## 1 Introduction  
Wireless sensor network (WSN) nodes of the past decade have traditionally been designed with a simple system concept in mind: application-specific sensors attached to a general platform that provides computational resources and wireless connectivity. Successful platforms, such as the MICAz [1] and TelosB [2] motes, are representative examples of this architecture, as both employ simple microcontrollers and integrated short-range radio frequency (RF) transceiver chips. The ultra-low-power requirement of these WSN nodes is usually addressed by optimizing the consumption of the microcontroller and the RF chip. The 30-60 mW order power demand of wireless communication, however, gradually overwhelms that of the microprocessor performing the sensor level digital signal processing, which is typically below 10 mW [3; 4; 5]. RF-related energy saving in multi-hop networks is almost exclusively achieved in the medium access control (MAC) layer by efficient scheduling of single-hop packet transmission and reception, e.g., via duty cycling. One major reason for this is that commonly used RF chips come as application-specific integrated circuits (ASIC). They primarily provide a packet level interface and prescribe a carrier sense multiple access (CSMA) or time division multiple access (TDMA)-based scheme, restricting access to the lower layers, hence, imposing inflexibility. Software-defined radio (SDR), on the other hand, is a well developed technology that emphasizes the design flexibility of communication protocols. Commercially available SDR platforms connected to desktop computers enable the development of complex and full vertical radio communication stacks with acceptable engineering effort. However, the design flexibility of such SDR platforms comes at a price in the power consumption budget (typically over 10 W) and in physical dimensions, which generally prohibits experimentation in deployed or mobile scenarios with no wall power for any reasonable length of time. We argue that communication-related power saving in WSNs can go beyond designing scheduling schemes defined over packet transmissions; energy consumption may be reduced significantly by tailoring the MAC and physical (PHY) layers of the communication protocol stack to a specific WSN application. Ideas can be borrowed from other fields, such as cellular telephony or satellite communication, and design flexibility can be increased by following the approach of field-programmable gate array (FPGA)-based software-defined radios. We also suggest that WSN communication-related research should not be restricted to computer simulations, often posing oversimplifying assumptions. Rather, real-world experiments should be encouraged to provide the proof of a given concept. Hence, we present a WSN research platform that provides access to the lower layers of the communication protocol stack with the design speed and flexibility of FPGA-based SDRs and, at the same time, allows for deployed battery-based operation spanning multiple days. Leveraging the flexibility of this platform, we implemented a simple frequency shift keying (FSK) PHY layer design to provide a power consumption comparison with existing commercial integrated RF chips and SDRs in typical WSN operating modes. We present the design and experimental evaluation of the PHY layer of a more sophisticated spread-spectrum WSN communication protocol. In particular, we propose the use of a direct-sequence spread spectrum (DSSS)-based code division multiple access (DS-CDMA) scheme to provide asynchronous, collision-free medium access for the sensor nodes in thebase station vicinity, as the experimental evaluation of such schemes has been largely unaddressed in the WSN literature. In the remainder of the paper, we first discuss the background and related work. In Section 3, we present the hardware design of the Marmote SDR platform. We present a preliminary evaluation of the platform's power consumption in Section 4. Then, we discuss the DSSS communication protocol in Section 5 and conclude in Section 6.