have tradeoffs that need to be recognized and understood. These tradeoffs (potential drawbacks) include greater harmonic distortion, radio frequency interference and other idiosyncrasies that can make typically used AC drives unfriendly.
In most instances, new construction design can consider these issues; however, elevator modernization in existing buildings requires thoughtful consideration. It is important to have a basic understanding of the tradeoffs that are determining factors in the drive selection process. MCE Technical Publications "Harmonic Analysis and Comparison" and "Motor Generator vs SCR" explored considerations for drive selection for a particular elevator control application.
Issues addressed were applied to all static drives, including the typical AC inverter drive. AC inverter drives can produce sufficient amounts of Radio Frequency noise (RFI) that affect the operation of equipment susceptible to Radio Frequency noise. This is particularly true in older buildings when grounding is lacking or otherwise inadequate.
The experiments were conducted in order to determine statistics related to the random variation of a continuous wave (CW) signal received in indoor environments [7, 8, 1]. Empirical models which allow the path loss between the transmitting and receiving antennas to be predicted have also been developed and are presented in this work. This additional loss mainly depends on a large number of factors with various degrees of importance. Among them are the transmission frequencies, the distance between the transmitter and receiver, the building construction material, and the nature of the elevators. Several researchers have studied the problem of receiving radio signals inside buildings and model it as the distance dependency of the path loss when the mobile is outside a building, plus a building loss factor. The building loss factor is included in the model to account for the increase in attenuation of the received signal observed when the mobile is moved from outside a building to inside. This model of path loss propagation loss was first proposed by Rice  in 1959, and used in most subsequent investigations, De Toledo  in 1998, recently, this model is applied within NEU campus in 2009 . The model is also modified and extended to include elevators in the NEU. In addition to penetration loss, system designers are also interested in learning about the received signal variability and the effects of building height, conditions of transmission, construction materials, and frequency of operation. Several research activities that deal with these aspects have been reported in the literature [7, 1, and 9 - 11]. However, so far, works done in these fields' elevators have not been taken into their considerations for calculating the path loss of signal.
d) Experiments Including Elevators
The tests were undertaken using a fixed base station transmitter and a mobile receiver. The signal transmitted from the base station was received using a purpose-built data logging system, which was moved into the elevator. The base station consisted of a CW transmitter feeding a collinear antenna, raised clear of local obstructions. It produced an effective radiated power (ERP) of 29 dBm at 900 MHz and 24 dBm at 1.8 GHz. A vertically polarized omni-directional antenna was also used at the mobile, with a ground plane 1.65 m above the floor. Each sample of data collected in a particular floor in the elevator was normalized by the average signal strength within that floor. The normalized data for each floor were then collated to form a data file consisting of fast fading only. The distribution of this component describes the small-scale signal variations. The local mean was estimated by averaging the signal strength over 32 samples symmetrically adjacent to every point (i.e., the process known as moving average) as shown in Figure 2. The large-scale signal distribution was determined by testing the departure (in decibels) of the average signal strength of each floor from the average signal strength for the whole building. The inside average signal strength was determined for each floor of the measured elevator. The outside signal strength was measured at street level around the perimeter of the building, along the closest available path to the elevator's isolated walls.
Fifteen tests were conducted in the NEU of Cyprus in order to assess the effect of transmission condition on propagation of signals into elevator at 900, and 1800 MHz in two buildings at the NEU campus to examine the values of penetration loss at ground-floor level with 1800 MHz only. The two distinct regions where penetration loss measurements, at ground-floor level, took place can be characterized as a highly built-up area (i.e., the FOM) and a medium built-up area. Two different locations for the transmitter were selected for each area. For the first set of trials, the transmitter was located on the roof of the FOM (TX1), which is five floors high. For the second set of measurements in the Faculty of Dentistry FOD which is also 5 floors high as shown in Figure 3, the transmitter was set up on the roof of the DD (TX3), which is approximately 25 m high. Fifteen experiments were conducted for each transmitter location.
The ten buildings selected in the NEU campus precinct for the 20 penetration loss tests are described in Tables 4, and Table 5, respectively. Their relative positions are shown in Figure 3 using 1800 MHz are described in Tables 4 and 5 respectively; the relative positions of the buildings are presented in Figure 4. The FOD, five floors, 25 m high, floor area of 900m2, steel-framed construction with offices, large laboratories, lecture and research halls, 2Ã-3 glass windows, overlooking to the FOM, and large glass entrance. Offices on all floors of all buildings were crowded with typical office furniture and the teaching or research laboratories contained experimental equipment according to the specialized demand of each area of study, and The NEU Techno Park which is the complex that will host the joint venture between NEU and IBM (NEU IITC).
In our experiments conducted in the NEU campus, the average values of the penetration loss at ground-floor level were found to be significantly different (20.39 dB and 17.4 dB) for the two transmitter locations. The difference of approximately 2.99 dB was due to the Internet receiver dish mounted on the roof of the Engineering department. The two sets of field's trials yielded an average value of penetration loss nearly 18.8957 dB .
In the NEU there were important changes in the relative position of the transmitter concerning the changeable buildings measured. However, it is necessary to remember that the validity of most outdoor propagation models, such as those of Okumura , Hata  and Ibrahim [4, and 5] have been developed for large cells, whereas for personal communication the suitable cell diameter is often less than 700 m. Whereas, the experiments conducted within the elevators and outside it while its moving from floor to another and while its standby in each floor from the mentioned ones and the data taken from each floor within and outside the Elevators while the transmitter was placed on the roof of the building, were listed below in Tables 6 - 8, respectively.
Therefore, those models cannot be fully trusted when used for the elevator environments without further investigations. In addition, predicting first the signal outside the elevator of interest and then, from that result, determining the signals inside the elevator leads to an inevitable reduction in accuracy. The separation distance between each floor is almost 37cm, since this distance is completely isolated by concrete without any part of window or door for the propagation. Therefore, prediction of the path loss for radio transmissions into elevator may be more accurate if it has been undertaken directly and not merely as an extension of outdoor propagation models. Propagation into (and within) buildings involves a more easy multipath structure than that of the elevator land-mobile radio channel, which is dependent on path length, effective base station antenna height, and the environment local to the mobile. In addition to these variables, inside the elevator propagation is also affected by other empirically observed variables such as elevator structure and layout of the space separating the elevator and the building.
After collecting all the survey measurements inside the NEU buildings, we investigate the relationships between a large number of variables. The best results, for the into building case was obtained when three variables were present in the regression (see Eqs. 1 and 2).
In referring to the Alexander et al model , Toledo and Turkmani [6, and 7] selected a number of other variables for the within building path loss models. The resulting general models within multistory buildings, at 900, and 1800 MHz, were found to be
where the RMSE values determined are 11.6 dB at 900 MHz, and 10.9 dB at 1800 MHz, respectively.
The above equations contain only the variables that have mostly contributed to the path loss and can be described as follows: represents the distance between transmitter and receiver; is the number of floors separating transmitter and receiver; the variable , also called sight, represents the amount of signal leaving and returning to the building, complemented with some considerations on the ability of the signal to propagate on the floor where the transmitter was located; represents the tendency exhibited by the signals to be higher on the first two floors of a building; and is the floor area. In addition, some other variables had either a positive or negative influence, or even no effect at all on the signal strength.
Therefore, high values of RMSE and poor coefficients of determination existed in the final general models. It can be also observed that different conditions of signal transmission and reception are more important in determining the values of each model coefficient than the frequency variation. Nevertheless, better predictions are possible in buildings where the features and transmission conditions are well known because; particular models can be applied yielding, smaller errors.
The weakness of the global models can be explained by the random spread of the coefficients when all experiments were put together, thereby decreasing the importance of the added variables in the global model. Furthermore, the conclusion that some of the variables considered in the models influenced the values of the path loss more than any other variable is not necessarily always correct, especially if the models are to be used in environments which are completely different from that considered in this study.
Therefore, it must be emphasized that there is still a great need for additional narrowband measurements over different types of building, and further statistical modeling should be performed as elevator which is important in real life. Here our proposed models include the elevator and also taking into consideration the selected number of other variables for within building path loss. The resulting general model for path loss within elevators at 1800 MHz takes the form:
where the determined RMSE value was 6.89 dB at 1800 MHz.
The above equation contains only one new variable which is the elevator factor that has mostly contributed to the path loss.
Our model can always be applied when at least the distance and number of floors between transmitter and receiver are small.
3. Calculations on the Propagation Path Loss
3.1 Radio Wave Propagation
The theory of EM propagation that predicts the existence of radio wave is formulated in . The propagation of EMW due to many factors such as reflection, scattering, diffraction by wall, terrain, building, etc, and the calculations of radio wave propagation are difficult since the necessary parameters are often not available.
To calculate propagation path loss by means of received power showing the power at the receiver for both okumura's and our model shown in Figure 4, with the given base station antenna of 70 m in height, mobile antenna of 1.7 m average height, distance 3 Km, and 900 MHz and 1800 MHz frequencies.
It is found that the propagation loss is 39.3488 dB for Okumura's model , however, it is -47.5013 dB for our model.
3.2 Free Space Path Loss
Free space path loss is the signal propagation along a line-of-sight (LOS) between transmitter and receiver with a distance, d. There is no obstruction between the transmitter and receiver. Free space path loss introduces a complex scale factor, resulting in the received signal, and the present work. We have used a parametric generalization version of the elevator derived for any building to obtain the approximate solutions for the penetration loss.
In accordance with the data given by TELSIM Company, the parameters are listed in Table 9.
The receiver signal level is
and then we calculate the free space loss. For the sake of comparison with our results we can use
to compare with those of Okumura's one as shown in Figure 5.
For example, we may compute the free space loss as
and the waveguide loss at the transmitter as
Over more, the waveguide loss at the receiver can be calculated as
and the total waveguide loss as
The total connector loss is found to be
and the total Radome loss
Also, the total fixed losses at the transmitter:
The total fixed losses at the receiver can be calculated as
and thus the total losses:
Further, the total antenna gain is
whereas the antenna gain is
3.3 Plane Earth Model
For calculating plane earth propagation loss where the distance d ranges from 1 - 10 km, antenna height 123 m, and mobile antenna height 1.7 m; the results are illustrated in Figure 6 (a), and 6 (b).
3.4 Cellular Propagation Models
1. Okumura's Model
Okumura carried out extensive drive test measurements, with a range in terms of clutter type, frequency, transmitter height, and transmitter power. The obtained results showed that the signal strength decreases at a much greater rate with distance than that predicted by free space loss. In the following approximation conditions, we take the frequency = 150 - 1500 MHz, communication range = 1 - 20 Km, the base station antenna height = 30 - 200 m, and the mobile station antenna height = 1 - 10 m. Hence, the propagation loss approximation becomes
where the parameters and have the same values for each area. Whereas, and C have different values for each area. However, in large cities, the value differs according to whether the working frequency is above or below 400 MHz.
Using Eq. 7 which is commonly used for all areas, and comparing them with our model, provides us the following results illustrated in Figure 7 (a), and (b), for = 900 and 1800 MHz frequencies respectively. The communication range is taken 1 - 20 Km, the base station antenna height = 30 m and the mobile station antenna height = 1.7 m.
The remaining values of parameters differ, however, according to the choice of place.
Here we list several places of interest:
1) In open land:
2) In suburbs:
3) In medium city:
4) In large city:
2. Hata's Model
Below we list the limitations on Hata model due to range of test results:
Carrier Frequency: = 150 - 1500 MHz,
Distance from the base station: = 1 - 20 km,
Height of base station antenna: = 30 - 200 m,
Height of mobile antenna: = 1- 10 m.
The path loss has been calculated for different places by using Eqs. 12 - 14.
By varying distances = 1 - 20 Km and for the sake of comparing with our model, we obtain the results shown in Figure 9 below for = 1500 MHz in according to Hata's model at 1500 MHz, and our model at 1800 MHz. Comparing with our model, gives us the following results as shown in Figure 8.
(1) Urban clutter:
(2) Suburban clutter:
(3) Open country:
3. COST 231-Hata Model
The COST 231 - Hata extends the Okumura-Hata model for medium to small cities to cover the 1500 - 2000 MHz band.
By using Eq. 15 with = 3 dB for metropolitan centers, = 0 dB for medium sized cities and suburban areas, we obtained the results as shown in Figure 9.
where = 3 dB for metropolitan centers, = 0 dB for medium sized cities and suburban areas.
The model is not valid for (i.e. base station below roof height) so it is not suitable for micro-cell planning.
4. Sakagami Kuboi Model
Moreover, we can calculate the path loss in dB by means of Eq. 16 to compare later with our model, and the results shown in Figure 10.
where is width of the road at the receiving point 5 -50 m, Ï• is orientation of road relative to base station direction 0 to 90o, is height of the building on the base station side of the receiving point 20 -100 m, <> is average height of the buildings near the receiving point 5 -50 m, and is average height of the buildings around the base station.
IV. Future Work
The following factors should be considered in the propagation of radio waves above 10 GHz:
The contribution of in homogeneities in the atmosphere.
The gaseous contribution of the homogeneous atmosphere due to resonant and non-resonant polarization mechanism.
Contribution due to rain, fog, mist, and haze (dust, smoke, and salt particles on air).
Elevators propagation loss also will increase.
Propagation through atmosphere gets affected due to several molecular resonance such as water vapor at 22 and 183 GHz, Oxygen with lines around 60 GHz. Other gasses like N2O2, SO2, O3, NO2, and NH3. The displays resonances but do not have much effect on propagation of radio waves. The major offender is precipitation. It can exceed that of all other sources of attenuation in atmosphere above 10 GHz. The total transmission loss is thus given by
where, in and is in .
It can be also expressed as:
where is the excess attenuation due to water vapor, is the excess attenuation due to mist and fog, is the excess attenuation due to O2, is the sum of absorption losses in dB due to other gasses, and is excess attenuation due to rainfall.
In this article we have investigated the signal strength, path loss and modeling of radio propagation at various frequencies 900, and 1800 MHz for the case into and within building scenarios, and into and within elevator. Our measurements of signal strength and signal variability have been done using many buildings at the NEU campus in TRNC.
It is interesting to observe that the Barry and Williamson models, derived from experiments carried out in Auckland, New Zealand, yielded slightly worse values of RMSEs: 3.9 dB for the line of sight case, and 7.2 dB for the obstructed path case . This is also true for DE Toledo models carried out in the University of Liverpool of RMSEs: 2.4, and 2.2 dB. However, our model has shown its effectiveness and thus provided better (i.e., approximately to the value of nearly 2.99 dB). This was due to the Internet receiver dish mounted on the roof of the FOE. The two sets of field's trials yielded an average value of penetration loss equal to nearly 18.8957 dB. Thus, predicting using the models presented in this article could produce more precise results for propagation path loss than those mentioned above. On the other hand, the work also includes measurements on path loss including elevators.
It is worthy to mention that these tests were undertaken using a fixed base station transmitter and a mobile receiver. The signal transmitted from the base station was received using a purpose-built data logging system, which was moved into the elevator. It produced an effective radiated power (ERP) of nearly 29 dBm at 900 MHz and 24 dBm at 1800 MHz.
 P. J. Barry and A. G. Williamson, "Statistical Model for UHF Radio-wave Signals within Externally Illuminated Multistory Buildings," IEE Proc. -Part I, Aug. 1991, vol. 138, no. 4, pp. 307-18.
 Y. Okumura et al., "Field Strength and Its Variability in VHF and UHF Land Mobile Radio Service," Rev. Elec. Commun. Lab., 1968, 16, pp. 825-73.
 M. Hata, "Empirical Formula for Propagation Loss in Land Mobile Radio Services," IEEE Trans., 1980, VT-29, no. 3, pp. 317-25.
 M. F. Ibrahim and J. D. Parsons, "Signal Strength Prediction in Built-Up Areas, Part 1: Median Signal Strength," Proc. IEE, Part F, 130, no. 5, 1983, pp. 377-84.
 J. D. Parsons, The Mobile Radio Propagation Channel, Pentech Press, 1992.
 A. M. D. Turkmani and A. F. Toledo, "Modeling of Radio Transmissions into and within Multistory Buildings at 900, 1800 and 2300 MHz," IEE Proc. - Part I, vol. 140, no. 6, Dec. 1993, pp. 462-70.
 A. F. Toledo, "Narrowband Characterization of Radio Transmissions into and within Buildings at 900, 1800, and 2300 MHz," Ph.D. thesis, Dept. Elec. Eng. and Electron., Univ. of Liverpool, U.K., May 1992.
 Jamal F. Abu Hasna, "Estimating Coverage of Radio Transmission into and within Buildings for Line of sight visibility between two points in terrain by Linear Prediction Filter" MIC-CSC2009, 400019, Mosharaka International Conference on Communications, Signals and Coding - SISP - Signal, Image and Speech Processing.
 L. P. Rice, "Radio Transmission into Buildings at 35 and 150 MHz," Bell Sys. Tech. J., 1959, 38, no. 1, pp. 197-210.
 J. M. Durante, "Building Penetration Loss at 900 MHz," Proc. IEEE VTC., 1973, pp. 1-7.
 A. M. D. Turkmani, J. D. Parsons, and D. G. Lewis, "Measurement of Building Penetration Loss on Radio Signals at 441, 900 and 1400 MHz," J. IERE, vol. 58, no. 6 (supp.), 1988, pp. S169-74.
 S. E. Alexander, "Radio Propagation within Buildings at 900 MHz," Elect. Lett., 18, no. 21, 1982, pp. 913-14.
 The RFID certification textbook, Google Books Result, Radio Frequency Identification (RFID).