| 1. |
- Haghighi, Kasra, 1978, et al.
(författare)
-
An LLR-based Cognitive Transmission Strategy for Higher Spectrum Reutilization
- 2011
-
Ingår i: IEEE GLOBECOM 2011 - Cognitive Radio and Networks Symposium. - 9781424492688
-
Konferensbidrag (refereegranskat)abstract
- Reutilization of the spectrum licensed to services with low occupancy is of great interest for cognitive radios (CRs). To achieve this goal, we introduce a simple hidden Markov model which captures the primary users activity, signal uncertainties, and noise. For evaluating the performance of any CR, two new criteria are presented entitled spectrum utilization ratio (UR) and interference ratio (IR). Based on this model and new measures, a new a-posterior log-likelihood-ratio based CR is designed and implemented. Its performance is compared with standard energy-detection based spectrum-sensing CR. We demonstrate more than 300% increase in UR for up to 1% allowed interference at the SNR of -5dB.
|
|
| 2. |
- Haghighi, Kasra, 1978, et al.
(författare)
-
On Optimum Causal Cognitive Spectrum Reutilization Strategy
- 2012
-
Ingår i: IEEE Journal on Selected Areas in Communications. - 0733-8716 .- 1558-0008. ; 30:10, s. 1911-1921
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we study opportunistic transmission strategies for cognitive radios (CR) in which causal noisy observation from a primary user(s) (PU) state is available. PU is assumed to be operating in a slotted manner, according to a two-state Markov model. The objective is to maximize utilization ratio (UR), i.e., relative number of the PU-idle slots that are used by CR, subject to interference ratio (IR), i.e., relative number of the PU-active slots that are used by CR, below a certain level. We introduce an a-posteriori LLR-based cognitive transmission strategy and show that this strategy is optimum in the sense of maximizing UR given a certain maximum allowed IR. Two methods for calculating threshold for this strategy in practical situations are presented. One of them performs well in higher SNRs but might have too large IR at low SNRs and low PU activity levels, and the other is proven to never violate the allowed IR at the price of a reduced UR. In addition, an upper-bound for the UR of any CR strategy operating in the presence of Markovian PU is presented. Simulation results have shown a more than 116% improvement in UR at SNR of -3dB and IR level of 10% with PU state estimation. Thus, this opportunistic CR mechanism possesses a high potential in practical scenarios in which there exists no information about true states of PU.
|
|
| 3. |
- Agrell, Erik, 1965, et al.
(författare)
-
Achieving the Shannon Limit with Probabilistically Shaped BICM
- 2012
-
Ingår i: IEEE International Symposium on Information Theory - Proceedings. - 2157-8095. - 9781467325806 ; , s. 2421-2425
-
Konferensbidrag (refereegranskat)abstract
- Probabilistic shaping for bit-interleaved coded modulation (BICM) systems at low signal-to-noise ratio (SNR) is investigated. Using known results for BICM systems with a uniform input distribution, the combinations of input alphabet, input distribution, and binary labeling that achieve the Shannon limit -1.59 dB are fully characterized. It is found that a BICM system achieves the Shannon limit at low SNR if and only if it can be represented as a zero-mean linear projection of a hypercube, which is the same condition as for uniform input distributions. Hence, probabilistic shaping offers no extra degrees of freedom to optimize the low-SNR mutual information of BICM systems, in addition to what is provided by geometrical shaping.
|
|
| 4. |
|
|
| 5. |
- Agrell, Erik, 1965, et al.
(författare)
-
First-order asymptotics of the BICM mutual information: Uniform vs. nonuniform distributions
- 2012
-
Ingår i: Information Theory and Applications Workshop (ITA) 2012, San Diego, CA [Invited]. - 9781467314732 ; , s. 306 - 310
-
Konferensbidrag (refereegranskat)abstract
- A linear, invertible transform is defined between two vectors or matrices as a tool for analyzing the bit-interleaved coded modulation (BICM) mutual information in the wideband regime. The transform coefficients depend on a set of real values, which can be interpreted as probabilities. The transform relates any BICM system with a nonuniform input distribution to another BICM system with a uniform distribution. Numerical evidence suggests that the two systems have the same first-order behavior, which would make possible to analyze nonuniform BICM systems based on known properties of uniform BICM systems.
|
|
| 6. |
- Agrell, Erik, 1965
(författare)
-
Nonlinear fiber capacity
- 2013
-
Ingår i: 39th European Conference and Exhibition on Optical Communication, ECOC 2013; London; United Kingdom; 22 September 2013 through 26 September 2013. - : Institution of Engineering and Technology. - 9781849197595 ; :622 CP, s. 630-632
-
Konferensbidrag (refereegranskat)abstract
- In this semi-tutorial presentation, we review fundamental information theory for links with and without memory, in the linear and nonlinear regimes. A comparison between channel models with long (but finite) memory and infinite memory yields an unexpected result.
|
|
| 7. |
- Agrell, Erik, 1965, et al.
(författare)
-
On the symbol error probability of regular polytopes
- 2011
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 57:6, s. 3411-3415
-
Tidskriftsartikel (refereegranskat)abstract
- An exact expression is derived for the symbol error probability when the vertices of the four-dimensional 24-cell are used for digital transmission in Gaussian noise. Corresponding expressions for other regular convex polytopes are summarized. Numerically stable versions of these error probabilities are also obtained. © 2011 IEEE.
|
|
| 8. |
- Agrell, Erik, 1965, et al.
(författare)
-
Optimal Alphabets and Binary Labelings for BICM at Low SNR
- 2011
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 57:10, s. 6650-6672
-
Tidskriftsartikel (refereegranskat)abstract
- Optimal binary labelings, input distributions, and input alphabets are analyzed for the so-called bit-interleaved coded modulation (BICM) capacity, paying special attention to the low signal-to-noise ratio (SNR) regime. For 8-ary pulse amplitude modulation (PAM) and for 0.75 bit/symbol, the folded binary code results in a higher capacity than the binary reflected gray code (BRGC) and the natural binary code (NBC). The 1 dB gap between the additive white Gaussian noise (AWGN) capacity and the BICM capacity with the BRGC can be almost completely removed if the input symbol distribution is properly selected. First-order asymptotics of the BICM capacity for arbitrary input alphabets and distributions, dimensions, mean, variance, and binary labeling are developed. These asymptotics are used to define first-order optimal (FOO) constellations for BICM, i.e. constellations that make BICM achieve the Shannon limit $-1.59 \tr{dB}$. It is shown that the $\Eb/N_0$ required for reliable transmission at asymptotically low rates in BICM can be as high as infinity, that for uniform input distributions and 8-PAM there are only 72 classes of binary labelings with a different first-order asymptotic behavior, and that this number is reduced to only 26 for 8-ary phase shift keying (PSK). A general answer to the question of FOO constellations for BICM is also given: using the Hadamard transform, it is found that for uniform input distributions, a constellation for BICM is FOO if and only if it is a linear projection of a hypercube. A constellation based on PAM or quadrature amplitude modulation input alphabets is FOO if and only if they are labeled by the NBC; if the constellation is based on PSK input alphabets instead, it can never be FOO if the input alphabet has more than four points, regardless of the labeling.
|
|
| 9. |
- Agrell, Erik, 1965, et al.
(författare)
-
Satellite constellations: Towards the nonlinear channel capacity
- 2012
-
Ingår i: 25th IEEE Photonics Conference, IPC 2012, Burlingame, 23 - 27 September 2012 [Invited]. - 9781457707315 ; , s. 316-317
-
Konferensbidrag (refereegranskat)abstract
- We present a family of adaptive constellations that, in a nonlinear optical fiber channel model, has a mutual information (modulation-constrained capacity) that does not decrease with signal power.
|
|
| 10. |
- Agrell, Erik, 1965, et al.
(författare)
-
Signal Shaping for BICM at Low SNR
- 2013
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 59:4, s. 2396-2410
-
Tidskriftsartikel (refereegranskat)abstract
- The generalized mutual information (GMI) of bit-interleaved coded modulation (BICM) systems, sometimes called the BICM capacity, is investigated at low signal-to-noise ratio (SNR). The combinations of input alphabet, input distribution, and binary labeling that achieve the Shannon limit are completely characterized. The main conclusion is that a BICM system with probabilistic shaping achieves the Shannon limit at low SNR if and only if it can be represented as a zero-mean linear projection of a hypercube. Hence, probabilistic shaping offers no extra degrees of freedom to optimize the low-SNR BICM-GMI, in addition to what is provided by geometrical shaping. The analytical conclusions are confirmed by numerical results, which also show that for a fixed input alphabet, probabilistic shaping can improve the BICM-GMI in the low and medium SNR range.
|
|
