In traditional wireless networks, two nodes cannot simultaneously transmit their packets to each other with one radio device (having no machinery enabling full duplex). To ensure bidirectional communications, we need a medium access protocol to coordinate neighboring nodes' transmissions. Two conflicting design goals for the medium access protocol are the ease of implementation and performance guarantees. We propose a novel medium access protocol which is easily implementable (not requiring clock synchronization) and guarantees performance (the fraction of available slots). Our basic idea is to exploit a set of binary sequences having provably low cross-correlation. Each node has its own code sequence and determines whether to transmit or receive a packet by sequentially examining each bit of the code sequence. As an example, we consider the application of Gold code sequences and theoretically analyze the the fraction of available slots that a Gold-code- based MAC can provide. Our simulation verifies our analysis and shows that a Gold-code-based MAC guarantees the fraction of available slots even on a short time scale.