Improvements in use of congested spectrum for land mobile radio service by adoption of bandsharing spread-spectrum system with TV broadcast channels

The paper examines the possibility of private mobile radio users sharing the RF spectrum with the television broadcast channels on a mutual noninterference basis, with the aim of accommodating greater spectrum efficiency. A comparison is made between the use of conventional narrowband modulation methods, such as AM, FM and SSB, and the broadband direct-sequence spread-spectrum modulation schemes as contenders for a bandshared system. These comparisons are made using the criteria for establishing imperceptible visual interference on the TV screen for both modulation schemes. The results show that, for a base station sited in Central London with a service area typically that of the Greater London region, 16 MHz of bandwidth may be gained by the use of bandsharing, with either narrowband or spread-spectrum modulation schemes, for guaranteed interference-free TV reception. The paper also shows that, for the same base-station site covering the same area, a further 72 MHz of bandwidth could be opened up by the use of direct-sequence spread-spectrum transmissions, which would otherwise be unavailable for narrowband use. Such a system would also have the advantage that it could be made covert to eavesdroppers using conventional AM or FM receivers with only limited modification to existing transmitters and receivers, It is argued that spread-spectrum techniques can open up significant amounts of bandwidth, by bandsharing, not available to conventional modulation schemes in many urban environments where the normal TV-channel allocations are almost fully utilised, as they appear to be in the Greater London area.