Spectral Keying (SK) modulation is a unique communication signaling method which combines the simplicity of an impulse radio design with the spectral flexibility of a multi-band radio. The result is an extremely low latency UWB radio capable of long range, robust operation in dynamic multipath environments.

As mentioned above, SK uses a multiband approach which divides the allocated spectrum (3.1 GHz -10.6 GHz) into multiple sub-bands of 500 MHz minimum bandwidth each. This allows for the selection of suitable bands (spectral agility) depending on the presence of interference or international regulatory constraints.  This increases the range of the system without relying on complex RAKE receivers. Figure 1 below shows the 5-band frequency spectrum of Aspen.  There is no Aspen transmission in the UNII band.

Figure 1: Aspen Multiband Waveform

SK modulation is an innovative approach that gives higher number of bits per symbol, giving a high data rate for a relatively low symbol rate. This ensures enough guard time between symbols to mitigate any multipath effects without relying on expensive equalizers. The number of bits per symbol scales up quickly as the number of sub-bands is increased.

How SK works:
As shown above, the Aspen system utilizes 5 sub-bands. An Aspen information symbol consists of 5 pulses.  Each of these pulses is sent in a different UWB sub-band, with each pulse having a minimum bandwidth of 500 MHz. Such a pulse is shown in figure 2 below.

Figure 2: Waveform for a SK Pulse

Unlike other modulations which encode each pulse separately, SK encodes the information collectively, based on the order in which the sub-band pulses are sent. Figure 3 below shows two typical SK symbols. The first symbol uses the sequence of sub-bands F1, F2, F3, F4, F5 to create a unique symbol, while the second uses F2, F5, F1, F3, F4 to create another symbol. Note each sub-band is used only once in a symbol.

Figure 3: Typical SK Symbols

With 5 bands, the number of unique symbols available is 5! =120. Hence, the number of possible bits per symbol is log2 (120) = 6.9 bits. These bits can be carried in the symbol in addition to polarity bits (BPSK or QPSK) in theses pulses. This gives the potential to create a uniquely large number of bits per symbol. As mentioned above, this allows the inclusion of sufficient guard time between symbols to mitigate any multipath effects