Network Deployment (Installation and Maintenance)
As fixed WiMAX network operators join the existing wireless carriers in the deployment of RF equipment to create a network, they face the challenge of a new undertaking: installation and maintenance of wireless networks. With fixed WiMAX solutions now being deployed and new trials for mobile WiMAX beginning each week, adequately addressing that challenge is all the more imperative.

When deploying a base station, the operator must comply with government regulations for radio frequency use, such as transmitter power levels within regulation range, correct frequency use, not causing interference with other users, and assuring correct operation of the network equipment. All this should satisfy both the regulatory agencies as to the correct use of the frequency band as well as, of course, the end users with the provided service quality. A list of the items which must be tested, and specific test requirements for each, is as follows:
 

Frequency Availability
Prior to base station deployment, the engineer must verify the availability of the spectrum via a site survey. This is done to determine whether anything will interfere with the assigned transmit frequency, or if anybody else is transmitting on the base station's assigned receive frequency.

The Interference Analyzer option available in the Anritsu BTS Master MT8222A (see Figure 4) is specifically designed to assist the engineer in finding signals that can cause problems. Using a spectrogram capture mode, it monitors any transmission across a specified frequency band for a period of time to make sure that the spectrum is clean. When an interfering signal is found, the Interference Analyzer can locate it with a directional antenna based on signal strength.

The Transmit Frequency Regulatory agencies dictate that a wireless operator must deploy its services only on the approved, designated frequency. This needs to be verified with a certain level of accuracy, not just for the purpose of compliance, but also to guarantee interoperability with the end user's stations. Due to a high accuracy internal clock which verifies its accuracy whenever the Global Positioning System (GPS) functionality is used, the BTS Master can make the required frequency measurement with an accuracy of better than 0.025 ppm.

Power Level
The power level of a base station greatly affects the usable range. To ensure correct coverage in the designated area, the power level needs to be measured with high precision. The WiMAX standard also requires that the adjacent channel power ratio (ACPR) be verified to make sure that the transmitter does not put out significant power on neighboring channels. And, the crest factor should be measured to verify the correct ratio of peak to average power over the frame. The High Accuracy Power Sensor option of BTS Master features a specified accuracy of better than 0.16 dB. This capability enables the engineer to fine-tune the transmit level with great accuracy.

Signal Quality
Signal quality has many aspects which need to be verified. The main issue, relative constellation error (RCE), is the error vector of the received modulation constellation expressed in dB versus in % as done in an error vector magnitude (EVM) measurement. RCE and EVM are important metrics for measuring the signal quality in the digital communications.

Other Tests
These measurements, however, are only an indication of the total signal quality. The engineer must also look for more detailed quality problems by analyzing the individual subchannels and symbols. For instance, it is also important to see the actual EVM versus subchannel (frequency domain) and EVM versus symbols (time domain) displays, as well as the spectral flatness of the channel to assure consistent quality for all users. The actual constellation should also be viewed to verify correct placement of the points in a single display with all modulation types.

Once these measurements are complete, the engineer will have confirmed that the WiMAX base station is performing as required. However, before it is actually put on the air, the correct installation and performance of the cabling and antenna system must be verified. This can be accomplished using a cable and antenna analyzer (there is one built in to the BTS Master). This functionality provides measurement capabilities such as return loss, cable loss, VSWR, and the ability to find a problem location with the distance-to-fault (DTF) function. Thanks to a true 2-port capability (both magnitude and phase), BTS Master's Cable and Antenna analyzer can also be used in situations where verification of duplex filters, amplifiers, repeaters, or an antenna isolation measurement is required.

Understanding Fixed WiMAX Measurements
Verifying fixed WiMAX performance requires field technicians to measure many different types of parameters. The key WiMAX measurements to make include:
 

• Channel Power (RSSI): measures the average time domain power within the selected bandwidth and is expressed in dBm.
• Received Signal Strength Indicator (RSSI): 'vendor-defined' in the standard and is typically the same as Channel Power.
• Occupied Bandwidth (BW): calculated as the bandwidth containing 99% of the transmitted power in the given RF span.
• Adjacent Channel Power Ratio (ACPR): The filtered power across the bandwidth of the signal is compared with the power in the adjacent upper and lower channels; the differences are measured in dB.
• Data Burst Power: the RMS power over the data burst part of the WiMAX frame.
• Preamble Power: the RMS power over the preamble part of the WiMAX frame.
• Crest Factor: the ratio of the peak-to-average power over the entire frame.
• Frequency Error: the difference between the received frequency and the specified frequency. This measurement is only as accurate as the frequency reference used. Frequency error is displayed in both Hz and ppm.
• Error Vector Magnitude (EVM): the ratio, in percent, of the magnitude of the error vector (e.g. difference between the measured waveform and the reference waveform) to the magnitude of the reference waveform. From this, the rms and peak EVM can then be derived.
• Relative Constellation Error (RCE): similar to EVM, but expressed in dB. RCE = 20 log(EVM in %/100) Both RMS and peak values over an entire frame are displayed.
• Carrier Frequency: the measured frequency of the received signal. It is equal to the specified frequency plus the Frequency Error described above.
• Base Station ID: Each transmitter has a unique ID. BTS Master displays it as Base Station ID by decoding the Frame Control Header (FCH) part of the downlink frame.
• Adjacent Subcarrier Flatness (Peak): the absolute difference between adjacent subcarriers.