A PHS Test System for Japan
Noise Com Inc.
The personal handyphone system (PHS) provides wireless communication to more than three million subscribers in Asia. One of the major PHS operators in Japan plans to employ maximum ratio-combining diversity with four (and optionally eight) antennas to increase capacity and reduce the effects of multipath propagation. In response to a request to provide the test solution for this customer's PHS application, a PHS interoperability lab, or PHS test system, was developed.
PHS provides equal forward and reverse link coverage even though the cell station transmitter power (500 mW, max) is 50-times larger than the personal station power (10 mW, max). This consistent coverage is possible using signal-processing techniques in the cell stations, including a low noise figure transceiver design, optional use of maximum ratio-combining diversity with up to eight antennas, sectored transceivers with directional antennas and coherent demodulation.
The diversity antenna system, sometimes referred to as a smart antenna system, provides a directional steerable antenna, which attenuates the signals that arrive from directions other than those intended. The attenuation is accomplished by combining the inputs from the four (or eight) antenna elements with a specific phase relationship. Undesired input signals with an out-of-phase relationship tend to cancel rather than add during this process. Paragraph 3.4.4 of the RCR STD-28 PHS standard allows the use of smart antennas by incorporating the phrase: "...in cases where the effective radiated power is less than the value when the specified antenna power is applied to an antenna of absolute gain 10 dBi (2.14 dBi for personal stations), the portion by which it is lower may be compensated by the gain of the antenna."
PHS cell stations can control the output power level of the personal stations by broadcasting the power control and handoff levels on the control channel. Interference between personal stations is reduced when the power levels are decreased. The gain of the directional antenna compensates for the reduced power and thus maintains a valid combination (in accordance with Paragraph 3.4.4) with a reduced interference level and, therefore, larger channel capacity. This combination can serve more subscribers than a cell station with a conventional antenna.
The maximum ratio-combining diversity antenna and the transmitter power control functions of PHS must operate in the real world, which means under multipath fading propagation conditions. The PHS test system provides a comprehensive emulation of the real-world wireless environment, including several impairments. The system is group delay and amplitude equalized at the factory for each antenna input since phase and delay characteristics are essential for diversity antenna testing.
Full Duplex Fading in TDD Systems
The forward and reverse links for each diversity antenna input may be modeled using the same one channel of a multipath fading emulator when testing PHS and the Digital European Cordless Telephone (DECT) system because these are time-division duplex (TDD) systems. The four channels of PHS are transmitted in time slots comprising a 5 ms frame. The first 2.5 ms are spent transmitting the four channels on the forward link; the next 2.5 ms transmit the reverse link. A 1.875 ms pause exists between the forward and reverse link transmissions on each channel.
The PHS test system takes advantage of the pause between forward and reverse link transmissions by first using the multipath fading emulator channel for the forward link, and then using the same channel for the reverse link without any switching or disconnecting, as shown in Figure 1 . A set of PHS duplexers and dividers provide the forward and reverse link signal separation. The correct propagation loss and power levels for the instruments are controlled by a series of programmable step attenuators. Each multipath fading emulator is also equipped with a second channel and built-in combiner for an interference signal with full propagation emulation.
Fig. 1: The TDD connection for PHS and DECT applications.
Interference Susceptibility Testing
PHS aims at assuring quality equal to or better than that of the existing analog cordless telephone systems, with improved encryption features and effective use of frequencies. The effect of interference on the cell stations and personal stations should be minimized in order to increase the capacity of PHS.
Both the personal and cell stations have a slot unit interference detection function and can allot channels automatically where little interference exists. If interference is received during communication, the personal and cell stations can avoid interference in slot units by channel switching, automatic reconnection or temporarily stopping transmission. The interference susceptibility of PHS is commonly tested with one interfering signal that is 50 dB larger than the desired signal but at the alternate channel, 600 kHz offset in frequency, or by two interfering signals 47 dB above the desired signal but at 600 and 1200 kHz offsets. Such large differences between the power levels of the interferer and the desired signal require special noise-reduction techniques when the interferer is faded as in the PHS test system. Optionally, the test system allows noise-reduction circuitry to be switched in for the interference test to reduce the noise of the interferer at 450 to 1350 kHz offsets.
The carrier-to-noise (C/N) ratio is set individually for each antenna input by means of a precision C/N generator (model UFX-BER-1850). The noise must be uncorrelated from channel to channel, hence the need for four precision C/N generators. The noise must also be added clearly after the multipath fading since C/N is the ratio of average signal power relative to the constant level of additive white Gaussian noise (AWGN). The precision C/N generator provides an extremely accurate C/N ratio since it uses the substitution calibration method to set the ratio. In the substitution calibration method, the signal is measured first and then the noise is set to produce the same reading on the power meter, resulting in a C/N ratio of 0 dB. Thereafter, the noise level is increased or decreased to provide the desired ratio. The power meter of the precision C/N generator is a special design since most commercially available power meters are designed and characterized for sine wave and quadrature phase-shift keying (QPSK) modulated signals only.
The comprehensive PHS diversity antenna test system is provided in two 19-inch racks. The complete test system allows the PHS operator to test diversity antennas, which will increase the capacity of the network.
Noise Com Inc.,
Paramus, NJ (201) 261-8797.
The Personal Handyphone System
The standard document for PHS is RCR STD-28, issued by the Research & Development Center for Radio Systems in Tokyo, Japan. The assigned PHS frequency band (in Japan) is 1895 to 1918.1 MHz, providing 77 frequency channels with 300 kHz channel spacing. The radio access method for PHS is four-channel multiplex multicarrier TDMA-TDD, as shown in Figure 2 . Maximum output power of the cell station is 500 mW, while the personal station generates 10 mW (max). The modulation method is p/4 QPSK with a roll-off rate of 0.5 and transmission rate at 384 kbps.
Fig. 2: The four-channel TDMA TDD used by PHS.
Typically, PHS personal stations offer approximately five hours of talk time and up to 550 hours of standby while using ordinary batteries. As of July 1996, there were more than 3.2 million PHS subscribers in Japan alone.