In systems of the type described above, range extension has been introduced in a non-time-varying manner. The IEEE 802.11ah standard introduced a range extension mode for data transmission using a Modulation and Coding Scheme (MCS) level, MCS0 rep2, which features a 1 MHz transmission based on a Fast Fourier Transform (FFT) of size 32, enabling a WiFi transmitter to repeat each Orthogonal Frequency-Division Multiplexing (OFDM) symbol two times in the frequency domain. Additionally, the IEEE 802.11ah standard also introduced a 1 MHz preamble to enable range extension, similarly by repeating an OFDM symbol two times in the frequency domain.
The processes for extending the range of a transmission discussed above allow for the repetition of a signal in the frequency domain. However, due to the frequency-selective nature of slow-varying channels, such as WiFi channels, these processes do not provide time diversity, which may be used to extend transmission range. Also, given that the signals are repeated in frequency and not in time, the use of early stopping schemes for improving power savings cannot be accomplished.
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