| 1. |
- Gezici, S, et al.
(författare)
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The trade-off between processing gains of impulse radio systems in the presence of timing jitter
- 2004
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Ingår i: 2004 IEEE International Conference on Communications. - 0780385330 ; 6, s. 3596-3600
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Konferensbidrag (refereegranskat)abstract
- In time hopping impulse radio, N/sub f/ pulses of duration T/sub c/ are transmitted for each symbol. This gives rise to two types of processing gain: (i) pulse combining gain, which is a factor N/sub f/, and (ii) pulse spreading gain, which is N/sub c/ = T/sub f//T/sub c/, where T/sub f/ is the mean interval between two subsequent pulses. This paper investigates the trade-off between these two types of processing gain with and without random polarity codes in the presence of timing jitter. Approximate expressions for bit error probability are derived for both coded and uncoded systems over additive white Gaussian noise channels and are used as the criterion to choose optimal N/sub f/ and N/sub c/ values. The effects of timing jitter and multiple access interference on the selection of optimal system parameters are explained through theoretical analysis. Simulation studies support the theoretical results.
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| 2. |
- Sehlstedt, Martin, et al.
(författare)
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Distribution of multiresolution data using orthogonal broadcast channels : some design issues
- 2004
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Ingår i: Gate to the global information society. - Piscataway, NJ : IEEE Communications Society. - 0780385330 ; , s. 2837-2841
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Konferensbidrag (refereegranskat)abstract
- Optimization of an N-layer digital multiresolution broadcasting system with respect to some form of quality/spl times/population product objective function appears to he an N-dimensional problem. However, with a generalized broadcasting model using N orthogonal channels with BPSK signaling under the assumption of uniform population distribution the problem can be narrowed down to three cases. This paper identifies each of these three cases and explains their meaning for system performance optimization.
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| 3. |
- Tacca, Marco, et al.
(författare)
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The disjoint path-pair matrix approach for online routing in reliable WDM networks
- 2004
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Ingår i: 2004 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-7. - 0780385330 ; , s. 1187-1191
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Konferensbidrag (refereegranskat)abstract
- In reliable wavelength division multiplexed (WDM) networks, the optimal choice of routing and wavelength assignment (RWA) for the working and protection path-pair is often a complex problem to solve. To cope with such problem complexity, this paper presents the disjoint path-pair matrix (DPM) approach. With the DPM approach, the RWA problem complexity - i.e., the size of the solution space - can be significantly reduced by limiting the number of candidate path-pairs considered in the optimization. Simulation results are collected using the DPM approach to solve the online RWA problem in a network based on the shared path protection switching scheme with differentiated reliability. When compared to the conventional k-shortest paths approach, the DPM approach requires up to one order of magnitude less candidate path-pairs. In addition, the DPM approach finds solutions with reduced hop length of both the working and protection paths by up to 3% and 14%, respectively.
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| 4. |
- Toyserkani, Arash T., et al.
(författare)
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Sub-carrier based adaptive modulation in HIPERLAN/2 system
- 2004
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Ingår i: 2004 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-7. - NEW YORK : IEEE. - 0780385330 ; , s. 3460-3464
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Konferensbidrag (refereegranskat)abstract
- The paper investigates the performance of HIPER-LAN/2 (H/2) with sub-carrier based adaptive modulation, with respect to average throughput. The results obtained are compared with the case where the same modulation is used across all the sub-carriers as in the current H/2. We suggest a bit loading algorithm "Simple Rate Adaptive Algorithm" (SRA) based on the criteria of maximizing the throughput. Our work displays that this bit loading scheme improves the throughput compared to the existent H/2. The cost incurred is the complexity added by the adaptive modulation scheme. However, the total complexity of the system is decreased due to simplification in channel coding.
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| 5. |
- Vehkaperä, Mikko, et al.
(författare)
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Layered space-frequency coding and receiver design for MIMO MC-CDMA
- 2004
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Ingår i: 2004 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS. - 0780385330 ; , s. 3005-3009
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Konferensbidrag (refereegranskat)abstract
- For the single-user communications pragmatic yet powerful methods known as layered space-time (LST) architectures provide means to increase the user data rate of a multiple-input multiple-output (MIMO) antenna system dramatically. To achieve better error rate performance, the LST transmission can be further accompanied with forward error correction coding. Future wireless communication systems will, however, require high data rates also in multiuser environments. For broadband multiuser wireless communications, multicarrier code-division multiple-access (MC-CDMA) has emerged as an attractive technique due to its low equalization complexity and robust performance in multipath fading channels. Utilizing strong channel coding and LST architectures with MC-CDMA, high spectral efficiency and good error rate performance can be obtained in diverse environments. In this paper, single and multi-antenna turbo coded downlink MC-CDMA is combined with the concept of LST architectures. Due to the inevitable complexity restrictions, a suboptimal receiver interface for the underlying system is designed and its performance is evaluated in fading channels. The results demonstrate that significant improvement in both error rate performance and the system throughput can be achieved also with multiuser communications by using these MIMO techniques.
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| 6. |
- Velayos, Hector, et al.
(författare)
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Load balancing in overlapping wireless LAN cells
- 2004
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Ingår i: 2004 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-7. - : IEEE. - 0780385330 ; , s. 3833-3836
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Konferensbidrag (refereegranskat)abstract
- We propose a load-balancing scheme for overlapping wireless LAN cells. Agents running in each access point broadcast periodically the local load level via the Ethernet backbone and determine whether the access point is overloaded, balanced or under-loaded by comparing it with received reports. The load metric is the access point throughput. Overloaded access points force the handoff of some stations to balance the load. Only under-loaded access points accept roaming stations to minimize the number of handoffs. We show via experimental evaluation that our balancing scheme increases the total wireless network throughput and decreases the cell delay.
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| 7. |
- Velayos, Hector, et al.
(författare)
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Techniques to reduce IEEE 802.11b handoff time
- 2004
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Ingår i: 2004 IEEE International Conference on Communications, Vols 1-7. - : IEEE. - 0780385330 ; , s. 3844-3848
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Konferensbidrag (refereegranskat)abstract
- We analyze the link-layer handoff process in wireless LANs based on the IEEE 802.11b standard and suggest how to reduce its duration. Firstly, we divide the process into three phases: detection, search and execution. Our performance measurements indicate that the detection and search phases are the main contributors to the handoff time. We show that the link-layer detection time can be reduced to three consecutive lost frames. We also show that the search time can be reduced at least by 20% using active scanning with the two timers that control its duration set to 1 ms and 10.24 ms. Several simulations illustrate the achieved reduction in handoff time.
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| 8. |
- Veronesi, Riccardo, et al.
(författare)
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Distributed dynamic resource allocation for multicell SDMA packet access networks
- 2004
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Ingår i: 2004 IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS, VOLS 1-7. - New York : IEEE. - 0780385330 ; , s. 202-207
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Konferensbidrag (refereegranskat)abstract
- This paper addresses dynamic resource allocation (DRA) for the downlink of a SDMA broadband wireless packet network with multiple access ports and adaptive antennas. The main issue for distributed DRA is how to manage the intercell interference, which is very difficult to be handled in an uncoordinated environment, due to packet access and downlink beamforming. Here, we investigate the performance and the feasibility of some distributed DRA strategies to gain insight on the impact of the lack of coordination among cells.We propose to estimate the intercell interference on the basis of cell load and worst case (taking into account adaptive antennas) measured interference and implement some greedy strategies suitable for a distributed implementation. We show that the gap between random allocation (previously proposed for distributed packet allocation) and a centralized algorithm can be significantly reduced, although the implementation of an efficient distributed power control still appears a hard task.
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| 9. |
- Wehinger, Joachim, et al.
(författare)
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On channel estimators for iterative CDMA multiuser receivers in flat Rayleigh fading
- 2004
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Ingår i: IEEE International Conference on Communications, 2004. - 0780385330 ; , s. 2497-2501
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Konferensbidrag (refereegranskat)abstract
- In this work we compare channel estimation algorithms for use in an iterative CDMA receiver in a block fading environment. The receiver consists of a soft multiuser data estimator, a bank of single user decoders, and a multiuser channel estimator. The multiuser data estimator is implemented as parallel interference canceler with unconditional post-MMSE filtering (PIC-MMSE) and the decoder is a soft-in soft-out MAP decoder. In the channel estimator we make use of dedicated pilot symbols and fed back soft-code symbols which are exploited as additional soft pilot symbols when the iterations proceed. We show that using extrinsic information increases the receiver performance significantly compared to using a posteriori information in the feedback for channel estimation. We introduce a linear MMSE (LMMSE) estimator which takes into account the variances of fed back code symbols and compare it to approximations of the least squares (ALS) estimator and the linear minimum-mean-square error (ALMMSE) estimator. Performance results are illustrated in terms of bit error rate (BER) and average normalized square error (ANSE) of the channel estimators. They show that the newly proposed LMMSE algorithm outperforms the ALS and ALMMSE algorithms.
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