Code Division Multiple Access (CDMA) is used in spread spectrum systems to achieve multiple access. The frequency spectrum of a signal is spread by using a code that is not correlated with the data signal. This code is unique to each recipient. A distinction can be made among signals because the codes are chosen to have low cross-correlation values. By matching the send and receive codes, the receiver is able to properly translate the message or data.
This function includes a measurement display with eight fixed markers. The marker values are displayed below the measurement and include the power and frequency at each marker along with a pass fail indication.
This function displays from 1 to 5 channels (carriers) and two adjacent channels on each side. The power of each channel is displayed below the measurement graph and includes the frequency and power in each of the 4 adjacent channels.
Code Domain Power (CDP) displays how much of the channel power is in each Walsh Code. Power is normalized to the channel power and, therefore, a code reading of –10 dB means that the power in that code is 1/10th of the channel power. The upper half of the measurement display shows all of the channels, and the lower half shows a zoom view of selected channels. The number of zoom channels and the starting position (relative to the upper display) are set with CDP submenu keys for Zoom and Zoom Start Index. A colored background in the upper display indicates the channels that are in the lower, zoom display. Colors are applied according to
Table: “Code Domain Power”:
Frequency error is the difference between the received center frequency and the specified center frequency. This is only as accurate as the frequency reference that is used and is typically only useful with a good quality external frequency reference.
Rho is a measure of modulation quality and measures the amount of power that is correctly transmitted. An rho of 1.0 indicates a perfect signal. The standard requires > 0.912, and common measurements are > 0.94. Due to environmental factors, rho values are typically poor (< 0.9) when measured over the air, and are usually useful only when connected to a base station.
Tau is the timing error. It is the timing offset, or the difference in time between when the PN sequence (or short code) restarts, and when the PN sequence should restart. A base station needs to be aligned in time to an absolute reference such as GPS. This cannot be measured if the PN search is set to No Trigger.
Note that during OTA measurement, Tau increases with distance from the transmitter. If the instrument is connected to the base station, then a Tau of 10
μs or less is specified, and 5
μs is typical. If the timing error becomes too large, mobiles may have trouble when being handed off to this particular base station.
This function displays a table view of the channels with channel code number, status, power (dB and dBm), and whether multiple codes are used. Overall code utilization is displayed as a percent at the bottom of the measurement screen.
Pilot Power: Pilot power is the total power in the pilot channel. This is normally a constant for a base station. When the instrument is connected to the base station, if the measured power is unexpected, then check that the entered power offset is correct and that the connections are tight. Unexpected readings may indicate an incorrect power setting at the base station. For Over-The-Air (OTA) measurements, the pilot power will vary as the signal path from the transmitter to receiver varies.
Multipath Power: Multipath Power is a measure of the total power in the dominant signal (spread in time due to multipath echoes) relative to its power in the main transmission path. This value should be < 0.4 dB to make good measurements.
