-
[show abstract]
[hide abstract]
ABSTRACT: Secondary wide-band users have to guarantee they will not disturb the in-band narrow-band primary licensed users. There are techniques proposed to limit the interference by secondary wide-band users in the specified in-band narrow bands. Active Interference Cancelation (AIC) can flexibly solve this problem in the digital domain and hence remove the need for extra expensive analog hardware. The main challenge is the degradation in the final depth of a nulled band by radio impairments such as PA non-linearity, phase noise, and I/Q imbalance. In this paper, we analyze the effect of phase noise on the performance of active interference cancelation techniques and then propose a mitigation technique. Our analysis shows that phase noise can degrade the nulling depth performance by 11.4-19.6dB at the edge of nulled band. We propose an adaptive pre-filtering technique to improve the nulling performance by 3-11dB depending on system configuration. The proposed scheme utilizes the existing analog-to-digital converter in the device's receiver path for providing feedback when in transmit mode. This can leads to higher power consumption in transmit mode due to this additional analog-to-digital conversion in the feedback path. To address this limitation, we propose an adaptive tracking filter with low duty-cycle on/off periods. We show that sacrificing 0.5dB in the null-depth can reduce the ADC power consumption by 95%.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
-
[show abstract]
[hide abstract]
ABSTRACT: Space division multiple access (SDMA)-based medium access control (MAC) protocols have been proposed to enable concurrent communications and improve link throughput in multi-input-multi-output (MIMO) ad hoc networks. For the most part, the works appearing in the literature make idealized and simplifying assumptions about the underlying physical layer and some aspects of the link adaptation protocol. The result is that the performance predicted by such works may not necessarily be a good predictor of the actual performance in a fully deployed system. In this paper, we look to introduce elements into the SDMA-MAC concept that would allow us to better predict their performance under realistic operating conditions. Using a generic SDMA MAC, we look at how the network sum throughput changes with the introduction of the following: (1) use of the more practical MMSE algorithm, instead of the zero-forcing or singular-value-decomposition-based nulling algorithms used for receive beamnulling; (2) impact of channel estimation errors; (3) introduction of link adaptation mechanism specifically designed for concurrent SDMA MACs; and (4) incorporation of TX beamforming along with RX beamnulling. Following on the transmission window during which concurrent transmissions are allowed by the MAC, we qualify the impact of each of these four elements in isolation. At the conclusion, the performance of a system that incorporates elements 1-4 is presented and compared against the baseline system, showing an improvement of up to five times in the overall network sum throughput.
IEEE Transactions on Vehicular Technology 04/2011; · 1.92 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Secondary wide-band users have to guarantee that they will not disturb the in-band narrow-band primary users with allocated spectrum. There are techniques proposed to limit the leakage power of secondary wide-band users in the specified in-band narrow bands. Active Interference Cancelation (AIC) can flexibly solve this problem in the digital domain and hence remove the need for extra analog hardware. The main challenge is that the final depth of a nulled band is severely degraded by RF impairments such as PA non-linearity, phase noise, and I/Q mismatch. In this paper, we investigate the effect of I/Q imbalance on the performance of active interference cancelation techniques. Our analysis shows that for a transmitter with 3° phase imbalance and 0.1dB gain imbalance, the null depth will decrease from 40dB (provided in the digital domain) to 28.7dB (at the out of the radio transmitter). Then we introduce a compensation techniques and demonstrate an improvement of 5 - 10dB in the null depth depending on the severity of I/Q imbalance.
MILITARY COMMUNICATIONS CONFERENCE, 2010 - MILCOM 2010; 12/2010
-
[show abstract]
[hide abstract]
ABSTRACT: Reliable spectrum sensing is one of the main challenges in the development of cognitive radios. To achieve the high probability of detection at very low SNRs, some feature detection schemes have been proposed in the literature based on the cyclo-stationarity theorem. One limitation of such methods is their high sensitivity to RF and analog impairments. This paper focuses on analysis and compensation of sampling clock frequency offset (SCFO) between the DAC (at transmitter side) and the ADC (at detector side). We show that for an SCFO of 20 ppm and for a pilot-aided cyclo-stationary detector, the performance degradation in detection thresholds reach 7 dB and 12 dB for 50 ms and 100 ms sensing periods, respectively. We then present a method to compensate for such significant degradations in the presence of SCFO. Our analysis and simulations show that using this technique, we can achieve a performance within 1.5 dB of the no-offset ideal scenario.
Communications (ICC), 2010 IEEE International Conference on; 06/2010
-
[show abstract]
[hide abstract]
ABSTRACT: Reliable spectrum sensing is the key requirement for secondary users to utilize the spectrum in the absence of primary users. Given the widespread deployment of OFDM modulation in recent and future multimedia broadcasting and wireless broad-band services, reliable spectrum sensing of OFDM waveforms is of great interest for future cognitive radio systems. This paper presents a new detection method for OFDM waveforms that exploits the available embedded pilot tones without any simplifying assumption or constraints on the pilots. Analytical performance characterization is presented and supported by simulation results for both AWGN and fading channels. This study shows that the new method can improve the performance by 5dB in comparison with other feature detectors.
Communications (ICC), 2010 IEEE International Conference on; 06/2010
-
[show abstract]
[hide abstract]
ABSTRACT: Reliable spectrum sensing is the key requirement for secondary users to utilize the spectrum in the absence of primary users. Given the widespread deployment of OFDM modulation in recent and future multimedia broadcasting and wireless broadband services, reliable spectrum sensing of OFDM waveforms is of great interest in cognitive radios. Although feature detection methods outperform energy detection ones, however they are generally more sensitive to RF impairments. In this paper, we have investigated the effects of a non-ideal receiver RF chain on the performance of the Pilot-Aided Cyclo-Stationary Detection (PACSD). Our study demonstrates how the performance of PACSD degrades due phase noise and carrier frequency offset while it is more tolerant to I/Q imbalances.
Wireless Communications and Networking Conference (WCNC), 2010 IEEE; 05/2010
-
[show abstract]
[hide abstract]
ABSTRACT: The design and implementation of an accurate and low complexity MIMO channel emulator is presented in this paper. A mathematical analysis is used to verify the accuracy of the emulator over a wide range of SNRs (0 - 35 dBs). The complexity of the emulator is reduced by preprocessing of the channels and hardward/software partitioning. All 802.11n channel models can be emulated on our platform. A 3Ã4 10MHz version of the emulator is successfully running on a Virtex-II XC2V6000-4 FPGA. A 20MHz version was synthesized and simulated on an XC2V6000-6 FPGA.
Signals, Systems and Computers, 2009 Conference Record of the Forty-Third Asilomar Conference on; 12/2009
-
[show abstract]
[hide abstract]
ABSTRACT: Carrier sense multiple access (CSMA) is the conventional medium access method used in wireless ad hoc networks. it can be enhanced as a space-division based CSMA (namely, SD-CSMA) by using multi-antenna techniques, which allows concurrent link communications in the network. In this paper, we present a MAC design that can adaptively switch between CSMA and SD-CSMA in multi-antenna based ad hoc networks. We first investigate the CSMA mode and the SD-CSMA mode by considering several practical constraints from MIMO physical layer, which include imperfect channel estimation, modulation and coding scheme, MMSE detection, power constraint and packet length. We then propose a selection criterion that adaptively switches between CSMA and SD-CSMA based on current wireless channels among nodes. Simulation results verify that the proposed design can outperform both the CSMA mode and the SD-CSMA mode in terms of aggregate throughput.
Military Communications Conference, 2009. MILCOM 2009. IEEE; 11/2009
-
[show abstract]
[hide abstract]
ABSTRACT: A computationally efficient algorithm for the decoding of low-density parity check codes is introduced. Instead of updating all bit and check nodes at each decoding iteration, the developed algorithm only updates unreliable check and bit nodes. A simple reliability criteria is developed to determine the active bit and check nodes per decoding iteration. Based on the developed technique, significant computation reductions are achieved with very little or no loss in the BER performance of the LDPC codes. The proposed method can be implemented with a slight modification to the sum-product algorithm with negligible additional hardware complexity.
Electronics Letters 09/2009; · 0.96 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We introduce an importance sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved.
IEEE Transactions on Communications 08/2009; · 1.68 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: MIMO based MANET has received a great deal of attention in the past decade, resulting in a number of MAC layer protocols that aim to utilize the inherent capabilities afforded by multi-antenna enabled nodes. Many of these MAC protocols look to improve on the total number of concurrent links within the network. In this paper, we present a theoretical approach for identifying the maximum number of concurrent links (MNCL) in a MIMO enabled MANET, subject to the constraint that all links provide a minimum QoS measured in terms of signal to interference and noise ratio (SINR). We assume that each receiver uses an MMSE based detector which provides a balance between completely nulling of neighboring interference and hardware complexity. As a byproduct of the MNCL, the solution will provide the optimum power allocation for the nodes as well as the optimum concurrent link allocation. The proposed approach is readily applicable to MIMO systems using Receive Diversity, Transmit Beamforming, and Space Time Block Coding (STBC). The results show that the MNCL can be improved significantly by exploiting multi-user diversity and increasing the number of receiver antennas. But the improvement from allowing higher transmit power is not significant. The derived MNCL capacity can be used to highlight theoretical insights for practical designs.
Military Communications Conference, 2008. MILCOM 2008. IEEE; 12/2008
-
[show abstract]
[hide abstract]
ABSTRACT: The detection of MIMO signals presents a formidable computation challenge due to the inherent complexity incurred by matrix-type wireless fading channels. To reduce the hardware cost, linear MIMO detection algorithms can be used instead of non-linear space-time processing. Conventionally, adaptive linear algorithms such as RLS and LMS are treated separately in implementation from matrix inversion based algorithms such as ZF and MMSE. In this paper, we identify the similarities between the QR-decomposition based forms of RLS and MMSE MIMO detection and present a unified systolic array implementation that can accommodate both. In the meanwhile, some subtle but important differences also exist. As a result, the unified array must have a size of N<sub>r</sub> x N<sub>r</sub> (assuming N<sub>t</sub> les N<sub>r</sub>) and both array inputs and array operations need to be controlled accordingly. As special cases of RLS and MMSE, LMS and ZF can also be realized in the same array, making the array flexible enough for all linear MIMO detection algorithms.
Wireless Communications and Networking Conference, 2008. WCNC 2008. IEEE; 01/2008
-
[show abstract]
[hide abstract]
ABSTRACT: Theoretical capacity calculations and corresponding simulations show significant capacity/throughput gains from MIMO systems. Whether these gains are achievable in a real system, deployed in a practical environment, depends on a variety of factors, such as the choice of the communication algorithms, analog impairments and the "quality" of the wireless channel to sustain MEMO communications. In this paper, a 5.25 GHz broadband MIMO-OFDM testbed is described along with field measurements conducted with it. The MIMO-OFDM communication algorithms and also the impact of analog impairments on the performance of the system are described. Detailed system calibration results are described which serve as a baseline for results of field measurements. The results of wireless measurements are compared with the theoretical capacity, computed with the channel estimates obtained during the demodulation process. The average achievable capacity in the indoor wireless environment is shown to be 9.97 bps/Hz (bits per sec per Hz) while the capacity loss due to analog impairments and the choice of algorithms is about 2.33 bps/Hz. Also, field measurements conducted with the system in various environments are presented comparing the average throughput/capacity achieved in each of these environments.
IEEE Transactions on Wireless Communications 09/2007; · 2.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This paper presents an architecture that utilizes spectrum sensing with a Weaver architecture receiver to ease the requirements on the RF front-end components. With the ability to sense the environment, large interferers that overwhelm small desired signals can be avoided. The learning and adapting capabilities are enabled by means of a flexible receiver architecture employing variable local oscillators (LOs) at RF and intermediate frequency (IF). Avoiding large interferers can reduce image rejection ratio (IRR) requirements for a given performance and results in a greater tolerance to the Q of filters and receiver mismatches. The design approach is applied to an IEEE 802.11a receiver and the results show that for a given performance level, the proposed design requires an IRR that is 40 dB less than that required for conventional Weaver receivers
IEEE Journal on Selected Areas in Communications 05/2007; · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: MIMO-OFDM is being considered for communication systems where high throughput and spectral efficiency are important factors. Analog impairments like I/Q mismatch and phase noise significantly degrade and limit the performance of communication systems. In this paper, we analyze the impact of I/Q mismatch and phase noise in MIMO-OFDM systems as a function of the number of antennas. We show the improvement in performance that is possible when these impairments are cancelled. We also discuss the impact of correlated and uncorrelated phase noise in MIMO-OFDM systems. Finally, we show the results of I/Q mismatch and phase noise cancellation in wireless measurements performed using a 2times2 MIMO-OFDM testbed
IEEE Transactions on Wireless Communications 01/2007; · 2.59 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We introduce an Importance Sampling (IS) method that successfully simulates the performance of Low density Parity Check (LDPC) Codes in an AWGN channel at very low bit error rates (BERs). By effectively finding and biasing bit node combinations that are the dominant sources of error events, called trapping sets, the developed technique provokes more frequent decoder failures. Consequently, fewer simulation runs and higher simulation gains are achieved. Regardless of the block size of an LDPC code, only a few dominant trapping set classes cause decoder failures at low BER regions. Therefore, the proposed technique allows the performance evaluation for any size LDPC code at very low BER regions with remarkable simulation gains. For BERs of 10¿20, we observed simulation gains on the order of 1014.
Communications, 2006. ICC '06. IEEE International Conference on; 07/2006
-
[show abstract]
[hide abstract]
ABSTRACT: This paper presents an architecture that utilizes spectrum sensing with a Weaver architecture receiver to ease the requirements on the RF front-end components. With the ability to sense the environment, large interferers that overwhelm small desired signals can be avoided. The learning and adapting capabilities are enabled by means of a flexible receiver architecture with variable local oscillators (LOs) and tunable bandpass filters. Avoiding large interferers can reduce image rejection (IR) requirements for a given performance level and results in greater tolerance for low-Q filters and receiver mismatches. The design approach is applied to an IEEE 802.11a receiver and has shown significant performance improvements compared to conventional approaches despite an 18 dB lower image rejection
Circuits and Systems, 2006. ISCAS 2006. Proceedings. 2006 IEEE International Symposium on; 06/2006
-
[show abstract]
[hide abstract]
ABSTRACT: This paper presents a computationally efficient application-specific integrated circuit (ASIC) implementation for the decoding of space-time block codes (STBCs) . Alternative methods of evaluating the originally proposed maximum-likelihood decision metrics are explored at the algorithm and architectural level. At the algorithm level, unique decoding techniques are developed that result in computation savings of as much as 65%. At the architectural level, a low-computation symmetrical approach for the implementation of the proposed algorithm is presented. The proposed ASIC architecture offers considerable computation reductions leading to substantial power and area savings compared to a direct implementation of the original algorithm. The proposed architecture was realized in an ASIC referred to as the ST block decoder ASIC. The chip was fabricated using 0.18-μm CMOS technology and occupies a core area of 0.25 mm<sup>2</sup>. The ASIC architecture is highly scalable and can implement 2 × 2, 8 × 3, and 8 × 4 STBCs with modulation formats ranging from binary-phase shift keying (BPSK) to 16 quadrature amplitude modulation (QAM), and can operate at any symbol rate up to 20 Mbaud. Depending on the mode of operation, the decoder power consumption ranges from 0.54 mW for 2 × 2 BPSK systems to 1.89 mW for 8 × 4 16-QAM systems.
Circuits and Systems I: Regular Papers, IEEE Transactions on 02/2006; · 1.97 Impact Factor
-
US Patent 7,106,784. 01/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we introduce a computationally efficient selective node update algorithm for the decoding of low-density parity check codes. Unlike the standard sum-product algorithm, where all bit and check nodes are updated at each decoding iteration, the developed algorithm only updates unreliable check and bit nodes. A simple reliability criteria is developed to determine the active bit and check nodes per decoding iteration. Based on the developed technique, significant computation reductions are achieved with very little or no loss in the BER performance of the LDPC codes. At a WER of 10<sup>-5 </sup>, 91.8% and 72.7% check node and 80% and 41% bit node computation reductions are obtained for a (96, 48) and a (504, 252) LDPC code, respectively. The proposed method can be implemented with a slight modification to the standard sum-product decoding algorithm with negligible additional hardware complexity. From a hardware point of view, the proposed algorithm offers power reductions proportional to the computation savings and it leads to higher decoding speeds in serial implementations by decreasing the number of required memory accesses
Military Communications Conference, 2005. MILCOM 2005. IEEE; 11/2005
