Wireless Sensor Network (WSN) consists of a large number of tiny sensor nodes capable of being deployed in a range of environments in random or regular fashion. These networks are getting attention of the research community due their broad application domain. They can be applied in many fields like health care, homes, military, environment monitoring or in any commercial environment. Due to resource constrained nature of the nodes, these networks need energy efficient solutions. Since a network operation is many times more expensive than local operation, therefore very efficient and highly adaptable communication systems need to be developed for these networks in order to increase a node’s life time. To avoid resource limitation and achieve energy efficiency collaborative communication combines the power of multiple sensor nodes to transmit the same data at the same time to a base station. This research focuses on energy efficiency in Body Area Networks, sensor networks with multipath and wideband channels. All these models are investigated in the presence of Rayleigh Fading Additive White Gaussian Noise (AWGN). The received signals from collaborative nodes at the base station are considered to be out-of-phase (imperfect phase synchronization). The ultimate goal is to investigate the effect of collaborative communication on energy efficiency, channel capacity gain, received power, BER in Wireless Sensor Networks using narrow-band, multipath and wideband channels. A synchronization process is designed to reduce the phase and frequency synchronization errors among the transmitter (Collaborative) nodes and the receiver (base station). A theoretical model for unsynchronized phase using collaborative communication in the presence of Additive White Gaussian Noise (AWGN) and Rayleigh fading is proposed, analyzed and simulated. The performance of collaborative communication system is evaluated by investigating several figures of merits including “received power”, “BER”, “energy efficiency” and “channel capacity”. The theoretical findings of collaborative communication system are verified using Monte Carlo simulation by considering the parameters of “off-the-shelf” products i.e. “CC2420” and “AT86RF212”. Theoretical analysis of the derived models for received power gain, BER, energy efficiency and capacity gain show that an increase in the number of collaborative nodes also increases the gain in received power and capacity of the system whereas inversely effect BER. Simulation results for phase error intervals f?0:1 0:1g; f?0:2 0:2g; f?0:3 0:3g and f?0:4 0:4g in case of BAN show a 0:475N2 to 0:8N2 gain in received power whereas to achieve a BER of 10?3, the required transmited power decreases from 12:5dB to 10dB with an increase Anwar Ghani: 70-FBAS/PHDCS/F11 Page viii of 153 in the number of nodes from 5 to 11 over the phase error interval f?0:2 0:2g. This required power to achieve the desired BER raises 15dB to 12:5dB in case of phase error f?0:4 0:4g. In multipath communication the gain in received power improves from 0:49N2 to 0:83N2 whereas required power for BER of 10?3 in case of f?0:1 0:1g decreases from 10dB to 7:5dB and for f?0:3 0:3g the decrease is from 12dB to 9dB. In case of wideband communication the gain in received power ranges from 0:51N2 to 0:93N2 and the required power for BER of 10?3 for single node is 7:5dB and for nodes from 5 to 11 it is 3dB to 2dB. For trade-off-analysis of energy saving and transmission distances performed for off-the-shelf devices “CC2420” and “AT86RF212”, shows in all scenarios “CC2420” stabilizes before “AT86RF212”. On the basis of these results it can be concluded that collaborative communication is energy efficient and suitaWireless Sensor Network (WSN) consists of a large number of tiny sensor nodes capable of being deployed in a range of environments in random or regular fashion. These networks are getting attention of the research community due their broad application domain. They can be applied in many fields like health care, homes, military, environment monitoring or in any commercial environment. Due to resource constrained nature of the nodes, these networks need energy efficient solutions. Since a network operation is many times more expensive than local operation, therefore very efficient and highly adaptable communication systems need to be developed for these networks in order to increase a node’s life time. To avoid resource limitation and achieve energy efficiency collaborative communication combines the power of multiple sensor nodes to transmit the same data at the same time to a base station. This research focuses on energy efficiency in Body Area Networks, sensor networks with multipath and wideband channels. All these models are investigated in the presence of Rayleigh Fading Additive White Gaussian Noise (AWGN). The received signals from collaborative nodes at the base station are considered to be out-of-phase (imperfect phase synchronization). The ultimate goal is to investigate the effect of collaborative communication on energy efficiency, channel capacity gain, received power, BER in Wireless Sensor Networks using narrow-band, multipath and wideband channels. A synchronization process is designed to reduce the phase and frequency synchronization errors among the transmitter (Collaborative) nodes and the receiver (base station). A theoretical model for unsynchronized phase using collaborative communication in the presence of Additive White Gaussian Noise (AWGN) and Rayleigh fading is proposed, analyzed and simulated. The performance of collaborative communication system is evaluated by investigating several figures of merits including “received power”, “BER”, “energy efficiency” and “channel capacity”. The theoretical findings of collaborative communication system are verified using Monte Carlo simulation by considering the parameters of “off-the-shelf” products i.e. “CC2420” and “AT86RF212”. Theoretical analysis of the derived models for received power gain, BER, energy efficiency and capacity gain show that an increase in the number of collaborative nodes also increases the gain in received power and capacity of the system whereas inversely effect BER. Simulation results for phase error intervals f?0:1 0:1g; f?0:2 0:2g; f?0:3 0:3g and f?0:4 0:4g in case of BAN show a 0:475N2 to 0:8N2 gain in received power whereas to achieve a BER of 10?3, the required transmited power decreases from 12:5dB to 10dB with an increase Anwar Ghani: 70-FBAS/PHDCS/F11 Page viii of 153 in the number of nodes from 5 to 11 over the phase error interval f?0:2 0:2g. This required power to achieve the desired BER raises 15dB to 12:5dB in case of phase error f?0:4 0:4g. In multipath communication the gain in received power improves from 0:49N2 to 0:83N2 whereas required power for BER of 10?3 in case of f?0:1 0:1g decreases from 10dB to 7:5dB and for f?0:3 0:3g the decrease is from 12dB to 9dB. In case of wideband communication the gain in received power ranges from 0:51N2 to 0:93N2 and the required power for BER of 10?3 for single node is 7:5dB and for nodes from 5 to 11 it is 3dB to 2dB. For trade-off-analysis of energy saving and transmission distances performed for off-the-shelf devices “CC2420” and “AT86RF212”, shows in all scenarios “CC2420” stabilizes before “AT86RF212”. On the basis of these results it can be concluded that collaborative communication is energy efficient and suitable for resource limited networks like WSN.ble for resource limited networks like WSN.