1. Field
The present disclosure relates generally to communications, and more specifically, to systems and techniques for testing a wireless communications device.
2. Background
Modern communication systems are designed to allow multiple users to share a common communications medium. One such communications system is a Code Division Multiple Access (CDMA) system. The CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, and thereby, spreads the signal. The transmitted signals are separated in the receiver by a correlator that uses a corresponding code to despread the signal. The undesired signals, whose codes do not match, are not despread and contribute only to noise.
In a CDMA communications system, a subscriber station may access a network, or communicate with other subscriber stations, through one or more base stations. The term “subscriber station” is intended to encompass any type of mobile communications device including cellular or wireless phones, personal data assistants (PDA), laptops, external or internal modems, PC cards, or any other similar devices. Each base station may be configured to serve all subscriber stations in a specific geographic region generally referred to as a cell.
In the mobile environment, transmitted signals are reflected and scattered by obstacles in their path, often resulting in multiple copies of the signal arriving at the receiver at different times. Depending on the location of the receiving antenna relative to the transmitting antenna, and the obstacles in the signal path, the multiple copies of the signal may combine constructively or destructively at the receiving antenna. In narrow band mobile applications, this phenomenon can cause fluctuations in the signal when the subscriber station travels even a small distance. This is often referred to as “fading”. The use of a wide band CDMA signal can significantly reduce the impact of fast fading. Another technique to mitigate fading in mobile communications is to use multiple antennas to increase the gain of the signal due to spatial diversity.
Various test methods and devices have been developed over the years to verify the functionality of a subscriber station. One method for testing a subscriber station includes the use of an actual base station in the field. The problem with this approach is that all possible test scenarios cannot be easily accomplished.
A more practical method for testing a subscriber station is with a base station test set-up. The base station test set-up may be implemented with hardware and controlled by test software so as to mimic the functionality of an actual base station. The advantage of this approach is that virtually any test scenario may be programmed into the software. Moreover, the wireless medium can be controlled by use of a radio frequency (RF) cable between the base station test set-up and the subscriber station under test.
While the use of a base station test set-up provides an expanded range of test capabilities, this test method still fails to simulate real world propagation conditions, such as the ability of the subscriber station to deal with fading. This problem is not limited to CDMA systems. Accordingly, there is a need for a new testing methodology applicable to a wide range of applications.