One of the main challenges of the 4th Generationwireless systems is to provide high data rates in mobile environments. In these 4G products, WiMax (worldwide interoperability for microwave access) and LTE (Long Term Evolution) will be the platform technologies. The rapid development of the Internet with new services and applications has created new challenges for the development of mobile communications systems. Many wireless systems offer high data rate services (IEEE 802.11n, IEEE 802.15.3 and IEEE 802.16) [1, 4 and 5]. Possible migration of 3G to 4G (LTE) will increase data rates throughput in the range of 100Mbps. These high data rate applications may be difficult to support with the required quality-of-services (QoS) in high mobility environment. For example, the mobile WiMax can be used at mobile speeds of 60 km/h to 120 km/h sufficient for car applications but not for high speed trains. One possible solution is to use a main router for each coach connected with a single antenna to the external broadband network and to distribute the signal inside the coaches thanks to a repeater. An other architecture consider direct transmission to mobile cellular telephone networks using the 3G technology such as the HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) allow high-speed data rates, opening the door to a range of mobile usage. All these future and advanced systems are mainly based on orthogonal frequency division multiplexing (OFDM) known to be resistant to various impairments in the mobile channels. In this paper, a transmission system based on OFDM technique and a solution for channel estimation is proposed. The main results will be presented in the case of high mobility context. Index Terms-DopplerEffect, Channel estimation, OFDM, Rayleigh Channel.

In this paper, a transmission system based on OFDM (Orthogonal Frequency Division Multiplexing) technique and channel estimation is proposed. The idea is to enhance the assisted channel estimation without data rate loss. A solution based on frequency diversity is proposed. With the same number of pilots, the global system performances are better for higher mobility speed. The main results will be presented in the case of high mobility context.