1,083 research outputs found
Weyl Spreading Sequence Optimizing CDMA
This paper shows an optimal spreading sequence in the Weyl sequence class,
which is similar to the set of the Oppermann sequences for asynchronous CDMA
systems. Sequences in Weyl sequence class have the desired property that the
order of cross-correlation is low. Therefore, sequences in the Weyl sequence
class are expected to minimize the inter-symbol interference. We evaluate the
upper bound of cross-correlation and odd cross-correlation of spreading
sequences in the Weyl sequence class and construct the optimization problem:
minimize the upper bound of the absolute values of cross-correlation and odd
cross-correlation. Since our optimization problem is convex, we can derive the
optimal spreading sequences as the global solution of the problem. We show
their signal to interference plus noise ratio (SINR) in a special case. From
this result, we propose how the initial elements are assigned, that is, how
spreading sequences are assigned to each users. In an asynchronous CDMA system,
we also numerically compare our spreading sequences with other ones, the Gold
codes, the Oppermann sequences, the optimal Chebyshev spreading sequences and
the SP sequences in Bit Error Rate. Our spreading sequence, which yields the
global solution, has the highest performance among the other spreading
sequences tested
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
Minimum mean-squared error iterative successive parallel arbitrated decision feedback detectors for DS-CDMA systems
In this paper we propose minimum mean squared error (MMSE) iterative successive parallel arbitrated decision feedback (DF) receivers for direct sequence code division multiple access (DS-CDMA) systems. We describe the MMSE design criterion for DF multiuser detectors along with successive, parallel and iterative interference cancellation structures. A novel efficient DF structure that employs successive cancellation with parallel arbitrated branches and a near-optimal low complexity user ordering algorithm are presented. The proposed DF receiver structure and the ordering algorithm are then combined with iterative cascaded DF stages for mitigating the deleterious effects of error propagation for convolutionally encoded systems with both Viterbi and turbo decoding as well as for uncoded schemes. We mathematically study the relations between the MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback. Simulation results for an uplink scenario assess the new iterative DF detectors against linear receivers and evaluate the effects of error propagation of the new cancellation methods against existing ones
Performance Analysis of Iterative Channel Estimation and Multiuser Detection in Multipath DS-CDMA Channels
This paper examines the performance of decision feedback based iterative
channel estimation and multiuser detection in channel coded aperiodic DS-CDMA
systems operating over multipath fading channels. First, explicit expressions
describing the performance of channel estimation and parallel interference
cancellation based multiuser detection are developed. These results are then
combined to characterize the evolution of the performance of a system that
iterates among channel estimation, multiuser detection and channel decoding.
Sufficient conditions for convergence of this system to a unique fixed point
are developed.Comment: To appear in the IEEE Transactions on Signal Processin
Information theoretic analysis of LSD scheme
In this paper, the capacity region of Low Density Signature Multiple Access Channel (LDS-MAC) is calculated through information theoretic analysis. LDS Code Division Multiple Access (LDS-CDMA) uses spreading sequences of low density for spreading the data symbols in time domain. This technique benefits from a less complex Multiuser Detector (MUD) compared to conventional CDMA with optimum MUD; while keeping the performance close to the single user scenario for up to 200% loaded conditions. Also evaluated is the effect of different factors on the capacity of LDS MAC
- …