Towards a Quaternion Complex Logarithmic Number System

The well-known generalization of real to complex arithmetic (two reals) extends further to more obscure quaternion arithmetic (four reals), which has applications in signal processing, aerospace, graphics and virtual reality. Quaternion multiplication implements 3D rotation, but is expensive (usually 16 floating-point multiplications and 12 additions). This paper proposes an alternative quaternion representation using logarithms to reduce multiplication cost. The real Logarithmic Number System (LNS) allows fast and inexpensive multiplication and division in embedded and FPGA-based systems. Recent advances in the Complex LNS (CLNS) have made fast log-polar complex representation affordable. Although the quaternion logarithm function is also well-defined, it is not useful to simplify multiplication (in the same way real and complex logarithms are) because quaternion multiplication is not commutative but quaternion addition is. To overcome this, we propose a novel Quaternion Complex (QCLNS) representation using a pair of CLNS numbers. This representation implements quaternion multiplication using only the theoretical minimum, of 8 LNS multipliers (i.e., fixed-point adders) and two CLNS adders. Because CLNS numbers are more compact than ordinary rectangular complex representation, single-precision QCLNS occupies 10.9 percent less memory than conventional quaternion representation. Extrapolating conventional LNS and floating-point synthesis data from Fu et al., QCLNS saves on average 10 percent of FPGA resources for precisions between 13 and 45 bits.