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Nowadays army's data networks need robust and high immunity to jamming communications. Spread Spectrum is one of the most used modulation techniques. Typical CDMA systems are a good solution but it has some disadvantages:

• Relatively slow rate: Due to long sequences needed to maintain Multiuser Access Interference.
• Difficult rate adaptation: Difficult to share high rate services (images/data) with low rate services (voice/control).
• Reduced number of orthogonal sequences for high speed rates: Reduced number of simultaneous users.
• Reduced capacity for high speed vehicles: Doppler is a major disadvantage of CDMA networks reducing or even making impossible to communicate.

QOTDM techniques can obtain rates coming from very slow high immunity to jamming rates to very high efficiency rates communications with up to 10 bits/s/Hz without changing neither modulation technique nor bandwidth.

Advantages and key features of QOTDM for tactical communications

QOTDM stands from Quadrature Orthogonal Time Division Multiplexing, this means a fully time multiplexing of data channels without mutual interference in time sharing the same frequency spectrum. It is based on the use of sets of complementary sequences to encode data.

The main properties to improve tactical communications are based on two properties of these sequences: The sum of their autocorrelation function is an ideal Krönecker delta with no lateral lobes and the cross-correlation among sets is null independently of the time displacement among them.

It is also well known that a Krönecker delta is the ideal signal to identify a Linear Time Invariant channel (LTI). Nevertheless, the use of a delta should require a high energy impulse in order to characterize the channel with enough immunity to noise to be useful and also, the power stages limitations are a drawback to use this technique.

Block diagram of a radio communications system with new QOTDM added or modified

As reflected in figure above, any CDMA system can benefit from the advantages offered by QOTDM based techniques without changing RF, filters, QAM modulation/demodulation and PSD (Power Spectrum Density) masks, etc.

Just applying encoding, to current implementation main features can be achieved:

• Optimal symbol and carrier synchronization.
• Optimal equalization and crosstalk cancelation.
• Null MAI (Multi-Access Interference): It is possible to get simultaneous access to N simultaneous users with N bits sequences.
• Easy Rate Adaptation: The orthogonal in time features of sequences used allows easy rate adaptation without changing the user sequences set. This feature is very important for taking advantage of the bandwidth in all service and jamming situations.
• High immunity to frequency drift and Doppler effect: This feature allow high rates at high vehicle speeds.
• Fully compatibility with current implementations.
• Reduced consumption and power transmission.

For more information about licensing and partnership, please send us an e-mail at license@gcmcom.com