BACKGROUND
The Army is seeking technologies to reduce the RF signatures of its radios and make them less susceptible to electronic attack. To achieve this goal Alion proposes using Multiple Input Multiple Output (MIMO) technology and Orthogonal Frequency Division Multiplexing (OFDM). Our method allows radios to simultaneously re-use frequency channels, thereby decreasing the spectrum footprint by minimizing bandwidth which also makes the radio transmissions harder to detect. We apply this technique to a frequency hopping system to further decrease the probability of detection, while increasing communication throughput speed.
INTRODUCTION
Traditional wireless communication systems use a single antenna for transmission and a single antenna for reception. Such systems are known as single input single output (SISO) systems. In recent years, significant progress has been made in developing systems that use multiple antennas at the transmitter and the receiver to achieve better performance. Such systems are known as multiple input multiple output (MIMO) systems.
In a contested or congested electromagnetic environment (EME), RF systems do not have the available spectrum to use multiple frequency channels when necessary. To minimize the number of channels required to operate Alion uses a form of frequency division duplexing (FDD) called Orthogonal Frequency Division Multiplexing (OFDM). The Alion solution will allow systems to transmit and receive multiple OFDM frequency bands simultaneously in the same frequency bands, a design achieved by changing the phase modulation of the OFDM signals. While each carrier frequency is phase modulated and amplitude modulated within an OFDM signal, our solution re-modulates the OFDM signal phases by 90 degrees. For example, the first OFDM signal is modulated at 0 degrees, the second OFDM signal will be modulated at 90 degrees, the third OFDM signal will be modulated at 180 degrees, and the fourth OFDM signal will be modulated at 270 degrees. Therefore, on the receiver side we demodulate the phase modulation to separate the OFDM signals before we demodulate each OFDM packet. Using this process we lose about 6-10dB signal to noise ratio (SNR) but gain much higher communication speed on the order of 4.
Our solution allows us to simultaneously use 4 OFDM channels in the same frequency band, and achieves close to 4 times greater throughput speed. Re-use of the same frequency bands reduces our spectrum footprint by a magnitude of 4 since we use 4 times less spectrum to send 4 times more data in a given time period.
This type of MIMO technology is just starting to make its way into the commercial sector, however most arrangements only use 2 -3 OFDM channels, while we propose using 4 OFDM channels.