Spot-futures basis arbitrage: earning the premium without a direction
Buy the coin, short the perp, collect the basis. The mechanics are simple; the margin management is where the trade is actually won or lost.
Basis is the gap between the spot price of an asset and the price of its perpetual or dated future. When the perp trades above spot, you can buy spot and short the perp in equal size: the position has no net exposure, and the basis is what you are paid to hold it.
On a dated future the payoff is mechanical — the contract converges to spot at expiry, so the basis is the return, annualised over the days left. On a perpetual there is no expiry, so convergence is enforced by funding instead: a perp trading rich pays longs into shorts, and your short leg collects that until the premium decays.
The part that surprises people is not the entry, it is the margin. The spot leg does not move against you in any way that matters, but the short perp does: if the price doubles, the short is deep underwater and needs collateral long before the spot profit is available to post. The trade is delta-neutral in P&L and very much not neutral in liquidation risk.
Practical consequences: keep meaningful free margin on the futures leg, prefer venues where spot and futures share one margin account, and size against the move you can survive rather than the basis you want to earn. A 12% annualised basis is not worth a forced close at the wrong moment.
Arbitrex lists spot-futures routes alongside futures-futures and funding, so the same coin can be compared across all three before you choose which version of the trade to take.