Abstract
Time reversal (TR) beamforming, considered as one of most prominent linear precoders, has showed the feasibility in conventional communications. In this paper, we propose the TR-based transmit optimization for heterogeneous networks (HetNets) where multiple femtocells apply TR precoders over frequency selective channels, with the signal-to-inference-plus-noise ratio (SINR) and interference constraints at single-antenna femtocell user devices. In multi-femtocell network environments, we exhibit that TR outperforms the well-known zero-forcing beamforming. Particularly, we focus on the practical case of imperfect channel estimation at femtocell base stations. The robust optimization methodology is hence devised by taking into account the average effect of channel estimation error (CEE) on system performance. Under the assumption of estimation error in time-varying channels, we derive the exact closed-form expressions of the desired signal, inter-symbol interference (ISI), inter-user interference (IUI), co- and cross-layer interference terms, respectively. These derivations allow us to tackle the proposed robust algorithm by convex optimization techniques. In final, numerical results are shown to confirm the validation of our proposal. DOI: http://dx.doi.org/10.5755/j01.eee.21.3.10431
Highlights
The growths of energy demand and restricting electromagnetic pollution lead to the motivations of green communications [1]–[3]
FBSs are uniformly located in a circle of 100 m far from macrocell base station (MBS), and macrocell users (MUs) and femtocell users (FUs) are uniformly located in the served area
The radiated power of FBS is limited up to 20 dBm in femtocell environments [2]. It means that Time reversal (TR) beamforming technique can be evaluated as a more promising candidate than the very prominent zero-forcing for the future green heterogeneous networks (HetNets)
Summary
The growths of energy demand and restricting electromagnetic pollution lead to the motivations of green communications [1]–[3]. The experimental results in [16] have confirmed the validity of TR properties (i.e. focusing gain and increased average received power) in conventional bandwidths. There have been no prior works on joint TR beamforming and power allocation with CEE for TR multi-user networks, whereas the effect of CEE on TR-based system performance is a greatly desirable problem [7], [13]. We are interested in proposing the conventional TR prefilter in term of transmit optimization for multi-femtocell network and compare TR to zero-forcing beamforming. We develop a novel robust optimization methodology for TR-based systems when the estimated channel may not be accurate due to the insufficient time and bandwidth over fast fading environments practically. Note that the time-reversed form of CIR is utilized as a matched-filter to prefilter message-bearing signals which is represented as gikj [l] h*ijk [L 1 l]
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