The antenna test bed (ATB) uses an upgraded version of the ITS digital channel probe (DCP). The DCP transmits a maximal-length pseudo-noise (PN) code. The PN code is used to biphase shift key (BPSK) modulate the RF carrier. The transmitter is both frequency and bit-rate agile and can produce multiple PN codes and frequencies simultaneously. This is useful for polarization and spatial diversity studies.

The transmitted signal, modified by the radio channel, is received, down-converted to an intermediate frequency (IF), and then digitized. If desired, the in-phase and quadrature-phase components can then be determined via software. A complex impulse response can be software generated by cross-correlating a copy of the transmitted PN code with the received signal after down-conversion to baseband. Because the DCP digitizes at IF, it can measure an impulse response much faster than an analog sliding correlator. In particular, the time window for this digital system is the length of one PN code whereas the time window for an analog sliding correlator is some larger multiple (>1) of the PN code length [1]. This allows better characterization of a rapidly changing radio channel. It also avoids problems with nonlinear components when making wideband measurements and allows accurate wideband amplitude and phase calibration of the system. This is essential for digital beam-forming applications.

The probe is typically configured to transmit a 511 bit PN word at 10 Mb/s. The theoretical impulse signal power to correlation noise power level is 54 dB for the 511 bit PN sequence. The processing gain of the system is 27 dB when the signal power equals the noise power. Multipath signals separated by 100 ns can be resolved to a maximum delay of 51 m s using this configuration. The probe can also be configured for higher bit rates (higher bandwidth and time resolution), longer codes (longer maximum delays), different code types and other RF carriers. Additional features of the system include time of flight and Doppler measurements. System timing is maintained using rubidium oscillators at the transmitter and receiver. These clocks synchronize the PN code generators, phase lock all local oscillators and provide sampling clocks for the digitizers. Multiple digital signal processors (DSPs) and a host computer system control data acquisition. The DSPs can be configured for real time processing or data streaming.

The ATB is compact and well suited for mobile measurements. A typical configuration is that of a cellular link with a fixed base station and a mobile. ITS uses a specially equipped van for mobile channel sounding measurements.

Table 1 below summarizes the range of permissible values for the ATB system, as well as giving an example of a PCS band system configuration. Block diagrams for the system can be found in [2].

[1] R. F. Linfield, R. W. Hubbard and L. E. Pratt, "Transmission channel characterization by impulse response measurements," OT Report 76-96, Aug. 1976.

[2] P. B. Papazian, K. Allen and M. Cotton, "A test bed for the evaluation of adaptive antennas used for mobile communications," in Proc. IEEE Aerospace Conference, Snowmass, CO, 1998, paper #161.