Radix-8 full adder in QCA with single clock-zone carry propagation delay

We design a 3-bit adder or a radix-8 full adder (FA) in quantum-dot cellular automata (QCA), where the 3-bit carry propagation path can be accommodated in one clock-zone. To achieve this, we introduce group majority signals similar to group propagate and generate signals in parallel prefix computations, use them to reformulate the carry expressions of a previous radix-4 FA, and as such we could extend it to higher radix FAs. Applying the aforementioned new interpretation of carry expressions (via group majority signals) on 3-bit adders, results in that only a single clock cycle is required for 12-bit (vs. the previous 8-bit) carry propagation, across four radix-8 FAs. Based on the proposed radix-8 QCA-FA, we realized 8-, 16-, 32-, 64, and 128-bit QCA adders via QCADesigner. Comparison of these adders with the previous radix-4 experiment, showed 9-41% speed up, and 57-76% area saving, for 16-128-bit adders, respectively. On the other hand, compared to the best previous radix-2 design, for the same bit widths, we experienced 57-172% speed up, but at the cost of 138-4% area increase, except for the 64 and 128-bit cases, where we also experienced 19% and 41% area saving, respectively.