| Future military communications systems, on both national and international levels, will demand the capability of transmitting high data rates over self-organising ad-hoc networks. In the work presented in this paper we describe a high data rate signal in space (HDR-SiS) developed by Rohde & Schwarz, that enables the transmission of real time video, voice and data in a mobile ad-hoc network (MANET).
The transmission scheme uses OFDM at variable bandwidths up to 10MHz in order to be adaptable to various national and international spectral usage requirements. Although OFDM is already used in many standards such as WLAN, WiMAX, DVB-T, these systems were designed for either stationary and/or indoor applications. The presented SiS is optimised for mobile operators with relative speeds up to several hundred kilometres per hour and multi-path delays corresponding to relative distances of around 15km.
The transmission rate must be varied according to channel conditions, which can be done by varying the modulation type and the coding rate. The modulation type can range from BPSK (1 bit per sub-carrier) to 64QAM (6 bits per sub-carrier), whereby the net data rate per sub-carrier and per symbol depends on the amount of redundancy added by error correction codes. Error correction coding (or channel coding) is an essential part of broadband OFDM transmissions. In addition to noise at the receiver input, a certain proportion of sub-carriers ‘disappears’ at each transmission due to frequency selective mobile communications channels. Using capacity approaching concatenated (turbo) codes we are able to achieve near-optimal bit error rates for given signal to noise ratios. The channel codes are optimised using EXIT chart techniques, where characteristic curves of the component codes in the code concatenation are used to describe the overall decoder performance.
The system presented in this work offers a set of transmission types combining modulation type and error control coding rate. Under good channel conditions, 64QAM together with a low redundancy code can be used to achieve very high transmission rates. In contrast the most robust transmission scheme uses BPSK with a high redundancy code. Variations between these two extremes allow us to adapt to channel conditions as exactly as possible. |