Browsing by Subject "Code division multiple access"

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CDMA ad hoc networks: design and performance tradeoffs
(2005) Yang, Xiangying; De Veciana, Gustavo
This dissertation proposes new principles for designing and performance evaluation for spread spectrum based ad hoc networks. We ﬁrst highlight the advantages of spread spec trum, in the form of Code Division Multiple Access (CDMA), in handling quality of ser vice (QoS) requirements, enhancing energy efﬁciency, and enabling spatial multiplexing of bursty trafﬁc. Then, based on stochastic geometric models and simulation, we show the ALOHA-like random channel access and 802.11-like simple contention and handshaking based schemes are poor at achieving good capacity or efﬁcient spatial reuse, especially un der bursty and heavy load. We show that this is because the closest interferers severely penalize the performance of the network, particularly for a direct sequence CDMA (DS CDMA) system. Therefore, it is necessary to reconsider system design for spread spec trum ad hoc networks. To this end, we consider improving system performance at differ ent network layers. At the physical layer, we ﬁrst propose to use interference cancelation techniques, in particular, successive interference cancelation (SIC), at receivers to handle strong nearby interferers. Our analysis not only shows the signiﬁcant improvement on capacity from SIC but also indicates that just canceling a few nearest interferers will pro vide most of the performance gain. Therefore, SIC is particularly suitable for DS-CDMA ad hoc networks to enhance capacity, incurring only a small amount of extra complexity. In addition, at the MAC layer, we show how idealized contention resolution among ran domly distributed nodes results in clustering of successful transmitters and receivers, in turn leading to efﬁcient spatial reuse. This motivates explicitly inducing clustering among contending nodes to achieve even better spatial reuse. We propose two distributed mech anisms to realize such clustering and show substantial capacity gains over simple random access/ALOHA-like and even RTS/CTS based protocols – on the order of 100-700%. We examine under what regimes such gains can be achieved, and how clustering and contention resolution mechanisms should be optimized to do so. We further extend our MAC design for inducing clustered contention in ad hoc networks to support hop-by-hop relaying on different spatial scales. By allowing nodes to relay beyond the set of nearest neighbors using varying transmission ranges (scales), one can reduce the number of hops between a source and destination so as to meet end-to-end delay requirements. To that end we propose a multi-scale MAC clustering and power control mechanism to support transmissions with different ranges while achieving high spatial reuse. The considerations, analysis and sim ulations included in this thesis suggest that the principle of inducing spatial clustering in contention has substantial promise towards achieving high spatial reuse, QoS, and energy efﬁciency in spread spectrum ad hoc networks.
Rate-robustness tradeoffs in multicarrier wireless communications
(2006) Kim, Tae Yoon; Andrews, Jeffrey G.
Emerging wireless communication systems exploit various resources to increase their robustness and data rate. Since these resources are limited, there is a tradeoff between the need for robust communication and the desire for high throughput. The aim of this dissertation is to study and optimally balance this tradeoff for a few important cases in multicarrier communications. First, multi-code code division multiple access (CDMA) techniques tradeoff the number of supportable subscribers with the per subscriber data rate. However, the interference scales linearly with the data rate of each user since they use multiple codes. To resolve this interference problem, a novel multi-code multicarrier CDMA system is proposed, and this system clearly outperforms previous systems in terms of bit error probability and user capacity. This shows that flexible data rates can be successfully balanced with robustness in a multiuser multi-rate CDMA system by carefully choosing the data rates number of subcarriers. Second, in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), pilots are used to estimate the channel, but in addition to consuming bandwidth, they reduce the transmitted energy for data symbols under a fixed transmit power constraint. This suggests a tradeoff between the power allowed to data symbols and the accuracy of channel estimation. The optimal pilot-to-data power ratio (PDPR) for maximizing a capacity lower bound is formulated and derived for four likely pilot patterns and two different channel conditions. The optimal PDPR shows about 10%∼30% higher capacity lower bound than equal power allocation. Third, and closely related to the second contribution, adaptive M-QAM, spectral efficiency, and symbol error rate (SER) are considered since these are respectively the dominant modulation type and quality metrics in emerging standards. The effect of the system structure on the PDPR is analytically shown, and the optimal PDPR for minimizing the average SER and maximizing the spectral efficiency is derived for two well-known linear receivers; zero-forcing and minimum meansquare error. The distributions of the SNR (including channel estimation error) for these receivers are derived and used to find the optimal PDPR. Exact guidelines are provided for the power allocation between data and pilot symbols for these cases.