2,685 research outputs found
Spectral efficiency and optimal medium access control of random access systems over large random spreading CDMA
This paper analyzes the spectral efficiency as a function of medium access
control (MAC) for large random spreading CDMA random access systems that employ
a linear receiver. It is shown that located at higher than the physical layer,
MAC along with spreading and power allocation can effectively perform spectral
efficiency maximization and near-far mitigation.Comment: To appear in IEEE Trans. on Communication
Spectral Efficiency of Random Time-Hopping CDMA
Traditionally paired with impulsive communications, Time-Hopping CDMA
(TH-CDMA) is a multiple access technique that separates users in time by coding
their transmissions into pulses occupying a subset of chips out
of the total included in a symbol period, in contrast with traditional
Direct-Sequence CDMA (DS-CDMA) where . This work analyzes
TH-CDMA with random spreading, by determining whether peculiar theoretical
limits are identifiable, with both optimal and sub-optimal receiver structures,
in particular in the archetypal case of sparse spreading, that is,
. Results indicate that TH-CDMA has a fundamentally different
behavior than DS-CDMA, where the crucial role played by energy concentration,
typical of time-hopping, directly relates with its intrinsic "uneven" use of
degrees of freedom.Comment: 26 pages, 13 figure
Asynchronous CDMA Systems with Random Spreading-Part I: Fundamental Limits
Spectral efficiency for asynchronous code division multiple access (CDMA)
with random spreading is calculated in the large system limit allowing for
arbitrary chip waveforms and frequency-flat fading. Signal to interference and
noise ratios (SINRs) for suboptimal receivers, such as the linear minimum mean
square error (MMSE) detectors, are derived. The approach is general and
optionally allows even for statistics obtained by under-sampling the received
signal.
All performance measures are given as a function of the chip waveform and the
delay distribution of the users in the large system limit. It turns out that
synchronizing users on a chip level impairs performance for all chip waveforms
with bandwidth greater than the Nyquist bandwidth, e.g., positive roll-off
factors. For example, with the pulse shaping demanded in the UMTS standard,
user synchronization reduces spectral efficiency up to 12% at 10 dB normalized
signal-to-noise ratio. The benefits of asynchronism stem from the finding that
the excess bandwidth of chip waveforms actually spans additional dimensions in
signal space, if the users are de-synchronized on the chip-level. The analysis
of linear MMSE detectors shows that the limiting interference effects can be
decoupled both in the user domain and in the frequency domain such that the
concept of the effective interference spectral density arises. This generalizes
and refines Tse and Hanly's concept of effective interference.
In Part II, the analysis is extended to any linear detector that admits a
representation as multistage detector and guidelines for the design of low
complexity multistage detectors with universal weights are provided
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
Randomly Spread CDMA: Asymptotics via Statistical Physics
This paper studies randomly spread code-division multiple access (CDMA) and
multiuser detection in the large-system limit using the replica method
developed in statistical physics. Arbitrary input distributions and flat fading
are considered. A generic multiuser detector in the form of the posterior mean
estimator is applied before single-user decoding. The generic detector can be
particularized to the matched filter, decorrelator, linear MMSE detector, the
jointly or the individually optimal detector, and others. It is found that the
detection output for each user, although in general asymptotically non-Gaussian
conditioned on the transmitted symbol, converges as the number of users go to
infinity to a deterministic function of a "hidden" Gaussian statistic
independent of the interferers. Thus the multiuser channel can be decoupled:
Each user experiences an equivalent single-user Gaussian channel, whose
signal-to-noise ratio suffers a degradation due to the multiple-access
interference. The uncoded error performance (e.g., symbol-error-rate) and the
mutual information can then be fully characterized using the degradation
factor, also known as the multiuser efficiency, which can be obtained by
solving a pair of coupled fixed-point equations identified in this paper. Based
on a general linear vector channel model, the results are also applicable to
MIMO channels such as in multiantenna systems.Comment: To be published in IEEE Transactions on Information Theor
Large-System Analysis of Joint Channel and Data Estimation for MIMO DS-CDMA Systems
This paper presents a large-system analysis of the performance of joint
channel estimation, multiuser detection, and per-user decoding (CE-MUDD) for
randomly-spread multiple-input multiple-output (MIMO) direct-sequence
code-division multiple-access (DS-CDMA) systems. A suboptimal receiver based on
successive decoding in conjunction with linear minimum mean-squared error
(LMMSE) channel estimation is investigated. The replica method, developed in
statistical mechanics, is used to evaluate the performance in the large-system
limit, where the number of users and the spreading factor tend to infinity
while their ratio and the number of transmit and receive antennas are kept
constant. The performance of the joint CE-MUDD based on LMMSE channel
estimation is compared to the spectral efficiencies of several receivers based
on one-shot LMMSE channel estimation, in which the decoded data symbols are not
utilized to refine the initial channel estimates. The results imply that the
use of joint CE-MUDD significantly reduces rate loss due to transmission of
pilot signals, especially for multiple-antenna systems. As a result, joint
CE-MUDD can provide significant performance gains, compared to the receivers
based on one-shot channel estimation.Comment: The paper was resubmitted to IEEE Trans. Inf. Theor
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
- …