Sinja Brandes

Reduction of out-of-band radiation in OFDM systems by insertion of cancellation carriers

Orthogonal frequency-division multiplexing (OFDM) suffers from high out-of-band radiation. In this letter, we investigate a new method for sidelobe suppression characterized by the insertion of a few so-called cancellation carriers (CCs) at both sides of the OFDM spectrum. These special carriers are modulated with complex weighting factors which are optimized such that the sidelobes of the CCs cancel the sidelobes of the transmit signal. With this technique a significant reduction of out-of-band radiation is achieved at the cost of a small degradation in system performance

Subcarrier weighting: a method for sidelobe suppression in OFDM systems

In this letter, a method for sidelobe suppression in OFDM systems is proposed and investigated. The proposed method is based on the multiplication of the used subcarriers with subcarrier weights. The subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized according to an optimization algorithm which allows several optimization constraints. As a result, sidelobe suppression by subcarrier weighting reduces OFDM sidelobes by more than 10 dB in the average without requiring the transmission of any side information

Sidelobe suppression in OFDM systems by insertion of cancellation carriers

Orthogonal frequency-division multiplexing (OFDM) systems exhibit significant out-of-band radiation caused by high sidelobes of the modulated subcarriers. Existing techniques for reducing this undesirable effect have several drawbacks as they waste the scarce spectral resources or expand the signal in time domain. In this paper, we propose a new technique which overcomes these problems. A few so-called cancellation carriers are inserted on the left and right hand side of the used OFDM spectrum. These special subcarriers are not employed for data transmission, but carry complex weighting factors which are determined such that the sidelobes of transmission signal and cancellation carriers cancel each other. Simulation results show that with the proposed method a significant sidelobe suppression is achieved with only a small loss in bit error rate performance. This loss is due to the fact that a certain amount of the transmission power has to be spent on the cancellation carriers and is not available for data transmission.

OFDM-Based Overlay Systems: A Promising Approach for Enhancing Spectral Efficiency [Topics in Radio Communications]

Recently, several approaches for increasing spectral efficiency have been proposed to overcome the scarcity of spectral resources and to ensure that the ever-growing demand for high-data rate mobile communications can be met in the future. In this article, we focus on OFDM-based overlay systems that share a frequency band with already existing systems without degrading their performance. This is achieved by temporarily using those parts of the common frequency band that are currently not used by the licensed system. In particular, we focus on design issues for the physical layer as well as the medium access control layer and propose appropriate solutions for enabling coexistence. As a result, our investigations show that OFDM-based overlay systems are a promising approach for enhancing spectral efficiency.

A technique for sidelobe suppression in OFDM systems

In this paper, a new technique for sidelobe suppression in orthogonal frequency-division multiplexing (OFDM) systems is proposed and investigated. This technique can be applied to both pure OFDM and OFDM based multiple-access systems to improve spectral efficiency. Moreover, it can be utilized within OFDM based overlay systems to avoid interference towards the legacy systems operating in the same frequency band as the overlay system. The proposed sidelobe suppression technique is based on subcarrier weighting. The real-valued subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized using an optimization algorithm which is capable to take several optimization constraints into account. Results show that sidelobes can be easily suppressed by more than 10 dB with the proposed technique. These results are achieved by allowing only a moderate loss in bit-error rate performance.

Compensation of the Impact of Interference Mitigation by Pulse Blanking in OFDM Systems

When pulsed interference is mitigated by means of pulse blanking, interference is reduced significantly. However, at the same time certain fractions of the useful orthogonal frequency-division multiplexing (OFDM) signal are erased. Consequently, only moderate improvements are achieved with pulse blanking. When representing the pulse blanking operation as multiplication with a rectangular window exhibiting notches at the positions where blanks are inserted, it becomes obvious, that pulse blanking mainly leads to inter-carrier interference (ICI). In this paper, the compensation of the impact of pulse blanking is proposed by reconstructing and subtracting ICI. The required shape of the subcarrier spectra is derived from the pulse blanking window. For an estimation of the transmitted data symbols and the channel coefficients of each subcarrier an iterative receiver structure is proposed. Simulation results at hand of a realistic interference scenario show that with perfect channel estimation and known data symbols the impact of pulse blanking can be reduced to a small loss in signal-to-noise ratio resulting from the energy loss due to erasing a certain fraction of the OFDM signal. With real channel estimation and estimated data symbols, the ideal case with perfect channel estimation and known data symbols is approached by 1.2 dB after only three iterations.

Physical layer specification of the L-band Digital Aeronautical Communications System (L-DACS1)

L-DACS1 is the broadband candidate technology for the future L-band Digital Aeronautical Communications System (L-DACS). The flexible design of L-DACS1 allows the deployment as an inlay system in the spectral gaps between two adjacent channels used by the Distance Measuring Equipment (DME) as well as the non-inlay deployment in unused parts of the L-band. In this paper, the specification of the L-DACS1 physical layer enabling the deployment as an inlay and as a non-inlay or as a mixed inlay and non-inlay system is presented. Apart from the transmitter design, the design of the L-DACS1 receiver is addressed including methods for mitigating interference from other L-band systems. Special emphasis is put on channel estimation which has to be robust towards interference. The different proposed algorithms for channel estimation are evaluated in simulations of the overall L-DACS1 physical layer performance. The results show that L-DACS1 is capable of operating even under severe interference conditions, hence confirming the feasibility of the inlay concept.

B-AMC a system for future broadband aeronautical multi- carrier communications in the L-BAND

The Broadband Aeronautical Multi-carrier Communications (B-AMC) system is a candidate for a future aeronautical communications system to be operated in the L-Band (960-1164 MHz). It is based on the Broadband-VHF (B-VHF) system recently developed for aeronautical communications in the VHF band. As conditions in the L-Band significantly differ from those in the VHF band, basic physical layer parameters had to be adapted, which in turn required a re-design of the higher layers as well. B-AMC offers air/ground (A/G) as well as direct air/air (A/A) communication capabilities without ground relay. The physical layer has been designed to coexist with other systems located in the aeronautical L-Band. The B-AMC data link layer is optimized for low latency and low duty cycle data communication. The B-AMC study is funded by EUROCONTROL.

B-AMC — broadband aeronautical multi-carrier communications

The broadband aeronautical multi-carrier communications (B-AMC) system is a promising candidate for the future L-band radio system called L-band digital aeronautical communications system (L-DACS). In this paper, the design of the physical (PHY) as well as of the data link layer (DLL) is addressed. As B-AMC is intended to be operated in the L-band between two adjacent distance measuring equipment (DME) channels, the avoidance of mutual interference between existing L-band systems and B-AMC has been in the focus of the PHY layer design. In order to demonstrate the feasibility of the coexistence with DME, a draft frequency planning has been performed for Europe, resulting in successful frequency assignments in wide parts of Europe. The B-AMC DLL supports data link communication with low latency and high throughput. It is designed to be highly configurable and to support different service requirement sets and traffic profiles. In this paper, the current DLL configuration featuring a low RL duty-cycle and graceful degradation is discussed in detail.

Interference mitigation for broadband L-DACS

One deployment option for the future broadband L-band digital aeronautical communications system (L-DACS) is operating as an inlay system between two adjacent channels of the distance measuring equipment (DME) system. Investigations for the broadband aeronautical multi-carrier communications (B-AMC) [1] system, one candidate for the broadband L-DACS, have shown that interference originating from DME systems operating in adjacent channels has a strong impact on the B-AMC system. To enable the utilization of spectral gaps between two adjacent DME channels, two efficient methods for mitigating the impact of interference are proposed and investigated for the B-AMC system, namely pulse blanking and erasure based decoding. Simulations show that the impact of DME interference onto the L-DACS can be reduced considerably by choosing an appropriate coding scheme to make the transmit signal robust against interference. The impact of interference is mitigated by means of the proposed methods, resulting in a performance close to the performance in the interference-free case.