# A Multiprotocol Wireless Sensor Network for High Performance Sport Applications
## 2 Materials and Methods
### Choice of Radio Transmission Device  
For a proper assessment of the performance of a flatwater kayak team, information on both dynamic and kinematic quantities is required. The former ones represent the measure of the forces applied by the athletes while the latter are the result of the action of such forces on the motion of the boat. Each paddle and footrest, which represent the slave nodes for the WSN, has therefore been equipped with strain gauges to measure the intensity and the coordination among these forces. By contrast, the velocity and the triaxial accelerations are measured, respectively, by a high frequency GPS and an IMU placed on the master node. As a result, each boat will be equipped with a master node and two slave nodes for each paddler.  
The range constraints in this application are particularly demanding: A racing kayak can have up to four paddlers (K4), each having a paddle and a footrest. Each paddle and footrest, equipped with a force sensor, is a slave node.  
Unlike in rowing, the paddle is not attached to the boat, so a wireless connection is required to avoid hindering the motion of the athlete. A typical K4 boat is about 11 m long, so even placing the master node in the middle of the hull, the distance between it and the farthest slave node can be up to 4-5 m, in a very noisy environment with bulky obstacles, namely the bodies of the athletes (Figure 2).  
This is outside the typical range of both Bluetooth and ANT+, the most widely used protocols for sport applications (see Table 1). A comparative test of the range of some commercially available wireless modules has shown that the one with longer range is the nRF24L01P+PA+LNA, (by Chengdu Ebyte Electronic Technology Co., Ltd., Chengdu, China) (Figure 3).  
Figure 1: Wireless sensor network (WSN) architecture.  
Figure 2: A K4 racing kayak.  
The module is a low power single chip transceiver for the global, license-free 2.4 GHz ISM band with high-speed communications (up to 2 Mbit/s). It consists of a fully integrated frequency synthesizer (Gaussian frequency shift keying--GFSK modulation), a power amplifier, andd a modulator-demodulator block, and it is based on the Enhanced ShockBurst protocol engine [32]. This makes it ideal for building wireless networks in a wide range of applications. Equipped with an external antenna, this module can cover a distance, measured in an open field, in excess of 100 m, with data loss <1% on a data rate of 1 kBps (declared range of the module is in fact 2500 m). Its power consumption of 100 mW, while higher than other modules, is still compatible with the two hours of battery time required by the project specifications.  
All system parameters (e.g., output power, frequency channels, protocol setup, and air data rate) are programmable through the serial peripheral interface (SPI) serial link. The data rate of 250 kbps, 1 Mbps, or 2 Mbps is configurable on air. The same modules can be mounted on master and slave nodes; the network can be composed up to eight peripheral nodes. At power on, all slave nodes transmit and receive over a \"handshake\" frequency, while the master node scans continuously for new slave nodes on that frequency; once the slave has been detected, its frequency is switched to the \"running\" frequency by the master node. Using multiple \"running\" frequencies, it is possible to operate different systems without interference.