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This paper studies the performance of a Medium Access Control (MAC) protocol for an Impulse Radio Ultra Wide Band (IR-UWB) based Wireless Body Area Network (WBAN). Unlike conventional UWB wireless schemes, the proposed UWB based WBAN uses UWB pulses for transmission and a narrowband signal for receiving at the sensor node to achieve a low-power network. To investigate the performance of the UWB-WBAN system in a large environment like hospitals, two network topologies have been considered. One topology uses a router as an intermediate node to collect vital physiological information collected by on-body sensor nodes. In the second scenario, the sensors directly transmit the collected data to a central control node. Binary Pulse Position Modulation (BPPM) is used as the modulation scheme for UWB transmission and Frequency Shift Keying (FSK) is used for narrow band transmission. Another unique feature in the proposed WBAN network is that the sensor nodes utilize multiple pulses per bit during UWB transmission. Number of pulses sent per bit are dynamically varied according to received bit error rate and used to optimize wireless transmission, thus providing low-power consumption.