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    Testing your strength

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    Prof Dr Matthias Wuschek, University of Applied Sciences Deggendorf, Dipl.-Phy, and  Jurgen Kausche, Product Management EMC Systems and Projects, Rohde & Schwarz explain the considerations required for testing field strength of a UMTS base station.

    Networks for third-generation mobile radio (UMTS) are currently being devel-oped in most European countries. Emissions radiated by broadband radio systems, such as UMTS, DAB, DVB-T, or WLAN, must be measured precisely. The modulation and multiple access procedures used in UMTS mobile radio require that the electromagnetic field (EMF) measurement systems display specific characteristics. Measuring instruments as well as measurement procedures must provide results that allow correct extrapolation of the field strength that a base station would generate at the test site at maximum load.

    In contrast to GSM, UMTS is a single-frequency network in which all antennas of an operator transmit at the same carrier frequency. Each base station encodes its signal with a scrambling code so that a terminal (telephone) can distinguish between the various stations. In addition, spread codes separate the various signaling and data channels of a station. Some of the signaling channels of a station, such as the common pilot channel (CPICH), operate at a constant transmit power, while other signaling channels, as well as the data channels, either become active as needed or permanently modify their power output.

    EMF measurement must take several important characteristics of the UMTS signal into consideration. Typical for this signal are a bandwidth of about 5 MHz, the noise-like signal with a crest factor of typically 10 dB, and a transmit power that varies by about 6 dB to 10 dB.

    EMF measurement methods

    Some basic steps included in every standardized EMF measurement must also be taken for UMTS. First, reflection and shadowing effects can cause significant spatial fluctuations in field strength. Therefore, any emission measurement in the area under test (for example, a room in a house) must measure the maximum field strength at that location. In addition to performing localized measurements, the stirring method or the multipoint method are also used for this purpose. The measurement results must be extrapolated correctly to find the value that represents the base station’s field strength at maximum load. Broadband measurements using isotropic E-field sensors are of only limited usefulness for this task because of their lack of selectivity and relative lack of sensitivity. Much more suitable are spectral or code-selective measurements using calibrated measurement antennas.

    The simplest method is based on a spectral measurement of the signal output by the station. To ensure that the measurement is valid, it is important that the settings on the spectrum analyzer are correct and suitable for the signal. For example, an adequate IF bandwidth and an rms detector are absolutely necessary for measuring rms  values. However, this procedure has some basic disadvantages. The measurement results show the emissions from all antennas of an operator, i.e. not only the station under test, but also all surrounding stations. A proportionate assignment of emissions is not possible. Moreover, the load on the individual antennas is not known at the time of the measurement. This makes precise calculation of the maximum load value impossible.

    Another option is code-selective measurement. This test measures the power of every CPICH that is present, which every base station transmits at a constant level. The scrambling code, which is automatically included in the measurement, can then be used to assign the measurement values to each base station. The extrapolation is done using the ratio of CPICH to the possible maximum power. This ratio is dependent on the settings on the base station. Typically, these are 2 W for the CPICH and 20 W for the overall transmit power of a UMTS cell, resulting in a factor of 10 dB. The code-selective method thus allows the maximum system load to be extrapolated correctly, regardless of what the actual load was at the time of the measurement.

    The requirements for this type of measurement system (Fig. 1, see page 28) include not only the ability to measure all CPICH signals that are present, but also high sensitivity and wide dynamic range, a high rate of measurement (5 to 10 measurements per second), the MaxHold function for finding the maximum spatial value, as well as simple operation and assessment. To meet these requirements, the system must be specially designed for CPICH decoding via the air interface. When used for UMTS, the spectral measurement method previously used for EMF limits the ability to assign measurement values to the base station as well as the ability to extrapolate the maximum system load.

    An alternative is decoding the signaling channel CPICH using measurement systems designed for this purpose, some of which can also be used for spectral EMF measurement.