COMMERCIAL RADIO TECHNOLOGY is moving away from single-carrier technologies, in which one digital symbol is transmitted at a time. At the heart of the new broadband connection is a modulation scheme called Orthogonal Frequency Division Multiplexing (OFDM). Aside from offering good spectral efficiency, OFDM is very tolerant of interference. It transmits hundreds of symbols simultaneously, but at a low rate per symbol. Keithley Instruments (Cleveland, OH) examines the transition to OFDM and its implications in "SISO to MIMO: Moving Communications from Single-Input Single-Output to Multiple- Input Multiple-Output."
The white paper begins by depicting a typical single-input single-output (SISO) radio, which uses one radio and one antenna at a time. In contrast, multiple-input multiple-output (MIMO) takes four independent OFDM carriers and puts them on top of each other. All of them are independently modulated. The result is four separate transmissions that share the same frequency. A challenge exists at the receivers, however, as they have to get back to four independent radio signals.
MIMO configurations vary. A 2 X 2 system, for example, contains two transmitters and two receivers. In contrast, a 4 X 4 system contains four transmitters and four receivers. The transmission of multiple signals also demands the accurate synchronization of multiple channels in phase and sampling alignment. The paper shows how such synchronization can be achieved with vector signal generators and a MIMO synchronization unit. In addition, two vector signal analyzers can be synchronized in order to independently receive MIMO signals.
Keithley Instruments, Inc., 28775 Aurora Rd., Cleveland, OH 44139; (440) 248-0400, FAX: (440) 248-6168, Internet: www.keithley.com.