Delay Line Multipath Simulator

In wireless communications, multipath signals are received in both terrestrial and indoor environments where different forms of propagation are present and the signals arrive at the receiver from transmitter via a variety of paths. The results are called multipath interference, causing multi-path fading. It is advantageous to create repeatable multi-path fading in a controlled lab environment so that various radio designs can be compared in terms of performance such as throughput and receive sensitivity. IEEE has standardized channel fading into models A to F for varying degrees of severity. Based on these models, products in the past have used elaborate DSP to simulate these models. However, many of these products proved to be too expensive and too cumbersome to operate.

Adaura Technologies’ OptiMeasure Series Switchable Delay Line Simulator (DLMS) uses shorted cables in a unique patent pending configuration to create a multi-path model that is extremely repeatable. The device utilizes shorted cables of precise lengths to create delayed signal reflection where RF power dividers and switches are used in an arrangement such that all of the geometrical combinations of the additive delays are created. The delayed path is switchable via computer or manual control so that a direct comparison of the performance can be made between multi-path and non-multi-path. The simulator is housed in a versatile 2U chassis. There are 5 additive 10ns delays creating a first order total of 50ns. Second order reflection creates a combined delay of 100ns. Subsequent reflections from the shorted transmission line cables create all geometrical combinations of the delays. This is analogous to the IEEE B model.

Specifications

Application Notes
In the “Through Bypass” mode, the RF signal does not go through any signal conditioning and is simply routed from the RF IN port to the RF OUT port. In the “Multipath” mode, the RF signal is routed to the common ports of the 2-way power dividers 2WPD_1 and 2WPD_8. Through the 2 power dividers, the “Main Path” is the RF signal traveling the shortest distance. The other output of 2WPD_1 is routed to the common port of 2WPD_2. First output port of 2WPD_2 is shorted creating the first reflection bouncing back to 2WPD_1 common port and travel to 2WPD_8 common port. Second output port of 2WPD_2 is connected to a cable of 80” in length which provides a time delay of 10ns.

Keep in mind the time delay is constant regardless of the RF frequency thus making it usable in the entire frequency range of the 2WPD which is 1 to 6GHz. This first cable is connected to the common port of 2WPD_3. One port of the 2WPD_3 is shorted thus creating another reflection with 10ns delay which travels back toward the “Main Path” becomes the additive delay component. The other port of the 2WPD_3 is connected to another cable of 80” in length which provides another time delay of 10ns. Again, this cable is connected to the common port of 2WPD_4 where one port is shorted to create reflection and the other port is fed into yet another cable to create additional time delay. As shown in the block diagram, each shorted port provides an incremental time delay of 10ns each when connected with the 5 cables. The symmetrical design of the delays makes the RF input ports reciprocal in nature. A total of 100ns delay is created when the RF input signal travels down the entire 5 cables and reflects back. Subsequently all geometrical combination of 10ns delays are created. Since each cable exhibits insertion loss, the longer length the signal travels the more it will be attenuated. The entire process simulates a real life situation where a transmitted signal in a room will reflect upon many objects and furniture. The combination of all these reflections will eventually reach the receiver with the multipath characteristics.

Mechanical Design