## 3 Related Work  
There is a significant amount of literature that contains studies of the performance and coexistence of the different IEEE standards. The authors in [17], for instance, examine the reliability of a point to point communication with a real IEEE 802.15.4 hardware by measuring the Packet Error Rate (PER) and the Received Signal Strength Indicator (RSSI), both in indoor and outdoor RF environments. The obtained results are used to calibrate the error model in the ns-2 network simulator in order to produce a more real simulation environment and evaluate the IEEE 802.15.4 network performance in a more reliable way. Furthermore, the coexistence between IEEE 802.15.4 and IEEE 802.11 networks is addressed, measuring the impact these two wireless technologies have on each other when operating concurrently and in range. It is concluded that the IEEE 802.15.4 network operation has practically no negative influence on a concurrent IEEE 802.11 communication, but if no care is taken about the operational channels of the two technologies, the IEEE 802.11 network will have a negative effect on the performance of the IEEE 802.15.4 network. From the performed measurements, there should be at least 7 MHz offset between the operational frequencies for a satisfactory performance of the IEEE 802.15.4 network. In [1], the coexistence between IEEE 802.11, IEEE 802.15.1 and IEEE 802.15.4 is examined through mathematical analysis. For different combinations of affected and interfering networks, the PER caused by cross-technology interference is calculated. This PER is calculated from the Signal to Noise Ratio (SNR) at the affected wireless network receiver, and it is evaluated in terms of distance between interfering and affected networks, packet interval and channel separation. When varying the interferer distance from the receiver or the packet interval, the transmitter and interferer channels are chosen as follows: IEEE 802.11 on channel 1 (2412 MHz), IEEE 802.15.1 on channel 3 (2410 MHz) and IEEE 802.15.4 on channel 12 (2410 MHz), thus constituting co-channel interference. When varying the channel separation, the interferer channel separation from the transmitter is varied from \(-\)15 to 15 MHz, thus constituting adjacent channel interference. It is concluded that the IEEE 802.15.1 is more affected by IEEE 802.15.4 interference than vice versa. On the other hand, the IEEE 802.15.1 results to be more resistant than IEEE 802.15.4 against IEEE 802.11 interference. Nevertheless, these studies do not deepen in interference avoidance mechanisms such as frequency hopping. Some other work can be found in the literature related to models and simulators of frequency hopping systems [18; 19; 20]. The authors in 18 modelled a frequency hopping wireless communication system based on the Signal Processing Worksystem (SPW) developed by CoWare (acquired by Synosys in 2010) [21]. The designed system is tested in terms of its Bit Error Rate (BER) performance under broadband and partial-band noise. The interfering noise is generated and modelled as the sum of a broadband Additive White Gaussian Noise (AWGN) and a multi-tone interference noise, where the jammer spreads some interference signals on a number of discrete frequency points. The system defines 32 frequency channels, and the frequency hopping pattern is generated according to a certain pseudo-random algorithm. In [19], a simulator is designed to simulate and evaluate a frequency hopping spread spectrum communication systems using VisSim Comm software [22], which is mainly designed to simulate and analyse communication systems. The presented simulator is capable of simulating a complete system (transmitter, receiver and medium) operating under a noisy environment. The carrier frequencies for the FHSS system are varied in a pseudorandom manner within a wideband channel. The performance of the communication system is evaluated in terms of BER, and it is concluded that the FHSS significantly reduces the probability of error of a system operating under narrow band jamming conditions.