Results 1  10
of
18
CDMA Codeword Optimization: interference avoidance and convergence via class warfare
 IEEE Transactions on Information Theory
, 2001
"... Interference avoidance has been shown to reduce total square correlation (TSC) for given ensembles of user signature waveforms (codewords) in a synchronous CDMA system. In all experiments we have conducted, sequential application of interference avoidance produces an optimal codeword set when starti ..."
Abstract

Cited by 60 (17 self)
 Add to MetaCart
Interference avoidance has been shown to reduce total square correlation (TSC) for given ensembles of user signature waveforms (codewords) in a synchronous CDMA system. In all experiments we have conducted, sequential application of interference avoidance produces an optimal codeword set when starting from randomly chosen initial codewords. Here we provide the rst formal proof of convergence to optimal codeword ensembles for greedy interference avoidance algorithms augmented by a technique called \class warfare" whereby users which reside in more heavily loaded areas of the signal space purposely interfere with (attack) the reception of users in less crowded areas. Coordination of deliberate interference by a complete class of aggrieved user is also sometimes necessary. Such \attacks" and subsequent codeword adjustment by attacked users are shown to strictly decrease TSC. Along the way we also show, using linear algebra and a variant of stochastic ordering, equivalence between minimiz...
Ensuring Convergence of the MMSE Iteration for Interference Avoidance to the Global Optimum
, 2003
"... Viswanath and Anantharam [1] characterize the sum capacity of multiaccess vector channels. For a given number of users, received powers, spreading gain, and noise covariance matrix in a codedivision multipleaccess (CDMA) system, the authors of [1] present a combinatorial algorithm to generate a se ..."
Abstract

Cited by 29 (1 self)
 Add to MetaCart
(Show Context)
Viswanath and Anantharam [1] characterize the sum capacity of multiaccess vector channels. For a given number of users, received powers, spreading gain, and noise covariance matrix in a codedivision multipleaccess (CDMA) system, the authors of [1] present a combinatorial algorithm to generate a set of signature sequences that achieves the maximum sum capacity. These sets also minimize a performance measure called generalized total square correlation (TSC ).
Water Filling May Not Good Neighbors Make
 In Proceedings 2003 IEEE Global Telecommunications Conference  GLOBECOM ’03
, 2003
"... Abstract — Consider a wireless system with multiple and independent userbase pairings over some region. Each user tries to greedily optimize its performance, and eventually a simultaneous waterfilling fixed point is reached. Here we seek to analytically understand properties of different waterfil ..."
Abstract

Cited by 22 (5 self)
 Add to MetaCart
(Show Context)
Abstract — Consider a wireless system with multiple and independent userbase pairings over some region. Each user tries to greedily optimize its performance, and eventually a simultaneous waterfilling fixed point is reached. Here we seek to analytically understand properties of different waterfilling fixed points. In so doing we show that waterfilling alone does not generally result in optimum resource sharing and in some cases is a poor solution. We close by suggesting dynamic strategies for performance enhancement.
Interference Avoidance and Dispersive Channels. A New Look at Multicarrier Modulation
 In Proc. 37th Allerton Conf. on Communication, Control, and Computing
, 1999
"... The availability of software radios and application of interference avoidance may offer a new perspective on communication over dispersive channels. By both lengthening the transmission interval so that ISI is almost inconsequential and by using combinations of channel eigenfunctions to convey the b ..."
Abstract

Cited by 13 (7 self)
 Add to MetaCart
(Show Context)
The availability of software radios and application of interference avoidance may offer a new perspective on communication over dispersive channels. By both lengthening the transmission interval so that ISI is almost inconsequential and by using combinations of channel eigenfunctions to convey the bits, communications over dispersive channels becomes (not surprisingly) a multiuser detection problem where each bit is assigned its own waveform. By choosing these waveforms to meet the Welch Bound with equality – through interference avoidance or some other numerical method – a uniform maximum SINR can be achieved for each bit. In addition by using Gaussian signaling and decoding with this codeword ensemble, the channel capacity can be (theoretically) met. A trivial extension of the single user case can be used for multiple users over identical channels. Multiple users with nonidentical channels is more subtle and is discussed in the interference avoidance context with suggestions for future work. 1
Sum Capacity and Interference Avoidance: Convergence Via Class Warfare
 In CISS 2000
, 2000
"... Interference avoidance has been shown to reduce total square correlation (TSC) of user signature waveforms (codewords) in a synchronous CDMA system. From random initial codewords interference avoidance has empirically always converged to an optimal set. Here we provide the first formal proof of conv ..."
Abstract

Cited by 9 (5 self)
 Add to MetaCart
(Show Context)
Interference avoidance has been shown to reduce total square correlation (TSC) of user signature waveforms (codewords) in a synchronous CDMA system. From random initial codewords interference avoidance has empirically always converged to an optimal set. Here we provide the first formal proof of convergence to optimal codeword ensembles using a technique called "class warfare" whereby users which reside in more heavily loaded areas of the signal space "attack" users in less heavily loaded areas. Such attacks and subsequent codeword adjustment by attacked users are shown to strictly decrease TSC at all suboptimal fixed points. This coupled to a finite number of possible TSC values at fixed points guarantees convergence to an optimal set. I. INTRODUCTION Interference avoidance has been identified as a method to iteratively obtain optimal signature waveforms (codewords) in multiple access systems [15]. The notion behind interference avoidance is that each user, with feedback from the r...
Optimum signature sequence sets for asynchronous CDMA systems
 In 38th Annual Allerton Conference on Communications, Control and Computing
, 2000
"... In this paper, we characterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a xed processing gain, of a single cell symbol asynchronous CDMA system. Based on the delay pro le of the users, we identify a class of optimum signature sequences that ac ..."
Abstract

Cited by 6 (2 self)
 Add to MetaCart
(Show Context)
In this paper, we characterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a xed processing gain, of a single cell symbol asynchronous CDMA system. Based on the delay pro le of the users, we identify a class of optimum signature sequences that achieve alower bound on the total squared asynchronous correlation (TSAC) among the users. When the users ' signatures achieve this lower bound, the user capacity ofa singlecell asynchronous CDMA system becomes the same as that of a singlecell synchronous CDMA system � that is, there is no loss in user capacity due to asynchrony. Further, when the optimum signature sequences are used, the users ' received powers are all equal and the Mshot MMSE receiver lters turn out to be scaled matched lters. That is, the maximum user capacity isachieved by observing only one symbol interval of the received signal and using singleuser matched lters in that interval. It is a signi cant open question whether these optimal sequence sets exist for all delay pro les. However, we present iterative and distributed signature adaptation algorithms, which, when executed sequentially by all of the users, appear to converge to these optimum signature sequences. 1
Signature Sequence Optimization in Asynchronous CDMA Systems
 IN IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS
, 2001
"... We characterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a fixed processing gain, of a singlecell symbol asynchronous CDMA system. We show that the user capacity of an asynchronous system is the same as the user capacity of a synchronous sys ..."
Abstract

Cited by 6 (2 self)
 Add to MetaCart
We characterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a fixed processing gain, of a singlecell symbol asynchronous CDMA system. We show that the user capacity of an asynchronous system is the same as the user capacity of a synchronous system; that is there is no loss in user capacity due to asynchrony. Optimum signature sequences are those that minimize the total squared asynchronous correlation (TSAC) among the users, and depend on users' delay profile. Optimum received powers of the users are equal, and the optimum linear receiver filters in any observation window of size M 1 symbols (i.e., Mshot MMSE filters) are oneshot matched filters. We present iterative and distributed signature adaptation algorithms where, at each iteration, only one user updates its signature sequence to decrease the TSAC of the set.
Inteference avoidance and collaborative multibase systems, part ii: simplifications for flat channels
 IEEE Transactions on Wireless Communications
, 2002
"... We consider distributed iterative interference avoidance algorithms for uplink wireless systems where base stations share information (collaborate). Though the structure of the multiple base collaborative problem is significantly different from single base and single user multiple antenna systems, ..."
Abstract

Cited by 2 (2 self)
 Add to MetaCart
(Show Context)
We consider distributed iterative interference avoidance algorithms for uplink wireless systems where base stations share information (collaborate). Though the structure of the multiple base collaborative problem is significantly different from single base and single user multiple antenna systems, we show that if users are allowed to greedily optimize their own performance (SINR or rate), then maximum sum capacity solutions result. After providing interference avoidance algorithm variants, we numerically study the improvement afforded by interference avoidance over random codewords, the speed of convergence, and how closely sum capacity bounds are approached when each user is allowed exactly one signature – a currently open
Interference Avoidance, Sum Capacity and Convergence Via Class Warfare
 IEEE Transactions on Information
, 2000
"... Interference avoidance has been shown to reduce total average interference (TSC) for given ensembles of user signature waveforms (codewords) in a synchronous CDMA system. In all experiments we have conducted, sequential application of interference avoidance produces an optimal codeword set when star ..."
Abstract

Cited by 1 (0 self)
 Add to MetaCart
(Show Context)
Interference avoidance has been shown to reduce total average interference (TSC) for given ensembles of user signature waveforms (codewords) in a synchronous CDMA system. In all experiments we have conducted, sequential application of interference avoidance produces an optimal codeword set when starting from randomly chosen initial codewords. Here we provide the first formal proof of convergence to optimal codeword ensembles for greedy interference avoidance algorithms augmented by a technique called "class warfare" whereby users which reside in more heavily loaded areas of the signal space purposely interfere with the reception of users in less crowded areas. Coordination of deliberate interference by a complete class of aggrieved user is also sometimes necessary. Such "attacks" and subsequent codeword adjustment by attacked users are shown to strictly decrease TSC. Along the way we also show, using linear algebra and a variant of stochastic ordering, equivalence between minimization ...
Cooperative Game Theory and NonConvex Optimization for Distributed Spectrum Sharing
, 2007
"... Opportunistic spectrum access has become a high priority research area in the past few years. The motivation behind this actively researched area is the fact that the limited spectrum available is currently being utilized in an inefficient way. The complete wireless spectrum is assigned and reserved ..."
Abstract
 Add to MetaCart
(Show Context)
Opportunistic spectrum access has become a high priority research area in the past few years. The motivation behind this actively researched area is the fact that the limited spectrum available is currently being utilized in an inefficient way. The complete wireless spectrum is assigned and reserved, but not necessarily being used. At the same time, the demand for innovation in wireless technology is growing. Since there is no room in the wireless spectrum to allocate significant frequency bands for future wireless technologies, the only recourse is to increase utilization of the spectrum. To achieve this, we must find a way to share the spectrum. Spectrum sharing techniques will require coordination between all the layers of the protocol stack. The network and the wireless medium are inextricably linked and, thus, both must be considered when optimizing wireless network performance. Unfortunately, interactions in the wireless medium can lead to nonconvex problems which have been shown to be NPhard. Techniques must be developed to tackle the optimization problems that arise from wireless network analysis.