Cross-layer TCP/IP segmentation, re-routing and adaptive modulation techniques to exploit instantaneous BER variations on parallel subchannels

Abstract

This paper describes a cross-layer technique for TCP-enabled multichannel wireless communications systems such as Multiple-Input Multiple-Output (MIMO) links. The technique uses adaptive modulation (AM) and different packet re-routing strategies over physically decomposed sub-channels to exploit time-varying channel conditions. The paper will demonstrate that combining both AM and packet re-routing mechanisms can improve the overall performance of MIMO systems where sub-channels possess unbalanced or uneven time-varying error rate characteristics. A simulated wireless communications system is developed to evaluate packet re-routing techniques over four sub-channel connections made available to the link layer. It then considers the effect of cross-layer link modulation adaptation switching. The system is investigated primarily in terms of goodput for several channel arrangements and characteristics, for various data rates and mean bit error rates. Based on the investigation results, this paper will demonstrate that even simple re-routing and AM mechanisms may be beneficial for systems utilising multiple antennas, as long as these expose the underlying sub-channels and their different instantaneous error characteristics to the link layer.

Keywords:

• Adaptive modulation
• channel difference
• MIMO sub-channels
• packet re-routing
• packet switching

Notes

No potential conflict of interest was reported by the author(s).

1 Data streams conveyed by the physical layer are usually characterised by their mean BER. In reality the BER of each sub-channel will vary away from this mean in real deployed wireless systems (McLoughlin and Sirisena 2010): MIMO coding naturally exploits this variability. Similarly the techniques described in this paper also exploit the variability where it remains stable for one transmission frame or longer – which would be the norm in the majority of deployed systems that do not continually operate at or beyond their worse-case channel conditions.

2 At this point it should be observed that, apart from the normal time-varying instantaneous BER imbalance between sub-channels in real-world MIMO systems, there can sometimes be long-term or static imbalances, which may be caused by sub-optimal placement of one or more antennas, by obstructions, or by the tolerances of electronic components