9,131 research outputs found
Bounds on the Sum Capacity of Synchronous Binary CDMA Channels
In this paper, we obtain a family of lower bounds for the sum capacity of
Code Division Multiple Access (CDMA) channels assuming binary inputs and binary
signature codes in the presence of additive noise with an arbitrary
distribution. The envelope of this family gives a relatively tight lower bound
in terms of the number of users, spreading gain and the noise distribution. The
derivation methods for the noiseless and the noisy channels are different but
when the noise variance goes to zero, the noisy channel bound approaches the
noiseless case. The behavior of the lower bound shows that for small noise
power, the number of users can be much more than the spreading gain without any
significant loss of information (overloaded CDMA). A conjectured upper bound is
also derived under the usual assumption that the users send out equally likely
binary bits in the presence of additive noise with an arbitrary distribution.
As the noise level increases, and/or, the ratio of the number of users and the
spreading gain increases, the conjectured upper bound approaches the lower
bound. We have also derived asymptotic limits of our bounds that can be
compared to a formula that Tanaka obtained using techniques from statistical
physics; his bound is close to that of our conjectured upper bound for large
scale systems.Comment: to be published in IEEE Transactions on Information Theor
NOMA and interference limited satellite scenarios
This paper deals with the problem of non-orthogonal multiple access (NOMA) in multibeam satellite systems, where the signals are jointly precoded. It is considered that the number of frames that are simultaneously transmitted is higher than the number of feeds, reducing the precoding interference mitigation capabilities as the system becomes overloaded. In order to solve this problem, we assume that the satellite user terminals are able to perform multi-user detection to mitigate the interference. In the current NOMA approach, it
is assumed a successive interference cancellation (SIC) receiver.
To increase the spectral efficiency, this paper investigates NOMA
with simultaneous non-unique detection (SND). Compared to the
case where user terminals perform single user detection (SUD),
conventional scheduling heuristic rules do not longer apply in this
scenario. Therefore, different scheduling algorithms are proposed
considering both SIC and SND strategies. As the numerical evaluations show, SND yields larger average data rates than the SIC receiver. Concerning the scheduling, the best strategy is to pair users with highly correlated channels and the lowest channel gain difference. It is also shown that the sum-rate can be increased in overloaded satellite systems with respect to satellite scenarios, where the number of transmitted frames and feeds is the same.Peer ReviewedPostprint (author's final draft
Fundamental Limits of Low-Density Spreading NOMA with Fading
Spectral efficiency of low-density spreading non-orthogonal multiple access
channels in the presence of fading is derived for linear detection with
independent decoding as well as optimum decoding. The large system limit, where
both the number of users and number of signal dimensions grow with fixed ratio,
called load, is considered. In the case of optimum decoding, it is found that
low-density spreading underperforms dense spreading for all loads. Conversely,
linear detection is characterized by different behaviors in the underloaded vs.
overloaded regimes. In particular, it is shown that spectral efficiency changes
smoothly as load increases. However, in the overloaded regime, the spectral
efficiency of low- density spreading is higher than that of dense spreading
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An Adaptive Soft Handover Scheme Using Fuzzy Load Balancing for WCDMA Systems
In cellular systems, user distribution variations can cause load imbalance between cells. Embedding a load balancing strategy within the handover scheme means that ensuing traffic congestion can be alleviated by dynamically reallocating load between neighbouring cells. An adaptive soft handover scheme for multimedia cellular communication systems is proposed in this paper, that considers both the cell load factors as well as the pilot channel signal-to-interference-and-noise-ratio (SINR) for soft handovers. By using fuzzy principles, the soft handover thresholds and time hysteresis are adapted dependent upon the loads of the neighbouring cells. Simulation results show that the new algorithm provides improved system performance in terms of a more evenly distributed load, lower blocking probabilities and higher throughput
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