An Inequality on the Capacity Region of Multiaccess Multipath Channels

Gallager, Robert G.

doi:10.1007/978-1-4615-2694-0_14

Cited by 61 publications

(44 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Unless and are deterministic, the total rate sum achieved by TDMA is no longer optimum, and its achievability region only intersects the capacity region at the trivial points where one of the users is silent. This property, which is one of the multiuser diversity mechanisms by means of which total capacity can be higher in the multiaccess channel than in the single-user channel with the same aggregate power (e.g., [3]), is a straightforward consequence of Jensen's inequality and appears to have been pointed out for the first time in [2]. Nevertheless, even in the presence of fading with an arbitrary distribution, it is easy to show that Theorem 1 holds, and thus TDMA is optimum as far as requiring the same minimum energies per bit as superposition.…”

Section: Theoremmentioning

confidence: 88%

“…By substitution of the first and second derivatives at of the rate functions in (2) and (1), we see that the minimum received energies per bit are (91) (92) achieved by both TDMA and (88). The slope region boundary achieved by TDMA is the following generalization of (79)):…”

Section: Theoremmentioning

confidence: 99%

“…The presence of fading tends to tilt the balance back in favor of superposition. Indeed, when users are affected by independent fading, they can achieve strictly higher total aggregate rate with superposition than with TDMA, as a simple consequence of the concavity of channel capacity as a function of signal-to-noise ratio [2]. Other practical effects that favor superposition include the presence of channel distortion (which destroys the orthogonality of the TDMA signals) and out-of-cell interference [3].…”

mentioning

confidence: 99%

See 2 more Smart Citations

Suboptimality of TDMA in the Low-Power Regime

Caire

Tuninetti

Verdú

2004

IEEE Trans. Inform. Theory

114

View full text Add to dashboard Cite

Abstract-We consider multiaccess, broadcast, and interference channels with additive Gaussian noise. Although the set of rate pairs achievable by time-division multiple access (TDMA) is not equal to the capacity region, the TDMA achievable region converges to the capacity region as the power decreases. Furthermore, TDMA achieves the optimum minimum energy per bit. Despite those features, this paper shows that the growth of TDMA-achievable rates with the energy per bit is suboptimal in the low-power regime except in special cases: multiaccess channels where the users' energy per bit are identical and broadcast channels where the receivers have identical signal-to-noise ratios. For the additive Gaussian noise interference channel, we identify a small region of interference parameters outside of which TDMA is also shown to be suboptimal. The effect of fading (known to the receiver) on the suboptimality of TDMA is also explored.Index Terms-Broadcast channels, channel capacity, code-division multiple access (CDMA), interference channels, low-power communication, multiple-access channels, time-division multiple access (TDMA), wideband regime.

show abstract

Section: Theoremmentioning

confidence: 88%

Section: Theoremmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

Suboptimality of TDMA in the Low-Power Regime

Caire

Tuninetti

Verdú

2004

IEEE Trans. Inform. Theory

114

View full text Add to dashboard Cite

show abstract

“…Strict orthogonality is utilized in Multicarrier (MC) schemes [1]- [3], orthogonal space-time block codes [4], and frequencydivision multiple access (FDMA). Although the strict orthogonality reduces the complexity, it can degrade the performance, as it is known [5] that FDMA is inferior to code-division multiple access (CDMA). A better trade-off between complexity and performance is provided by adopting statistical orthogonality.…”

Section: Introductionmentioning

confidence: 99%

Asymptotic Analysis of General Multiuser Detectors in MIMO DS-CDMA Channels

Takeuchi¹,

Tanaka²,

Yano

2008

IEEE J. Select. Areas Commun.

View full text Add to dashboard Cite

We analyze a MIMO DS-CDMA channel with a general multiuser detector including a nonlinear multiuser detector, using the replica method. In the many-user, limit the MIMO DS-CDMA channel with the multiuser detector is decoupled into a bank of single-user SIMO Gaussian channels if a spatial spreading scheme is employed. On the other hand, it is decoupled into a bank of single-user MIMO Gaussian channels if a spatial spreading scheme is not employed. The spectral efficiency of the MIMO DS-CDMA channel with the spatial spreading scheme is comparable with that of the MIMO DS-CDMA channel using an optimal space-time block code without the spatial spreading scheme. In the case of the QPSK data modulation scheme the spectral efficiency of the MIMO DS-CDMA channel with the MMSE detector shows waterfall behavior and is very close to the corresponding sum capacity when the system load is just below the transition point of the waterfall behavior. Our result implies that the performance of a multiuser detector taking the data modulation scheme into consideration can be far superior to that of linear multiuser detectors. Index TermsMultiple-input multiple-output (MIMO) systems, direct-sequence code-division multiple access (DS-CDMA), asymptotic analysis, multiuser detection, replica method.K. Takeuchi is with the

show abstract

“…So the codewords can not be selected as a function of the state of the channel but the decoding is able to use such information. The capacity region of time varying fading MAC is known (Gallager [9], Shamai and Wyner [10]) and given by:…”

Section: Capacity Region Of Lds Macmentioning

confidence: 99%