32,568 research outputs found
Slow Frequency Hopping Assisted MC DS-CDMA using Large Area Synchronised Spreading Sequences
The family of Multi-Carrier Direct-Sequence CDMA (MC DS- CDMA) systems exhibits numerous attractive properties, which render them attractive candidates for next-generation wireless communications. We demonstrate that spreading codes exhibiting a so-called interference-free window (IFW) are capable of outperforming classic spreading codes, when the interfering multi-user and multipath components arrive within this IFW. The best possible quasi-synchronous timing of the spreading sequences has to be adjusted with the aid of accurate adaptive timing advance control, which has to be significantly more accurate than that used in the lower-bit-rate second-generation GSM system. Fortunately, the IFW duration may be extended with the advent of multi-carrier DS-CDMA proportionately to the number of subcarriers. Hence the resultant MC DS-CDMA system is capable of exhibiting a near-single-user performance without employing a multi-user detector. A deficiency of the resultant system is that the number of spreading codes exhibiting a certain IFW is limited and so is the IFW duration. This contribution sets out to mitigate the above-mentioned shortcomings so that when the users' delays are in the range of the IFW, we separate them with the aid of the unique, user-specific LAS spreading codes. By contrast, when the users roam at a high distance from the base-station and hence their received signal arrive outside the range of the IFW, we separate them using their unique frequency hopping patterns
Downlink SpaceâTime Spreading Using Interference Rejection Codes
In this paper, the authors will investigate the performance of a loosely synchronized (LS) code-based spaceâtime spreading (STS) scheme in comparison to that of classic Walsh code and pseudonoise code-based STS when communicating over dispersive Nakagami-m multipath channels. Closed-form formulas are derived for characterizing the bit-error-rate performance as a function of the number of resolvable paths L and the number of users K. Our numerical results suggest that the employment of LS code-based STS scheme is beneficial in a low-user-load and low-dispersion channel scenario, where a near-single-user performance can be achieved without a multiuser detector. Index TermsâCode-division multiple access (CDMA), Gaussian approximation, interference-free window (IFW), large area synchronized (LAS) codes, loosely synchronized (LS) codes, Nakagami-m fading
Multiuser Detection Assisted Time- and Frequency-Domain Spread Multicarrier Code-Division Multiple-Access
In this contribution, we study a reduced-complexity multiuser detection aided multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) scheme, which employs both time (T)-domain and frequency (F)-domain spreading. We investigate the achievable detection performance in the context of synchronous TF-domain spread MC DS-CDMA when communicating over an additive white Gaussian noise (AWGN) channel. Five detection schemes are investigated, which include the single-user correlation based detector, the joint TF-domain decorrelating multiuser detector (MUD), the joint TF-domain MMSEMUD, the separate TF-domain decorrelating/MMSE MUD, and the separate TF-domain MMSE/decorrelating MUD. Our simulation results show that the separate TF-domain MUD schemes are capable of achieving a similar bit error rate (BER) performance to that of the significantly more complex joint TF-domain MUD schemes. Index TermsâCode-division multiple-access (CDMA), decorrelating, frequency-domain spreading, joint detection, minimum mean square error (MMSE), multicarrier (MC), multiuser detection, separate detection, time-domain spreading
A Space-Time Spreading Assisted Multicarrier DS-CDMA System using OVSF Codes Employing Adaptive Mode Switching Thresholds and Adaptive Modulation
The Concept of Space-Time Spreading (STS) assisted, Adaptive Quadrature Amplitude Modulation (AQAM) aidedMC DS-CDMAis introduced. The AQAMmode-switching thresholds are determined using an on-line real-time Bit-Error-Ratio (BER) based learning procedure. Four different schemes were studied, designed for maintaining a given target BER of 10?2, 10?3, 10?4 and 10?5 both with and without turbo channel coding. The effect of using different number of STS antennas ranging from1 to 8 was also characterized in terms of the achievable effective throughput, characterising the associated system design trade-offs
Finite-Blocklength Bounds for Wiretap Channels
This paper investigates the maximal secrecy rate over a wiretap channel
subject to reliability and secrecy constraints at a given blocklength. New
achievability and converse bounds are derived, which are shown to be tighter
than existing bounds. The bounds also lead to the tightest second-order coding
rate for discrete memoryless and Gaussian wiretap channels.Comment: extended version of a paper submitted to ISIT 201
A Beta-Beta Achievability Bound with Applications
A channel coding achievability bound expressed in terms of the ratio between
two Neyman-Pearson functions is proposed. This bound is the dual of a
converse bound established earlier by Polyanskiy and Verd\'{u} (2014). The new
bound turns out to simplify considerably the analysis in situations where the
channel output distribution is not a product distribution, for example due to a
cost constraint or a structural constraint (such as orthogonality or constant
composition) on the channel inputs. Connections to existing bounds in the
literature are discussed. The bound is then used to derive 1) an achievability
bound on the channel dispersion of additive non-Gaussian noise channels with
random Gaussian codebooks, 2) the channel dispersion of the exponential-noise
channel, 3) a second-order expansion for the minimum energy per bit of an AWGN
channel, and 4) a lower bound on the maximum coding rate of a multiple-input
multiple-output Rayleigh-fading channel with perfect channel state information
at the receiver, which is the tightest known achievability result.Comment: extended version of a paper submitted to ISIT 201
Beta-Beta Bounds: Finite-Blocklength Analog of the Golden Formula
It is well known that the mutual information between two random variables can
be expressed as the difference of two relative entropies that depend on an
auxiliary distribution, a relation sometimes referred to as the golden formula.
This paper is concerned with a finite-blocklength extension of this relation.
This extension consists of two elements: 1) a finite-blocklength channel-coding
converse bound by Polyanskiy and Verd\'{u} (2014), which involves the ratio of
two Neyman-Pearson functions (beta-beta converse bound); and 2) a novel
beta-beta channel-coding achievability bound, expressed again as the ratio of
two Neyman-Pearson functions.
To demonstrate the usefulness of this finite-blocklength extension of the
golden formula, the beta-beta achievability and converse bounds are used to
obtain a finite-blocklength extension of Verd\'{u}'s (2002) wideband-slope
approximation. The proof parallels the derivation of the latter, with the
beta-beta bounds used in place of the golden formula.
The beta-beta (achievability) bound is also shown to be useful in cases where
the capacity-achieving output distribution is not a product distribution due
to, e.g., a cost constraint or structural constraints on the codebook, such as
orthogonality or constant composition. As an example, the bound is used to
characterize the channel dispersion of the additive exponential-noise channel
and to obtain a finite-blocklength achievability bound (the tightest to date)
for multiple-input multiple-output Rayleigh-fading channels with perfect
channel state information at the receiver.Comment: to appear in IEEE Transactions on Information Theor
MultiâDuality in Minimal SurfaceâType Problems
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113741/1/sapm1995952127.pd
- âŠ