Sub-carrier Switch Off in OFDM-based wireless local area networks

OFDM based wireless communication systems split the available frequency band into so-called sub-carriers, and data is transmitted on each of these sub-carriers in parallel. With frequency selective fading, sub-carriers may experience different channel qualities. Thus, choosing a different modulation and coding scheme (MCS) per sub-carrier improves performance. However, this comes at an increase in transceiver complexity and no current wireless system adapts the MCS at such a fine granularity. Some OFDMA based systems such as LTE allow to adapt the MCS per user, whereas wireless local area networks as specified by IEEE 802.11 use the same MCS on every sub-carrier. The performance of such wireless systems that use a single MCS in a frequency selective fading channel can be significantly improved through Sub-Carrier Switch Off (SSO), a simple but powerful alternative to adaptive MCS. SSO deactivates weak sub-carriers that excessively raise the error probability to improve the overall throughput. In this paper, we implement and test SSO in a software-defined radio testbed based on the Wireless Open Access Research Platform (WARP). We present a novel light-weight method for selecting the sub-carriers to be switched off based on the per-sub-carrier channel quality. The results we obtain from our measurements indicate that throughput increases of up to 250% are possible and thus SSO is a highly promising and very low complexity mechanism for future wireless local area networks.