N093-218 TITLE: Orthogonal Frequency-Division Multiplex (OFDM) Waveform Optimized for Power Limited Line of Sight (LOS) User Environments

TECHNOLOGY AREAS: Information Systems, Sensors, Weapons

ACQUISITION PROGRAM: JPEO JTRS, Network Enterprise Domain (NED), ACAT I

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), which controls the export and import of defense-related material and services. Offerors must disclose any proposed use of foreign nationals, their country of origin, and what tasks each would accomplish in the statement of work in accordance with section 3.5.b.(7) of the solicitation.

OBJECTIVE: Develop and validate a new or improved OFDM data symbol design that provides robust performance when used in a LOS data link with low Signal-to-Noise Ratio (SNR), high rates of Doppler shift, and interference.

DESCRIPTION: OFDM systems are designed using data symbol modulation chosen to satisfy user traffic requirements and spectral bandwidth constraints. Fixed microwave links with little or no multipath degradation can support OFDM systems with very high efficiency data symbols, e.g., 64-QAM giving 6 bits/symbol can be used on 802.16e in some cases. However, under realistic mobile radio link conditions involving ground operation with multipath, OFDM modulation efficiency may be forced down into the range of 1 to 3 bits/symbol. If even more link range is desired beyond what these symbols can support, then fractional bit/symbol alphabets must be considered. Simple symbol repetition is one such case; the symbols can be combined either coherently or non-coherently. Another class of symbols that might be fruitful in this low SNR case is cyclic shift correlation symbols such as the Zadoff-Chu symbols, because they can yield high processing gain and can operate in high Doppler and low SNR environments. In addition, these symbols have the characteristic of reduced peak to average power ratio , which allows the transmit power to be increased (by reducing "back-off") thus increasing the net link margin.

In a military LOS tactical communications environment, the users are most often ground-based or traveling in aircraft. Ground users (both dismounted and in vehicles) typically have significant limitations in antenna height and performance (height less than 10 feet), and rugged rural or urban terrain creates a high level of multipath. This analysis will need to be based upon several realistic operational scenarios in urban, desert, mountain, ocean, and forested regions.

The following are some of the key environmental characteristics of interest: Stationary white Gaussian noise, impulse noise and narrowband tones and Rayleigh fading up to 5 path fading at up to 25 Hz fading rates.

PHASE I: Develop three or more OFDM symbol designs and demonstrate performance using modeling and simulation. This demonstration will need to show performance characteristics in multiple types of fading environments, with various types of interference in the environment, and in the presence of residual (uncorrected) Doppler rates from 0 Hz 100 Hz. . Investigate benefits erasure decoding to combat single OFDM symbol loss. Consider the relationship of symbol choice and coding choice and provide an analysis to determine possible advantages and disadvantages of each. Generate a final report documented and supported with citations, analysis and simulation results.

PHASE II: Create a more comprehensive software implementation suitable for development and capability demonstration using software-defined radios development platforms. Demonstrate the performance of the proposed approach. In order to demonstrate practical viability, estimate the digital processing requirements and software code size required to port and adapt the software to available Software Defined Radio platforms.Perform field testing to validate the waveform�s performance and the accuracy of the model used in the simulations.

PHASE III: Enhance and port the software into the JTRS radio product lines. Support development and operational testing processes by users. Maintain and enhance capabilities of software to meet JTRS Enterprise requirements.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: OFDM is used by both military and commercial wireless systems such as WiMax. The techniques developed as part of this SBIR will be directly applicable to improving the performance and affordability of commercial mobile networks.

REFERENCES:
1. Popovic, B.M., "Generalized chirp-like polyphase sequences with optimum correlation properties", IEEE Transactons on Information Theory, v38, Issue 4, July 1992, pp1406-1409.

2. Liru & Dubey, "Extended orthogonal polyphase codes for multicarrier CDMA system", IEEE Communication Letters, V8, Issue 12, December 2004, pp 700-702.

3. IEEE Standard 802.16e-2005

4. IEEE Standard 802.16-2004

KEYWORDS: OFDM, JTRS, MANET

** TOPIC AUTHOR (TPOC) **

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