We analyze a low power, wideband CDMA system with multiple antennas. The joint "slope region" - of transmission rates at minimum energy per bit - is derived as a function of proportion between vanishing rates for multiple users. Vertices of the slope region may be achieved by a matched filter linear interface, followed by successive interference cancellation. We introduce and evaluate "robust slope region," the largest region which is inside every other slope region and is therefore independent from the relative proportion between transmission rates. Furthermore, we find the "robust slope," maximum slope which can always be guaranteed to all users.