Abstract:
Apparatus and methods facilitating a distributed approach to measuring the RF propagation characteristics of an indoor area or region are described. Such measurements are particularly useful during the installation and management of indoor wireless data communication networks, such as Wireless Local Area Networks (WLANs). A plurality of sounder units may be geographically distributed at arbitrary points in a two-dimensional area surrounding the indoor area or region whose RF propagation characteristics are to be measured. These sounder units are linked to a central controller, which functions to control all of the sounder units as well as to maintain a user interface that provides a user with a display of the measured propagation characteristics of the region. Each sounder unit is capable of independently injecting RF stimulus signals with some desired radiation pattern into the region being measured, as well as recording received signals from which the RF propagation characteristics may be calculated. The central controller co-ordinates the set of sounder units to ensure that they act as a logical whole, and also enables propagation measurements to be made over long periods of time in order to capture time-varying characteristics of the indoor environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from, and herein incorporates by reference, U.S. Provisional Patent Application Ser. No. 60/505,419 filed on Sep. 22, 2003. 

   FIELD OF THE INVENTION 
   The present invention generally relates to measurement of radio frequency propagation characteristics within a two or three-dimensional area, and, more specifically, to methods and apparatus for measuring and analyzing the radio frequency propagation characteristics of indoor environments for wireless data communication purposes. 
   BACKGROUND OF THE INVENTION 
   Wireless Local Area Networks (WLANs) have recently gained in popularity and importance. These networks are a special case of standard computer Local Area Networks (LANs), wherein the wires or optical fibers interconnecting computers have been partially or completely replaced by radio frequency (RF) data links operating at very high frequencies. WLANs may also be viewed as a special case of commonly encountered cellular telephone networks, where the relatively large distances (tens of miles) covered by cellular telephones have been significantly reduced (to hundreds of feet, in an indoor environment within buildings) in exchange for much higher data transmission rates. WLANs offer the possibility of interconnecting intra-building information technology devices such as computers, Personal Digital Assistants, printers, etc. at relatively high speeds without wires, and hence yield significant reductions in installation cost together with significant improvements in user convenience. 
   The design and installation of RF data links must factor in the propagation characteristics of the region covered by the data links. RF propagation characteristics of interest in indoor digital wireless communication systems include the following:
         (a) Attenuation (loss) as a function of distance, including shadowing caused by reflecting or absorbing bodies in the environment; and   (b) Multipath (echoes) caused by reflection or diffraction from one or more scatterers in the environment, which can result in inter-symbol interference or variations in signal strength (fading).   WLANs are significantly affected by the propagation characteristics of the indoor environment in which they are used. This results from:   (a) Increased data rates. WLANs typically operate at data rates of 1 Mb/s or more for a given channel; modern WLAN technologies transfer data at speeds of up to 54 Mb/s. This means that the symbol periods used must be quite short, and consequently they are affected by inter-symbol interference resulting from multipath. In addition, the higher data rates require wider channels, which are more susceptible to effects such as frequency fading.   (b) Reduced transmit power limits. WLAN equipment is constrained to use unlicensed frequency bands with strict limits on radiated power, and as a result cannot overcome attenuation problems with increased transmit power.   (c) Strict cost and size constraints. As WLAN systems are intended to support mobile and portable applications in a home or business environment, it is not possible to deal with attenuation and multipath problems by utilizing large antenna arrays or complex networks of repeaters.       

   The increased usage and reliance upon WLANs has in turn required a much greater emphasis to be placed on measuring the propagation characteristics of RF energy in an indoor environment. Heretofore, most RF propagation studies and analyses have focused on propagation characteristics in an outdoor (urban or regional) environment, in response to the needs of broadcasting, cellular telephony, and other fixed and mobile wireless systems. The indoor environment in which WLANs are placed, however, exhibits different propagation characteristics and requires measurements to be made in different ways than the outdoor environment. 
   The indoor RF propagation environment poses a number of challenges. Firstly, the environment is quite complex, containing a large number of scatterers as well as a high density of absorbing elements with diverse physical characteristics. Secondly, the environment typically changes frequently, as objects are moved about. Finally, the relatively large density of transmitters and receivers present in a WLAN environment results in a large number of interactions. 
   Several approaches have been implemented to date to enable these issues to be dealt with when implementing indoor data networks. These are:
         (a) Computer modeling of the propagation characteristics of the indoor space based on an exact representation of the dimensions and physical characteristics of the contents. This entails locating all of the objects present in a floor or building (walls, ceilings, furniture, etc.), creating a computer model of the entire space, and then using the model to predict the propagation characteristics of interest to WLAN designers. While this yields accurate results, it is complex, time-consuming and expensive.   (b) Extrapolation based on previous propagation studies. A large number of studies have been made of the propagation characteristics of various kinds of buildings, as well as of the physical characteristics of various types of building materials. It is possible to combine the results of these studies to produce a composite model or a set of composite models that can be applied to the building of interest. However, this approach can produce very inaccurate and unpredictable results because the geometry and contents of buildings vary widely, and RF propagation is greatly affected by small differences in layout and composition.   (c) Empirical deduction based on signal strengths. This consists of setting up an RF signal source at some location in a building and then measuring signal strengths at various points within the floor or building, and deducing the propagation characteristics based on the various measurements. However, this method is labor intensive, error prone, and frequently unrepeatable.   (d) Direct measurement of the propagation characteristics. An apparatus known as a channel sounder can be employed to directly measure the propagation characteristics of an RF channel. Channel sounders have been commonly employed in measuring the characteristics of point-to-point wireless links, but have also been utilized in the propagation studies previously referred to in order to measure the characteristics of buildings and materials. However, no means has been disclosed in the prior art of using this approach to perform propagation measurements of building spaces in two (or three) dimensions.       

   Accordingly, it is an object of the invention to provide an improved RF propagation measurement system for indoor environments. It is a further object of the invention to provide a propagation measurement system that enables measurements to be made over two-dimensional areas or regions. It is yet a further object of the invention to provide a propagation measurement system that allows the time-variant RF propagation characteristics of an indoor environment to be measured. 
   SUMMARY OF THE INVENTION 
   The invention provides for a distributed RF propagation measurement system for performing channel sounding in a two-dimensional space, comprising a plurality of independent sounder units, which are controlled by a user from a single central controller. In one preferred embodiment, each sounder unit contains: an antenna array and associated beamforming means for transmitting and receiving RF energy; channel sounder receiver and transmitter means; processing means for computing the propagation characteristics from the received RF energy; synchronization means for enabling multiple independent sounder units to function as a single distributed system; location determining means to measure the position of the sounder unit in three-dimensional space; communication means for transferring data and commands to and from the central controller; and control software means to control the functions of the sounder unit. The central controller is preferably implemented using software executed by a host computer, and performs user interface functions as well as communication and co-ordination of the sounder units. 
   The antenna array and beamforming means may preferably be operative to transmit and receive RF energy with well-defined directional characteristics, and may contain antenna means as well as means for phasing, gain control, power division and power combining (summing). 
   The channel sounder transmitter means may preferably be operative to generate RF stimulus signals that are transmitted into the environment. 
   The channel sounder receiver means may preferably be operative to process RF signals received from the environment and obtain power-delay profiles that are indicative of the characteristics of the RF propagation channel. 
   The location determining means may preferably be operative to measure the three-dimensional co-ordinates of the sounder unit relative to the central controller to a high degree of accuracy. The location determining means may use either the Global Positioning System (GPS) or an independent location determining capability that employs pseudolites to provide accurate spatial references from which three-dimensional vectors can be computed. The sounder units may communicate the location calculated by the location determining means to the central controller. 
   The communication means may preferably use a dedicated Ultra High Frequency (UHF) radio data link to communicate with the central controller. 
   The control software means may preferably be supported by an embedded controller or Central Processing Unit (CPU), and controls and co-ordinates the activities of the sounder unit. 
   Advantageously, the channel sounder transmitter and receiver means may be implemented using a sliding correlator function. 
   Advantageously, the synchronization means may employ clock synchronization signals derived from the location determining signals utilized by the location determining means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description of the preferred embodiments is taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a diagram illustrating the general arrangement of the sounder units and the central controller in relation to the region over which RF propagation measurements are to be made; 
       FIG. 2  is a further representation of a typical usage scenario of the two-dimensional channel sounder system within an example building or floor; 
       FIG. 3  is a schematic block diagram of the sounder unit; 
       FIG. 4  is a schematic block diagram of the beamformer circuitry; 
       FIG. 5  is a depiction of the mechanical arrangement of an antenna array used in accordance with one preferred embodiment of the sounder unit; 
       FIG. 6  is a representation of the radiation pattern (in the azimuthal direction) produced by the antenna array of  FIG. 5 ; 
       FIG. 7  is a schematic block diagram of the circuitry for the channel sounder transmitter; 
       FIG. 8  is a schematic block diagram of the circuitry for the channel sounder receiver; 
       FIG. 9  is a schematic block diagram of the circuitry for the pseudo-random bit sequence generator; 
       FIG. 10  is a schematic block diagram of the circuitry for the post-processing logic; 
       FIG. 11  is a schematic block diagram of the circuitry for the control processor; 
       FIG. 12  is a schematic block diagram of the circuitry for the communications processor that serves as the control link to the central controller; 
       FIG. 13  is a schematic block diagram of the circuitry for the location processor; 
       FIG. 14  is a representation of an example of a Graphical User Interface which may be employed by the central controller for presenting the measured propagation characteristics of a region to a user; 
       FIG. 15  depicts the process of setting up, initializing, configuring and operating the sounder units and analyzing the results; 
       FIG. 16  is a representation of an example power-delay profile obtained by the sounder unit; 
       FIG. 17  is a representation of the process used by the central controller to associate a specific direction of arrival or transmission with a peak in a measured power-delay profile; 
       FIG. 18  is a representation of the process used by the central controller to correlate the attenuation properties of the environment to elements of the power-delay profiles; 
       FIG. 19  is a representation of the process used by the central controller to correlate specific scatterers to elements of the power-delay profiles; 
       FIG. 20  is a representation of the process used by the central controller for accumulating results from multiple sounder units and correlating them into a single representation of the propagation characteristics of the region; 
       FIG. 21  is a depiction of the mechanical arrangement of an antenna array used in accordance with another preferred embodiment of the sounder unit; and 
       FIG. 22  is a representation of the radiation pattern (in the azimuthal direction) produced by the antenna array of  FIG. 21 . 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , the two-dimensional channel sounder system comprises a general-purpose computer  14  that is programmed to act as a central controller, and a plurality of identical sounder units  10 ,  11 , and  12  that perform the actual measurement of the RF propagation conditions within the region  13 . As all of the sounder units  10 ,  11 ,  12  are preferably identical, it is understood that a reference to an aspect of any specific sounder unit, e.g.  10 , shall be hereinafter taken to apply to all of the other sounder units in the system, e.g.,  11 ,  12 . It is further understood that the number of sounder units  10  in the system may range from 1 to any arbitrary number required to perform RF propagation measurements in the measurement region  13  to some predetermined level of accuracy. The level of accuracy increases as more sounder units  10  are placed in the measurement region  13 , especially when measurement region  13  contains a large number of scatterers or other artefacts that affect RF propagation. 
   Each sounder unit  10  is a compact device that can be placed at various locations around or within the measurement region  13 , and either generates RF stimulus energy within a specified frequency band into the measurement region  13 , or receives and processes RF energy in the same frequency band from the latter. To measure propagation characteristics affecting WLAN communications, the frequency band selected is preferably centered at 2.4 gigahertz (GHz). However, it is understood that the apparatus and method described herein can be applied to propagation measurements performed for any desired RF band. In addition, to attain a spatial resolution of 1 meter (3 feet), which is suitable for measurements in an indoor region, a bandwidth of 300 megahertz (MHz) is preferably employed. However, it is understood that any desired spatial resolution may be attained by employing a different bandwidth, with the spatial resolution R being approximately equal to the speed of light c divided by the bandwidth B.
 
 R=c/B   (1)
 
   The central controller  14  communicates with each sounder unit  10  via communications links  15 . The central controller  14  preferably utilizes a standard host computer or workstation, such as a personal computer, and performs user-interface, control, results processing and results display functions. A comprehensive control and analysis program may be implemented on the central controller  14  in order to control and co-ordinate the sounder units and analyze the data that they collect. User control of the sounder units is preferably accomplished through a Graphical User Interface (GUI), which performs user interface functions, including such elements as providing the user with access to propagation analysis tools and the ability to format and output reports based on data gathered by the sounder units and processed by the central controller. 
   With reference to  FIG. 2 , an example of the usage of the system to determine the RF propagation characteristics of a particular floor  20  of a building or structure would consist of placing an appropriate number of sounder units  21 ,  22 ,  23 ,  24  at various locations within the floor  20  of the building, situating central controller  25  within the same floor at a location permitting communications with sounder units  21 ,  22 ,  23 ,  24  via communication links  26 , and then making measurements of the propagation characteristics of the region enclosed by the sounder units  21 ,  22 ,  23 ,  24  within floor  20 . Advantageously, the communication links  26  may be low-speed Ultra High Frequency (UHF) wireless data links, that eliminate the need for physical connections between sounder units  21 ,  22 ,  23 ,  24  but do not interfere with the propagation measurements being performed. A user of the system may then be enabled to make various propagation measurements on floor  20  by means of commands input to central controller  25 ; the results of these propagation measurements are preferably displayed by central controller  25 . Sounder units  21 ,  22 ,  23 ,  24  may be re-positioned at various points within floor  20  and additional measurements performed if more precise results are required. 
   With reference to  FIG. 3 , an embodiment of sounder unit  10  preferably comprises antenna array  30 , which transmits and receives RF energy in a specific frequency band according to a predetermined radiation pattern; beamformer  31  coupled to antenna array  30  that shapes the radiation pattern of antenna array  30  to create a deep null (area of reduced RF radiation intensity) in a specific direction; channel sounder receiver  32  and channel sounder transmitter  33 , both of which are operatively coupled to clock generator  34  that is preferably synchronized to the clock generators of other sounder units via clock synchronization signal  42  from location processor  40 , and may also be synchronized using external synchronization interface  35 ; post-processing logic  36  that receives raw measured data from channel sounder receiver  32 , processes it, and transfers the results to control processor  37 ; communications processor  38  that supports a communications link  39 , which may advantageously be a UHF wireless data link, to the central controller; and location processor  40  coupled to location processor antenna  41  which serves to provide control processor  37  with accurate three-dimensional location information, as well as providing reference clock generator  34  with precise timing information via clock synchronization signal  42 . Central controller  37  controls the operation of beamformer  31 , channel sounder transmitter  33 , and channel sounder receiver  32  using channel sounder control interface  43 . 
   With reference to  FIG. 4 , beamformer  31  preferably comprises a set of transmit/receive switches  50 ,  51 ,  52 ,  53 ; a set of broadband RF power amplifiers  54 ,  55 ,  56 ,  57 ; a set of broadband RF low noise amplifiers  58 ,  59 ,  60 ,  61 ; a set of variable-gain RF amplifiers  62 ,  63 ,  64 ,  65 ,  66 ,  67 ,  68 ,  69 , of which one subset  62 ,  63 ,  64 ,  65  is used for transmit signals and another subset  66 ,  67 ,  68 ,  69  is used for receive signals; a set of RF phase shifters  70 ,  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77 , of which one subset  70 ,  71 ,  72 ,  73  is used for transmit signals and another subset  74 ,  75 ,  76 ,  77  is used for receive signals; phasing control block  78 , which controls the gain of variable-gain RF amplifiers  62 ,  63 ,  64 ,  65 ,  66 ,  67 ,  68 ,  69  and the phase lag or lead of phase shifters  70 ,  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77 ; power divider  80 , which accepts transmitted signal power on transmit RF signal  81  and divides it uniformly among the transmit phase shifters  70 ,  71 ,  72 ,  73 ; and a power combiner  82  that accepts receive RF power from receive phase shifters  74 ,  75 ,  76 ,  77  and sums it to produce the final received signal output on receive RF signal  83 . Preferably, beamformer  31  may further contain a set of broadband matching networks  84 ,  85 ,  86 ,  87  that serve to match impedances between the RF power amplifiers  54 ,  55 ,  56 ,  57  and antenna array  30 . Preferred embodiments of the present invention may use switched matching network circuits for broadband matching networks  84 ,  85 ,  86 , and  87 . Such matching network circuits are well known in the prior art and will not be described further. 
   The function of beamformer  31  is to control the relative phases and amplitudes of the transmitted RF signals driven to antenna array  30 , as well as to control the relative phases and amplitudes of the RF signals received from antenna array  30 , under control of control processor  37  via channel sounder control interface  43 . It is well known in the prior art that controlling amplitudes and phases in this manner enables antenna array  30  to exhibit various types of radiation patterns. An example of a radiation pattern obtained by this process is depicted in  FIG. 6 , and is described in detail in the accompanying text. 
   Beamformer  31  depicted in  FIG. 4  represents a four-channel beamformer circuit, comprising four identical sets of phase shifting, amplification, matching and switching units operatively coupled to four identical ports on power divider  80  and power combiner  82 . It is understood, however, that the beamformer  31  can be extended to any arbitrary number of channels by increasing or decreasing the number of sets of phase shifting, amplification, matching and switching units as well as the number of ports on the power divider and power combiner. 
   With reference to  FIG. 5 , antenna array  30  may comprise a set of vertical radiators  90 ,  91 ,  92 ,  93  preferably mounted at the corners of a square on a flat conducting ground plane  94 . The spacing between vertical radiators  90  and  91 ,  91  and  92 ,  92  and  93 ,  91  and  93  is preferably set to one-quarter of a wavelength at the frequency band of operation. For example, for WLAN propagation measurements in the 2.4 GHz frequency band, this is 31.25 mm. The vertical radiators are fed from the outputs of transmit/receive switches  50 ,  51 ,  52 , and  53  in beamformer  31  by means of coaxial cables  95 ,  96 ,  97 , and  98 . For example, the interconnection of transmit/receive switches  50 ,  51 ,  52 ,  53  to coaxial cables  95 ,  96 ,  97 ,  98  may be performed as follows:  50  to  95 ,  51  to  96 ,  52  to  97 ,  53  to  98 . 
   Each of the vertical radiators  90 ,  91 ,  92 ,  93  may hence be driven with independent phases and amplitudes with respect to the transmit RF input  81  to beamformer  31  during transmission of RF energy. In addition, RF energy received by each of the vertical radiators  90 ,  91 ,  92 ,  93  may be combined with different phase and amplitude relationships to generate a single receive RF output  83  from beamformer  31 . 
   With reference to  FIG. 6 , radiation pattern  100  generated by the antenna array  30  as plotted on azimuth plot  101  may be controlled by beamformer  31  to preferably produce a cardioid type of radiation pattern with a single deep null  103 . The direction of null  103  may be steered (rotated in azimuth) by adjusting the phase and amplitude relationships using phasing control  78  of beamformer  31 . The process of calculating the phase and amplitude adjustments in order to steer the direction of null  103  is well known in the prior art and will not be described further. 
   With reference to  FIG. 7 , channel sounder transmitter  33  preferably comprises transmit clock synthesizer  110 , transmit local oscillator (LO) Phase Locked Loop (PLL)  116 , Pseudo Random Bit Sequence generator (PRBS)  114 , baseband filter  115 , mixer  117 , RF bandpass filter  118 , variable-gain amplifier (VGA)  119 , and Automatic Level Control (ALC) circuitry  121 . 
   Transmit clock synthesizer  110  accepts a stable clock reference input  111 , which is preferably at a frequency of 10 MHz, and multiplies its frequency to generate two output reference clock signals  112  and  113 . Clock reference signal  112  is preferably at a frequency of 100 MHz, and clock reference signal  113  is preferably at a frequency of 300 MHz. Clock reference signal  113  is used by PRBS generator  114  to generate a continuous, repeating maximal-length pseudo-random bit sequence, preferably of length  2047 , which is input to baseband filter  115 . Baseband filter  115  preferably performs a low-pass filter function upon the pseudo-random bit sequence to produce a modulation signal of the desired bandwidth; in the preferred embodiments of the present invention, this bandwidth is set to 300 MHz. 
   Transmit LO PLL  116  multiplies clock reference signal  112  to generate a stable local oscillator signal with low phase noise. In one preferred embodiment, the local oscillator signal is at a frequency of 2,450 MHz, enabling sounder unit  10  to perform propagation measurements in the 2,300 MHz to 2,600 MHz frequency band. In another preferred embodiment, the local oscillator signal is switchable between a frequency of 5,250 MHz and a frequency of 5,775 MHz, enabling sounder unit  10  to perform propagation measurements in the 5,100 to 5,400 and 5,625 to 5,925 MHz frequency bands respectively. 
   The outputs of baseband filter  115  and transmit LO PLL  116  are input to mixer  117 . Mixer  117  mixes (multiplicatively combines) these two signals to produce an RF signal of bandwidth set by baseband filter  115  and centered on the center frequency set by transmit LO PLL  116 . The output of mixer  117  is coupled to RF bandpass filter  118 , which may filter the RF signal to shape its frequency characteristics and eliminate unwanted mixing products. 
   In the preferred embodiments, the output of RF bandpass filter  118  is passed to VGA  119 , which is operatively coupled to ALC circuit  121 . VGA  119  amplifies the RF signal produced by RF bandpass filter  118  and passes it to the transmit RF signal  81  of channel sounder transmitter  33 . ALC circuit  121  detects the output level of transmit RF signal  81 , and preferably controls the gain of VGA  119  to ensure that this output level remains constant regardless of the voltage levels of the outputs of baseband filter  115  and transmit LO PLL  116 . 
   With reference to  FIG. 8 , channel sounder receiver  32  preferably comprises receive clock synthesizer  130 , receive LO PLL  134 , VGA  135 , AGC circuit  145 , baseband filter  138 , PRBS generator  139 , divide-by-N counter  140 , mixer  142 , and lowpass filter  143 . 
   Receive clock synthesizer  130  accepts a stable clock reference input  111 , which is preferably at a frequency of 10 MHz, and multiplies its frequency to generate two output clock signals  132  and  133 . In the preferred embodiments, clock signal  132  is at a frequency of 300 MHz and drives PRBS generator  139 , and clock signal  133  is at a frequency of 100 MHz and is used as an input reference by receive LO PLL  134 . The frequency and phase of clock signals  132  and  133  in channel sounder receiver  32  are preferably very nearly identical to those of clock signals  112  and  113  respectively in channel sounder transmitter  31  in all channel sounder units  10 ,  11 ,  12 , to enable any of channel sounder units  10 ,  11 ,  12  to detect and demodulate the transmitted signal generated by any other of channel sounder units  10 ,  11 ,  12 . This may be done by ensuring that clock reference input  111  to receive clock synthesizer  130  in any of channel sounder units  10 ,  11 ,  12  is frequency and phase coherent with clock reference inputs  111  to transmit clock synthesizers  110  in all other channel sounder units  10 ,  11 ,  12 . 
   Receive LO PLL  134  multiplies clock reference input  111  to generate a stable local oscillator signal with low phase noise. In one preferred embodiment, the local oscillator signal is at a frequency of 2,450 MHz, enabling sounder unit  10  to perform propagation measurements in the 2,300 MHz to 2,600 MHz frequency band. In another preferred embodiment, the local oscillator signal is switchable between a frequency of 5,250 MHz and a frequency of 5,775 MHz, enabling sounder unit  10  to perform propagation measurements in the 5,100 to 5,400 and 5,625 to 5,925 MHz frequency bands respectively. 
   In the preferred embodiments, VGA  135  accepts receive RF signal  83  to channel sounder receiver  32  and amplifies the signal prior to passing it to mixer  137 . Mixer  137  mixes the amplified RF input signal with the local oscillator signal produced by receive LO PLL  134 , generating a baseband signal that is preferably low-pass filtered by baseband filter  138  to remove unwanted mixing products from mixer  137 . Baseband filter  138  may further ensure that the bandwidth of the baseband signal is limited to that of the frequency band of interest, preferably the same as the frequency band occupied by the RF signal generated by channel sounder transmitter  33 . Preferably, AGC circuit  145  is operatively coupled to VGA  135  and baseband filter  138 , and may adjust the gain of VGA  135  to ensure that the level of the output signal from baseband filter  138  maintains a constant average level to avoid overdriving mixer  142 . 
   PRBS generator  139  may accept clock input  132  and use it to generate a maximal-length pseudo-random bit sequence, preferably of identical length and value to that generated by PRBS generator  114  in channel sounder transmitter  33 . PRBS generator  139  is coupled to mixer  142 , which mixes (multiplies) the baseband signal from baseband filter  138  and the PRBS from PRBS generator  139 . This multiplication function is equivalent to a correlation performed between the incoming baseband signal and the locally generated PRBS; hence mixer  142  produces a high output level when the baseband signal matches the PRBS and a low output level when the baseband signal does not match the PRBS. The output of mixer  142  is preferably passed to lowpass filter  143 , which may filter out the higher-order components of the signal, leaving the lower frequency correlation products that are output to channel sounder receiver signal  144 . The bandwidth of the lowpass filter is preferably set to the frequency of clock signal  132  supplied to PRBS generator  139 , divided by the length of the PRBS sequence. For example, with a frequency of 300 MHz for clock signal  132  and a PRBS length of 2047, the bandwidth of lowpass filter  143  may be set to 146.6 kilohertz (kHz). 
   In the preferred embodiments, the locally generated PRBS may be progressively delayed with respect to the received baseband signal in order to sweep the correlation function being performed over the received baseband signal. This is preferably done by inserting a 1-bit delay into the PRBS sequence (i.e., preventing PRBS generator  139  from producing the next bit of the sequence until 1 extra bit time has elapsed) at regular intervals. The rate of the sweep may be controlled by divide-by-N counter  140 , which may be configured by control processor  37  to divide clock signal  132  by a specific ratio, such that a 1-clock pulse is produced on bin marker signal  141 . The pulse on bin marker signal  141  in turn causes PRBS generator  139  to freeze its internal state for the duration of bin marker signal  141 , thus causing the PRBS output by PRBS generator  139  to be delayed by one clock. 
   It is apparent that shifting the output of PRBS generator  139  by 1 clock cycle at periodic intervals results in progressively shifting the PRBS relative to the received baseband signal from baseband filter  138 , such that the correlation is performed with a delay increasing in steps of one clock cycle on clock signal  132 . The effect is to divide the correlation into bins, with each bin being of a constant width and having a different (progressively increasing) delay. The start of each bin is marked by the pulse on bin marker signal  141 , which may be used by post-processing logic  36  to determine the boundaries of each bin. 
   With reference to  FIG. 9 , PRBS generator  114  in channel sounder transmitter  33  and PRBS generator  139  in channel sounder receiver  32  may be implemented in the form of a Linear Feedback Shift Register (LFSR) using a set of D flip-flops and exclusive-OR (XOR) gates. For a PRBS of length (N−1), where N is a power of 2, a total of log 2 (N) D flip-flops are required, all connected in series to form a standard shift register. Further, for a PRBS generating polynomial having k terms, where k is less than log 2 (N), a total of k XOR gates are required, each XOR gate being connected at a tap point on the shift register of D flip-flops corresponding to the power of the corresponding polynomial term.  FIG. 9  shows an example of a PRBS generator with D flip-flops being represented by  160 ,  161 ,  162 ,  163 ,  164 ,  165  and XOR gates by  166 ,  167 ,  168 , with additional D flip-flops being connected in between  163  and  164 . It is understood that this representation may be extended to cover generating polynomials of any arbitrary size and complexity by adding more D flip-flops and XOR gates. It is further understood that such a structure may be implemented in both serial and parallel forms. The architecture and implementation of PRBS generators is well known in the prior art and will not be described further. 
   With reference to  FIG. 10 , post-processing logic  36  preferably comprises Analog-to-Digital (A/D) converter  180 , digital accumulator  181 , bin memory  182 , memory address generator  183  and control state machine  184 . A/D converter  180  accepts analog input from channel sounder receiver  32  on channel sounder receiver signal  144  and converts it to digital, with output in the form of a periodic sequence of digital words. The sampling rate of A/D converter  180  is preferably set to at least twice the bandwidth of lowpass filter  143  in channel sounder receiver  32 , and the reference signal used by A/D converter  180  is controlled by AGC circuit  145  to scale the digital output in accordance with the AGC voltage. The sequence of digital words is passed to accumulator  181 , which may accumulate (add) all of the digital samples within a given bin delineated by bin marker signal  141 . The accumulated result is then passed to bin memory  182  for storage in the memory address corresponding to the appropriate bin, as indicated by address generator  183 . Control state machine  184  controls the operation of accumulator  181  and address generator  183  according to the signal on bin marker  141 . Control processor  37  is enabled to read the data out of bin memory  182  via bin memory interface  188 . 
   In accordance with a preferred embodiment of the present invention, the operation of post-processing logic  36  is as follows. When a pulse is received on bin marker  141 , address generator  183  is caused to step to the next sequential memory address (corresponding to the next sequential bin), and accumulator  181  is cleared to zero. Subsequently, A/D converter  180  converts channel sounder receiver signal  144  to a sequence of digital words that may be accumulated into accumulator  181 . When the next pulse is received on bin marker  141 , the accumulated value in accumulator  181  may be written to bin memory  182  at the indicated address, and the cycle repeats. The number of cycles is N, where (N−1) is the length of the PRBS generated by PRBS generator  114  in channel sounder transmitter  33  and PRBS generator  139  in channel sounder receiver  32 . At the end of the process, bin memory  182  will contain N accumulated values, each value corresponding to a shifted correlation of PRBS generator  139  in channel sounder receiver  32  with receive RF signal  83 . 
   The values present in bin memory  182 , when arranged in increasing sequence of delay from 0 to (N−1), correspond to the power-delay profile of the RF propagation channel being measured by channel sounder unit  10  at that point in time. An example of a power-delay profile is provided in  FIG. 16  and described in the accompanying text. The present invention may use a large number of such power-delay profiles, obtained from all of channel sounder units  10 ,  11 ,  12  with different orientations for deep null  103  in antenna radiation pattern  100  on each channel sounder unit  10 ,  11 ,  12 , to compute the two-dimensional RF propagation characteristics of the indoor region in which the channel sounder units  10 ,  11 ,  12  are located. 
   With reference to  FIG. 11 , control processor  37  preferably comprises CPU  200 , which is operatively coupled to bus control logic  201  that is in turn coupled to program storage  202 , data storage  203 , and I/O logic  204 . I/O logic  204  couples to location processor  40  using location interface  205 ; to bin memory  182  using bin memory interface  188 ; to communications processor  38  using communications interface  207 ; and to beamformer  31 , channel sounder receiver  32 , and channel sounder transmitter  33  using channel sounder control interface  43 . 
   CPU  200  may exercise overall control and co-ordination of location processor  40 , beamformer  31 , channel sounder receiver  32 , and channel sounder transmitter  33 ; maintain communication links  15  to central controller  14  via communications processor  38  (preferably supporting a TCP/IP protocol stack in order to simplify the communications functions); and communicate with the central controller  14  to perform test set-up and report test results. The CPU  30  may also implement firmware programs required for performing RF propagation measurement functions. 
   A communications link is required between the sounder unit  10  and the central controller  14  in order for the central controller  14  to configure and control sounder unit  10  and also to receive test results. This communications link may preferably be implemented using a dedicated UHF radio link. The communications link is supported by implementing one instance of communications processor  38  in each sounder unit  10 , and one similar instance of communications processor  38  in central controller  14 . Central controller  14  may advantageously implement a polling or time-division-multiplexing protocol to allow communications with all of sounder units  10 ,  11 ,  12  without requiring multiple instances of communications processor  38  to be present at the central controller  14 . The realization of such polling or time-division-multiplexing protocols in radio links is well understood and will not be described further. 
   With reference to  FIG. 12 , communications processor  38  may be implemented using a dedicated UHF radio data link operating in a suitable frequency band, preferably 430 MHz. The dedicated UHF-radio data link comprises UHF antenna  225  coupled to RF filters and transmit/receive switch  224 , which is in turn coupled to UHF serial data transmitter  222  and UHF serial data receiver  221 . Serializer/deserializer (SERDES) and data processor  220  converts between parallel data transferred to or from CPU  200  via communications interface  207 , and serial data streams that are generated by UHF serial data receiver  221  and accepted by UHF serial data transmitter  222 . Clock generator  223  implements a clock synthesis function that generates the necessary bit-clock, carrier and frequency conversion signals required by UHF serial data transmitter  222  and UHF serial data receiver  221 . Multiple UHF channels may be supported by reconfiguring clock generator  223  to generate different carrier frequencies. 
   With reference to  FIG. 13 , location processor  40  may be advantageously implemented using the Global Positioning System (GPS) to determine the absolute three-dimensional spatial co-ordinates of sounder unit  10 , and subsequently computing the three-dimensional vector from the sounder unit  10  to the central controller  14  in order to ascertain the relative position of sounder unit  10 . Location processor  40  consists of GPS RF front end unit  231 , operatively coupled to location processor antenna  232 , and GPS baseband processor  230 . Standard GPS processing is performed on the GPS satellite navigation signals received by location processor antenna  232  to compute the three-dimensional co-ordinates of sounder unit  10 , which are passed to control processor  37  via location interface  205 . In addition, location processor  40  may preferably generate clock synchronization signal  42  derived from the GPS radio signal received by location processor antenna  232  for use by reference clock generator  34  to generate a 10 MHz clock reference  111 . 
   With reference to  FIG. 14 , the software program executed by central controller  14  displays and maintains a Graphical User Interface (GUI)  250  that interacts with the user of the two-dimensional channel sounder system and controls the operation of sounder units  10 ,  11 ,  12  through communication links  15 , together with an associated underlying control program supporting GUI  250 . The specific capabilities of GUI  250  and its associated control program preferably include: 
   (a) Detection, initialization and configuration of sounder units; 
   (b) Display of sounder unit status; 
   (c) Configuration of propagation measurement parameters; 
   (d) Display of sounder unit location in a 3-D window; 
   (e) Display of measured propagation characteristics as charts and contour maps; 
   (f) Saving and restoring of charts and sounder unit log files; and 
   (g) Download and update of firmware on the sounder units. 
   GUI  250  and its associated control program may advantageously enable the user to download firmware images stored on the central controller to sounder units  10 ,  11 ,  12  thereby allowing the sounder units to be upgraded in capabilities and features in the field. GUI  250  display preferably consists of menu bar  251  that displays menus of commonly used commands; sounder unit status window  252  that shows the current operational status of a plurality of sounder units  10  available to the user for the test; 3-D view window  253  that may display a three-dimensional view (as a 2-D projection) of the set of sounder units, preferably superimposed upon a floorplan or architectural projection of the building in which propagation is being measured; and measurement results window  254  that displays the results of the propagation measurements performed by sounder units  10  under command by the user. 
   Measurement results window  254  may preferably interpret and display the results of the propagation measurements as contours  255 . Contours  255  may represent RF propagation characteristics (for example, attenuation) or predicted traffic characteristics (for example, bit error rate for a specific transmitted power). Contours  255  may further be superimposed upon a floorplan  256  of the building or region in which measurements are being made. In the case of a three dimensional view, contours  255  may represent a projection of RF propagation characteristics or predicted traffic characteristics upon floorplan  256  representing a particular floor or area. 
   User interactions with GUI  250  are translated by the underlying control program into sets of instructions that are transferred to sounder units  10  via communications links  15 . Each set of instructions is executed by control processor  37  in the corresponding sounder unit  10  in order to perform a specific measurement or test. The results are returned to GUI  250  via communications links  15  and subsequently displayed in measurement result window  254  of GUI  250 . 
   It is understood that various modifications of GUI  250 , in particular relating to the representation of the measurement results as tables, lists, charts or graphs, will be apparent to those skilled in the art upon reference to this description. 
   Operation of the RF propagation measurement system depicted in  FIG. 1  is completely initiated and controlled via GUI  250  running on the host computer serving as central controller  14 . GUI  250  converts operator commands that are input via a keyboard and/or mouse into high-level command messages directed at one or more of the sounder units  10 ,  11 ,  12 ; these command messages are then passed to the specified sounder units via the appropriate communications links  15 . System operation preferably begins with an initialization phase, followed by the actual measurement execution phase. Post-processing and report generation may then follow the measurement phase, after the results of the measurements have been gathered by GUI  250  from the sounder units. 
   With reference to  FIG. 15 , a typical usage scenario may include the steps of:
         (a) At step  261 , setting up the sounder units  10 ,  11 , and  12  at the desired locations around the region for which RF propagation characteristics are to be measured, and powering them on;   (b) At step  262 , starting up GUI  250  on central controller  14  to display the top-level screen, search for and detect sounder units  10 ,  11 ,  12 , and ensure that the required sounder units  10 ,  11 ,  12  have been detected and initialized;   (c) At step  263 , configuring parameters, if necessary, for the RF propagation measurement to be performed by the set of sounder units  10 ,  11 ,  12 ;   (d) At step  264 , executing the propagation measurements on sounder units  10 ,  11 ,  12 ;   (e) At step  265 , reviewing the data captured by sounder units  10 ,  11 , and  12 , processed by central controller  14 , and presented on measurement results window  254 . Sounder unit status window  252  may also be reviewed in order to ascertain whether the system is functioning properly, and 3-D location window  253  may be inspected to determine if the sounder units have been properly placed. Step  265  may further comprise the step of invoking post-processing analysis and report generation functions on the measured data; and   (f) At step  266 , checking to see if more measurements need to be made; if not, at step  267  terminating the test procedure by exiting GUI  250  and powering down the sounder units  10 ,  11 ,  12 ; otherwise, returning to step  261  to perform additional measurements.       

   Initialization of the wireless data communication protocol test system at step  112  takes place immediately after GUI  250  is started, may include three stages: sounder unit polling and discovery, timing synchronization, and sounder unit location. The system initialization process preferably happens automatically (when GUI  250  is started); however, it may also be initiated and controlled by the user via GUI  250 . Also, the initialization phase may advantageously include firmware upgrades to the sounder units  10 ,  11 ,  12 ,  13  under user control. 
   The first stage in the initialization process preferably includes polling for and discovering all of the sounder units that are available. The set of sounder units thus found is reported to the user, who may then be allowed to modify the set by removing or reassigning sounder units that are not intended to participate in the subsequent measurements. The process of polling for sounder units may advantageously occur at regular intervals while GUI  250  is running, in order to detect when new sounder units have been added to the system, or to detect if an existing sounder unit has been removed or has failed during a test. 
   The initialization process preferably then ensures that reference clock generators  34  within each of the sounder units  10 ,  11 ,  12  are synchronized to each other. Synchronization may be performed within each sounder unit utilizing either clock synchronization signal  42  or external synchronization interface  35 . After synchronization has been performed, the final stage of initialization preferably includes obtaining the precise three-dimensional location of each of the sounder units by means of location processor  40 . Central controller  14  may poll for the three-dimensional coordinates of each sounder unit, and report the results to the user via 3-D location window  253 . The central controller  14  may advantageously improve location accuracy by transmitting differential GPS (DGPS) corrections to the sounder units at this time, if DGPS information is available. 
   Subsequent to initialization  262 , each sounder unit  10  is preferably configured from central controller  14  prior to running tests, as shown in step  263 . Configuration may preferably include the steps of selecting one or more frequency bands in which measurements are to be made, configuring PRBS patterns and transmit power levels to be used, selecting antenna pattern characteristics to be used, configuring measurement periods and repetition rates, and defining reporting options for measurement results to be sent back to central controller  14 . 
   During the actual RF propagation measurement process, the channel sounder transmitter  33  in any one sounder unit  10  is activated to transmit RF energy by means of antenna array  30 . At the same time, a plurality of other sounder units, for example  11 ,  12 , may have their channel sounder transmitters  33  disabled and their channel sounder receivers  32  and post-processing logic  36  activated to receive, demodulate, correlate and accumulate data from the RF energy received by their antenna arrays  30 . Data thus received and processed by the post-processing logic into bins representing elements of power-delay profiles are passed to the respective control processors in the receiving sounder units. These power-delay profiles are further processed and the resulting data is passed to central controller  14  via communications link  39  for final processing and presentation of results. 
   With reference to  FIG. 16 , an example of a power-delay profile measured by a sounder unit is represented in graphical form  280 . It is understood that this is a representation of the power-delay profile for purposes of description, and the actual power-delay profile comprises vectors of amplitude values stored in bin memory  182  in post-processing logic  36 . 
   The horizontal axis  282  represents an increasing excess delay in the propagation path between a transmitting sounder unit, for example  10 , and a receiving sounder unit, for example  11 . This excess delay arises as a consequence of RF energy being scattered from one or more scatterers in the region being measured, and corresponds to addresses in bin memory  182  in post-processing logic  36 , with an increasing excess delay corresponding to an increasing address. The vertical axis  283  represents an increasing power level for received RF energy. The power-delay profile is plotted as curve  281  corresponding to values stored in bin memory  182  as a function of address (i.e., excess delay). 
   The representation of the measured power-delay profile in this fashion normally contains a number of peaks  284 ,  285 ,  286 ,  287 ,  288 ,  289 . Each peak corresponds to one or more scatterers in the region surrounding the transmitting and receiving sounder units. The height of the peak corresponds to the total intensity of the reflection from the scatterer(s), and the width of the peak corresponds to cross-section of the scatterer(s). The first (lowest-delay) peak  284  corresponds to the direct, or line-of-sight, path between the transmitting and receiving sounder units, provided that such a line-of-sight path exists. It will be apparent to persons skilled in the art that analysis of the power-delay profile from any sounder unit will provide information such as the number of scatterers present in the surrounding region within the range of the sounder units  10 ,  11 ,  12 , as well as the properties of those scatterers. In addition, the lowest-delay peak  284  provides information about the attenuation experienced by the line-of-sight path through the region. 
   The RF propagation analysis process carried out by the measurement system requires not only identifying the nature of the scatterers within the surrounding region but also localizing their position within the region. With reference to  FIG. 17 , the process of localizing specific scatterers (corresponding to specific peaks in power-delay profile  280 ) with respect to their position in the surrounding region may comprise the steps of:
         (a) at step  301 , configuring the antenna radiation pattern  100  produced by antenna array  30  in both the transmitting and receiving sounder units to be an omnidirectional pattern;   (b) at step  302 , measuring and storing a base power-delay profile in the receiving sounder unit;   (c) at step  303 , configuring a null at an azimuth direction of 0 degrees in the antenna radiation pattern  100  produced by the receiving sounder unit;   (d) at step  304 , measuring a second power-delay profile in the receiving sounder unit;   (e) at step  305 , comparing the second power-delay profile with the base power-delay profile stored at step  302 ;   (f) at step  306 , checking whether any of the peaks in the second power-delay profile have dropped in amplitude relative to the same peak in the base power-delay profile by a predetermined threshold amount;   (g) at step  307 , if one or more peaks have dropped in amplitude, associating that peak in the power-delay profile with the azimuth direction of the null induced in antenna radiation pattern  100 ;   (h) at step  308 , checking to see if additional angles in the azimuth direction remain to be covered by the null induced in antenna radiation pattern  100 ;   (i) if additional angles remain, at step  309  rotating the direction of the null induced in antenna radiation pattern  100  by a predetermined angle, preferably 15 degrees, and repeating the process of steps  304 ,  305 ,  306 ,  307  and  308  until all possible directions have been covered; and   (j) at step  310 , repeating the process of steps  303 ,  304 ,  305 ,  306 ,  307 ,  308  and  309 , but this time with an omnidirectional radiation pattern  100  for antenna array  30  in the receiving sounder unit, and a null induced in radiation pattern  100  for antenna array  30  in the transmitting sounder unit.       

   The information gathered by the process depicted in  FIG. 17  preferably associates specific directions with specific scatterers (corresponding to peaks  284 ,  285 ,  286 ,  287 ,  288 ,  289  in example power-delay profile  280 ), relative to the positions of a transmitting sounder unit, for example  10 , and a receiving sounder unit, for example  11 . The process may further associate separate directions for a given peak (i.e., scatterer) relative to the transmitting sounder unit and receiving sounder unit. The absolute locations of these sounder units, for example  10  and  11 , within the region being measured may preferably be determined by means of location processors  40  within these sounder units. The absolute location of the specific scatterers can then be deduced by triangulation, preferably using a geometric method as depicted in  FIG. 19  and described in the accompanying text. 
   In some cases, multiple scatterers may correspond to a single peak of the power-delay profile measured at step  302  in  FIG. 17 . These scatterers can be separated to within the angular resolution of the sounder units by means of the triangulation process. The excess delay of the peak in the power-delay profile may preferably be used to further localize the position of the multiple scatterers. 
   In addition to localizing the positions of the scatterers within the region surrounding the sounder units  10 ,  11 ,  12 , it is necessary to determine the attenuation properties of the region in order to fully calculate the RF propagation characteristics of the space. It is well known in the prior art that the RF path loss within different indoor spaces may be approximated by standard propagation models having a number of site-specific parameters. The attenuation properties of the region are preferably characterized by calculating, using a process of successive refinement, the site-specific parameters of a particular propagation model from the actual attenuation measured over a plurality of line-of-sight paths within the region. 
   With reference to  FIG. 18 , the process of determining the attenuation properties of the region may comprise the steps of:
         (a) at step  321 , subdividing the region to be measured (for example, measurement region  13  enclosed by sounder units  10 ,  11 ,  12 ) into sub-regions, preferably in a regular fashion, and assigning initial values to the parameters of a predetermined path loss model;   (b) at step  322 , selecting a first sounder unit, for example  10 , to transmit RF energy into the region;   (c) at step  323 , computing line-of-sight paths from the sounder unit selected in step (b) above to a plurality of second sounder units, for example  11 ,  12 , that are configured to act as receivers;   (d) at step  324 , measuring power-delay profiles between the first sounder unit selected in step  322  above to each of the plurality of second sounder units for which line-of-sight paths are computed in step  323  above, and estimating the attenuation of the line-of-sight paths from the amplitudes of the lowest-delay peaks in the power-delay profiles;   (e) at step  325 , adjusting the values assigned to the parameters for each of the sub-regions along the various line-of-sight paths using a linear programming approach such that the overall attenuations computed by the propagation model corresponds to the actual attenuation measured by the sounder units acting as receivers;   (f) at step  326 , checking if all sounder units have been selected to act as transmitters; and   (g) at step  327 , if additional sounder units remain to be selected as transmitters, selecting a next sounder unit, for example  11 , to transmit RF energy into the region, and repeating the process of steps  323 ,  324 ,  325  and  326 , further adjusting the values assigned to the parameters of the propagation model for each of the sub-regions along the various line-of-sight paths.       

   It is necessary to determine whether a line-of-sight path exists between any pair of sounder units, for example  10  and  11 , before attempting to measure the attenuation and use it to compute the parameters of the propagation model. This may be done by locating the first peak in the power-delay profile, for example  284  in power-delay profile  280 , and comparing the excess delay of this peak to the expected run-time of the line-of-sight path between the sounder units as calculated from their geometric coordinates. If the excess delay of the first peak is substantially in excess of the expected run-time of the line-of-sight, then no line-of-sight path exists and this pair of sounder units cannot be used for adjusting the parameters of the propagation model. In this situation, the sounder units may be physically relocated until line-of-sight paths are found to exist. 
   The non line-of-sight peaks in each power-delay profile (i.e., the peaks corresponding to a path delay greater than that of the line-of-sight path) correspond to specific scatterers in the environment surrounding the sounder units, for example  10 ,  11 ,  12 . The possible locations of a given scatterer corresponding to the delay associated with a given peak lie on an ellipse, with the transmitting and receiving sounder units at the foci of the ellipse. If l represents the length of the non line-of-sight path as computed from the path delay, and d represents the distance between the transmitting and receiving sounder units, then the locus of the ellipse in the x-y plane is given by:
 
(4 x   2   /l   2 )+(4 y   2 /( l   2   −d   2 ))=1  (2)
 
   Once the locus of the ellipse corresponding to the position of a given scatterer has been determined, the exact location of the scatterer may preferably be found using the angle of arrival and departure information associated with the peak in the power-delay profile corresponding to the scatterer. This may be done by determining the intersection of rays drawn from the transmitting and receiving sounder units at the respective angles with the locus of the ellipse. With reference to  FIG. 19 , the procedure for using the power-delay profiles calculated by the post-processing units in sounder units  10 ,  11 ,  12  to determine the locations of the scattering elements within measurement region  13  preferably includes the steps of:
         (a) at step  341 , selecting a first sounder unit, for example  10 , to transmit RF energy into the region;   (b) at step  342 , selecting a second sounder unit, for example  11 , to receive and process the RF energy, and determining its distance from the first sounder unit selected in step  341 , preferably using data obtained from location processor  40 ;   (c) at step  343 , measuring the power-delay profile between the first sounder unit selected as in step  341  to the second sounder unit selected in step  342 , and associating specific angles of arrival for each peak in the power-delay profile with respect to the transmitting sounder unit, for example  10 , and the receiving sounder unit, for example  11 ;   (d) at step  344 , selecting a first non line-of-sight peak in the power-delay profile measured at step  343  above, by selecting the first peak with a delay greater than the estimated line-of-sight path delay between the first and second sounder units;   (e) at step  345 , calculating the total path delay between the first and second sounder units (corresponding to the delay of the peak selected in step  344 ), converting the path delay into a path length (by multiplying by the speed of electromagnetic waves), and then calculating the locus of the ellipse on which the scatterer must lie by using equation (2) above;   (f) at step  346 , locating the scatterer on the locus of the ellipse computed in step  345 , using the angle of departure at the transmitting sounder unit, for example  10 , and the angle of arrival at the receiving sounder unit, for example  11 ;   (g) at step  347 , checking if additional peaks remain to be selected in the power-delay profile;   (h) at step  348 , if additional peaks remain to be selected in the power-delay profile, then selecting the next peak and repeating steps  345 ,  346  and  347  to determine the locations of additional scatterers, until no more peaks remain;   (i) at step  349 , checking if additional receiving sounder units remain to be selected for participating in the measurement process;   (j) at step  350 , if additional receiving sounder units remain, then selecting the next sounder unit, for example  12 , and repeating steps  343 ,  344 ,  345 ,  346 ,  347  and  348  to determine the locations of additional scatterers, until no more sounder units remain;   (k) at step  351 , checking if additional transmitting sounder units remain to be selected for participating in the measurement process;   (l) at step  352 , if additional transmitting sounder units remain, then selecting the next sounder unit, for example  11 , and repeating steps  342 ,  343 ,  344 ,  345 ,  346 ,  347 ,  348 ,  349 ,  350  and  351  until no more sounder units remain; and   (m) at step  353 , terminating the process and reporting the results in terms of the number and positions of scatterers found in the environment.       

   In step  346 , if the intersection of the locus of the ellipse as computed in step  345  with the angle of departure at the transmitting sounder unit does not exactly match the intersection of the locus of the ellipse with the angle of arrival at the receiving sounder unit, the average of these intersections is taken as the location of the scatterer. 
   In some preferred embodiments of the present invention, after the attenuation properties of the measurement region  13  have been determined, and the scatterers within the region have been located, central controller  14  may preferably perform a final calculation procedure to combine the data from multiple sounder units, for example  10 ,  11 ,  12 , determine whether the measurement accuracy falls within user-defined limits, and present the results to the user. With reference to  FIG. 20 , this procedure may comprise the steps of:
         (a) at step  371 , reading in a general floorplan of the building, for example  20 , or otherwise determining the internal and external boundaries of measurement region  13 ;   (b) at step  372 , locating the sounder units  10 ,  11 ,  12  within the boundaries determined at step  371 , preferably using data obtained from location processor  40  within each sounder unit  10 ;   (c) at step  373 , subdividing measurement region  13  and calculating the parameters of an indoor propagation model for each sub-region as an average of the parameters obtained by measurements performed by a plurality of pairs of sounder units, said measurements being preferably made according to the process depicted in  FIG. 18  and described in the accompanying text;   (d) at step  374 , determining the locations of all of the scatterers within measurement region  13 , preferably according to the process depicted in  FIG. 19  and described in the accompanying text;   (e) at step  375 , using the relative amplitudes as well as the angles of arrival and departure associated with the peaks in the power-delay profiles measured by the sounder units to estimate the reflection properties of each scatterer within measurement region  13 ;   (f) at step  376 , partitioning the scatterers within measurement region  13  into groups, such that all scatterers within a given group are less than a predetermined distance from each other, and, for each group, coalescing the scatterers to create a larger virtual scatterer containing all of the scatterers in the group and having the averaged reflection properties of the scatterers in the group;   (g) at step  377 , performing a standard ray-tracing electromagnetic propagation simulation using the data generated during steps  373  and  376  to predict the power-delay profiles that should result at each of the sounder units, for example  10 ,  11 ,  12 , as a consequence of the attenuation and scattering properties of measurement region  13 ;   (h) at step  378 , comparing the predicted power-delay profiles from step  377  to the actual power-delay profiles measured by the sounder units;   (i) at step  379 , checking if the predicted power-delay profiles match the measured power-delay profiles to within predetermined limits of error;   (j) at step  380 , if the predicted power-delay profiles do not match the measured power-delay profiles, then requesting the user to reposition the sounder units  10 ,  11 ,  12  within measurement region  13  and take additional measurements to provide more data points for the attenuation and scattering calculations;   (k) at step  381 , if the predicted power-delay profiles match the measured power-delay profiles, then accepting the attenuation and scattering properties calculated for measurement region  13  during steps  373  and  376 , and using them to calculate and display a final propagation and scatterer map, preferably in measurement results window  254  of GUI  250 ; and   (l) at step  382 , terminating the process and waiting for further user input.       

   The process of performing a ray-tracing electromagnetic propagation simulation using the attenuation and scattering properties of a region is well known in the prior art and will not be described here further. The final propagation and scatterer map may calculate loci of constant attenuation within measurement region  13 , and draw these as contours  255  in measurement results window  254  of GUI  250 , preferably superimposed on a depiction of the floorplan, for example  256 . The scatterers are also depicted in their computed locations upon floorplan  256 . It is apparent that the detailed propagation information presented in measurement results window  254  is sufficient to enable prediction of all of the RF propagation effects within indoor measurement region  13 . This propagation information may be stored or further manipulated as desired by the user of the propagation measurement system. 
   Measurements over a period of time may be performed by repeating the measurement process multiple times at regular intervals. Central controller  14  may preferably receive the information for each measurement, perform calculations according to the process depicted in  FIG. 20  and described in the accompanying text, and store the resulting information as a record of the propagation characteristics of measurement region  13  at the corresponding instant in time. A series of such measurements may be stored to provide the user of the system with a view of the properties of measurement region  13  over time. 
   Each sounder unit  10  should preferably present its operational status continuously to the central controller  14  for display in the sounder unit status window  252  of GUI  250 . The status information displayed may include: the health of the sounder unit (whether running, idle or faulty); the current location of the sounder unit in three dimensions; and the current transmit power and receiver sensitivity settings. 
   In another embodiment of the present invention, antenna array  30  may generate a radiation pattern corresponding to a beam rather than a null. With reference to  FIG. 21 , antenna array  30  may comprise a large number of vertical radiators  404 ,  405 ,  406 ,  407 ,  408 ,  409 ,  410 ,  411 ,  412  mounted on a ground plane  400  in two rows  401  and  402 . Such a large array of radiators, when driven by a corresponding number of channels within beamformer  31  in sounder unit  10 , is capable of creating a beam-shaped radiation pattern, as depicted for example in  FIG. 22 . With reference to  FIG. 22 , adjusting the relative phases and amplitudes of the signals applied to vertical radiators  404 ,  405 ,  406 ,  407 ,  408 ,  409 ,  410 ,  411 ,  412  may create a beam-shaped radiation pattern with a main lobe  423  and a plurality of small sidelobes, for example  424  and  425 , as plotted on azimuth plot  421  with cardinal directions  422 . 
   This beam-shaped radiation pattern may be advantageously used to determine the arrival or departure direction associated with each peak in the power-delay profile, for example  280 , as measured by the sounder units, without creating unwanted radiation in other directions as may be encountered when using a cardioid type radiation pattern, for example  90 , and hence improving the sensitivity and accuracy of the measurements. The procedure of determining the arrival or departure directions may exactly be the same as that depicted in  FIG. 17  and described in the accompanying text, with the exception that instead of checking for a drop in the amplitude of a peak at step  306 , a rise in the amplitude of a peak is checked for instead. The remainder of the measurement and calculation procedures are unchanged. The process of calculating the phase and amplitude adjustments in order to steer the direction of beam  423  while minimizing the sidelobes, for example  424  and  425 , is well known in the prior art and will not be described here further. 
   In another embodiment of the present invention, antenna array  30  may be advantageously replaced with an antenna or antenna array having an omnidirectional pattern, for example a single vertical radiator mounted above a ground plane, and beamformer  31  may preferably be omitted. The RF output from channel sounder transmitter  33  and RF input to channel sounder receiver  32  may be connected directly to the antenna or antenna array. This eliminates the ability to determine the precise location of the scatterers in the measurement region  13 , but also enables a significant reduction in the overall cost and size of each sounder unit  10 . 
   In another embodiment of the present invention, beamformer  31  and antenna array  30  may advantageously be replaced by a simple passive directional antenna, for example a horn, dish or array, having a beam shaped radiation pattern, for example  423 . This directional antenna may be mechanically steered to orient the main lobe of the antenna radiation pattern in any desired direction, thereby achieving the same effect as that of electrically steering a beam or null but without the cost and complexity of beamformer  31 . The process of mechanically steering a directional antenna is well known in the prior art and will not be described here further. 
   In another embodiment of the present invention, sounder unit  10  may advantageously implement the functions necessary to perform propagation measurements on multiple frequency bands, for example 2.4 GHz and 5.6 GHz. A multi-band sounder unit may be realized in different ways. For example, a multi-band sounder unit may be created by duplicating the functions of antenna array  30 , beamformer  31 , channel sounder receiver  32  and channel sounder  33 , and enabling each duplicated set of functions to operate on a different frequency band. As another example, a multi-band sounder unit may be created by using a multi-band antenna array  30  and band-switching the components of beamformer  31 , channel sounder receiver  32  and channel sounder  33 . The use of multi-band sounder units enables the propagation characteristics of measurement region  13  to be measured at different frequency bands without requiring two or more independent sets of sounder units operating in the different frequency bands of interest. 
   In another embodiment of the present invention, the propagation measurements performed on multiple frequency bands by sounder unit  10  may advantageously be combined to obtain a more accurate measurement of the attenuation properties of the measurement region  13 . It is well known that the propagation characteristics of an indoor region are highly dependent on the frequency band being used. The data obtained by measuring the attenuation properties and scatterer locations of measurement region  13  in more than one frequency band may be combined during steps  373 ,  375  and  376  in the process depicted in  FIG. 20  and described in the accompanying text, to produce a more accurate representation of the propagation characteristics of the measurement region. 
   In another embodiment of the present invention, one or more of sounder units  10 ,  11 ,  12  may contain only channel sounder transmitter  33 , omitting channel sounder receiver  32 ; or one or more of sounder units  10 ,  11 ,  12  may contain only channel sounder receiver  32 , omitting channel sounder transmitter  33 . This enables additional sounder units to be located in measurement region  13  without significantly increasing the cost of the complete system. 
   In another embodiment of the present invention, antenna array  30  may advantageously be replaced with an antenna array that is capable of controlling its radiation pattern over three dimensions rather than two dimensions (i.e., in both azimuth and elevation, rather than only in azimuth). This may enable sounder units  10 ,  11 ,  12  to be used to determine the location of scatterers in a three-dimensional volume rather than over a two-dimensional area. The procedures for computing the attenuation characteristics and locating the scatterers are preferably the same as for the two-dimensional case, with the exception that the null or beam is steered in three dimensions. 
   In another embodiment of the present invention, the sliding correlator channel sounding method utilizing PRBS patterns, as used by channel sounder transmitter  33  and channel sounder receiver  32 , may be replaced by another channel sounding method that is capable of generating a power-delay profile. Examples of alternate channel sounding methods are impulse-based channel sounding and swept-frequency (chirp) channel sounding with an inverse Fourier transform. 
   It is apparent that the teachings of the present invention enable the RF propagation characteristics of an indoor environment to be measured in a simpler and more deterministic manner. It is further apparent that the present invention enables RF propagation measurements to be made over a two-dimensional area in an automatic manner using a relatively small number of measuring instruments. It is further apparent that the present invention provides for the time-varying RF propagation characteristics of an indoor environment to be measured and recorded. 
   Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of this invention, will be apparent to persons skilled in the art upon reference to this description without departing from the scope of the invention, which is defined solely by the claims appended hereto.