Multi-Carrier (OFDM) Cooperative Transmission in MANETs with Multiple Carrier Frequency Offsets

Abstract

Cooperative transmission, realized by aggregating several nodes to a virtual multiple input system, is an auspicious approach to establish a more robust and effective communication in MANETs. In such a setup, impairments, i. e. multiple timing and carrier frequency offsets (TO, CFO) will occur. While multi-carrier schemes, e. g. Orthogonal Frequency Division Multiplexing (OFDM), are well-known to mitigate the impact of multipath propagation and TO, multiple CFO causes inter-carrier-interference (ICI) which typically degrades the performance significantly. Within this paper, we propose an effective code and equalizer structure that allows to overcome this limitation. It mitigates the impact of multiple CFO that can be significantly larger than the subcarrier spacing with a reasonable computational effort. For that, we utilize inherent code properties of Linear-Scalable Dispersion Codes (LSDCs) and propose a communication system composed of an equalizer structure in combination with LSDCs that enables multi-carrier distributed cooperative transmission for practical MANETs with high node mobility. We demonstrate the benefits of cooperative transmission in comparison to classical non-cooperative multihop or concurrent transmission by outage simulations, which clearly indicate that our proposal can be of crucial importance for the overall MANET scalability. Lastly, we compare our OFDM system with a recently proposed time-domain equalization singlecarrier system and point out use cases, where the OFDM system can be more advantageous.

SDR-based Demonstration System and Applicability of SNR Aggregation for Multistage Distributed Cooperative Communication in MANETs

“…However, realworld systems suffer from imperfections due to hardware limitations. Besides, specific impairments arise due to aggregating several standalone nodes to a virtual transmit cluster, such as multiple carrier frequency and timing offsets [7], [8]. Transmit diversity schemes, e. g. Space-Time Block-Codes (STBCs), allow for a comparably low-complexity implementation of cooperative communication [1].…”

“…Nonetheless, adaptations are necessary to utilize them for distributed setups [1]. Recently, a single-carrier (time-domain equalization) [7] and multicarrier (OFDM) [8] communication system for distributed cooperative communication have been proposed that are tailored to the needs of practical systems and the associated impairments. Both systems utilize STBCs, in particular Linear-Scalable Dispersion Codes (LSDCs) [9], which allow to profit from both, distributed transmit diversity and SNR gain.…”

“…However, imperfections of practical systems due to hardware constraints and due to the aggregation of several nodes to a virtual MISO system are not considered therein. In our previous research, we introduced communication systems that particularly concentrate on these [7], [8]. Within this paper, we present a scalable and modular low-cost demonstration system based on software-defined radios (SDRs) that enables to investigate distributed multistage cooperative communication (broadcasting) in practical MANETs.…”

“…To the best of our knowledge, it is the first of its kind. To enhance the performance of the systems presented in [7], [8], we investigate the applicability of SNR aggregation and propose an implementation approach. We investigate the benefits of combining SNR aggregation with distributed cooperative transmission by means of measurements and simulations.…”

“…The latter build a virtual transmit cluster, while the isolated node continuously aggregates all packets, that it receives. In contrast to aforementioned scenario, the number of transmitting nodes is steadily rising during aggregation, which directly corresponds to the typical propagation of a message within a MANET (multistage distributed cooperative broadcasting) [1], [7], [8]. Fig.…”