In 1977, Cooper and Nettleton proposed a spread spectrum mobile radio system using frequency-hopping multiple access, Hadamard coding for error correction, and differential phase shift keyed (DPSK) modulation, and they claimed higher spectral efficiency than frequency-division (FD) FM systems. Subsequent analyses showed that the DPSK system has a spectral efficiency of 8.4 percent as compared to the efficiency of unity for a FD-FM system with 30-kHz channel spacings. Goodman et al. suggested an alternative modulation scheme in 1980, using multilevel frequency shift keying (MFSK), and a 30 percent efficiency was obtained. The research results in spread spectrum mobile radio are summarized, and the areas requiring further investigation before a commercial system can be implemented are identified.