The quest for higher data rates in WiFi is leading to the development of standards that make use of wide channels (e.g., 40MHz in 802.11n and 80MHz in 802.11ac). We argue against this trend of using wider channels, and instead advocate that radios should communicate concurrently over multiple narrow channels for efficient and fair spectrum utilization. We propose WiFi-NC, a novel PHY-MAC design that allows radios to use WiFi over multiple narrow channels simultaneously.
Krishna Chintalapudi, Bozidar Radunovic, Vlad Balan, Michael Buettener, Srinivas Yerramalli, Vishnu Navda, and Ramachandran Ramjee, WiFi-NC : WiFi Over Narrow Channels, in NSDI, April 2012
MIT tech Review : http://m.technologyreview.com/communications/39429/
The trend of using wider channels in WiFi
Shannon's rate sets the limit on the data rate in terms of bits/sec/Hz for a point to point link. Advanced modulation (e.g. OFDM) and coding (e.g. Turbo and LDPC codes) schemes strive to achieve this ideal rates. As we achieve the limits of achievable data rates, in order to increase data rates, upcoming standards are proposing the use of wider channels. For example, 802.11 proposed 80 or 160MHz channels.
Problem with using wide channels
While using wider channels can provide higher physical layer throughputs, the MAC layer performance will be poor because of three reasons :
MAC inefficieny due to constant overheads : Even though data rates increase with wider channels, constant overheads such as CSMA, preambles, ACKs etc. remain the same and become the bottleneck.
Incompatible coexistence of wide and narrow channels : Devices with different channel widths cannot co-exist and it results in starvation of the device with the sider band. For example, a 40MHz device cannot co-exist with two 20 MHz devices that operate in different halves of the 40MHz.
Fragmented Spectrum in Whitespaces : In whitespaces gaining access to a contiguous chunk of 160MHz of 80MHz will be a rare opportunity. Thus, devices will invariably be limited to using a narrower channel width.
"Uses several narrow independent low data rate channels concurrently, instead of one wide channel." This approach solves each of the problems mentioned above. The fraction of overheads is now less in each narrow channel this increasing the overall efficiency. Devices can share each part of the spectrum fairly, for example, when there are 2 20MHz and one 40Mhz device as mentioned above, the 40 MHz device can obtain access to each of the halves while fairly sharing with the 20 MHz devices. Finally, WiFi-NC can opportunistically use potentially non-contiguous pieces of the spectrum thus enabling higher spectral usage.
In order to enable WiFi-NC, we propose, design and implement the compound radio - a radio that uses a wide band analogue radio front end but performs digital signal processing in the base-band to provide the equivalence of several independent narrow-band channels to the MAC layer. A WiFi-NC prototype has been implemented on an FPGA based software defined radio.