R-3 Repository :: Browsing by Author "Rajan, Dinesh"

Browsing by Author "Rajan, Dinesh"

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Delay and Rate Constrained Transmission Policies over Wireless Channels
(2001-11-20) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we study delay and rate-constrained transmission of bursty traffic over wireless channels. We characterize the minimum power requirements via bounds for both single user and multiuser downlink problems, using a class of randomized first-come first-serve policies. We show that larger tolerable delay leads to power reduction, even for single-user Gaussian channels.
Delay bounded packet scheduling of bursty traffic over wireless channels
(2004-01-01) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we study minimal power transmission of bursty sources over wireless channels with constraints on mean queuing delay. The power minimizing schedulers adapt power and rate of transmission based on the queue and channel state. We show that packet scheduling based on queue state can be used to trade queuing delay with transmission power, even on additive white Gaussian noise channels. Our extensive simulations show that small increases in average delay can lead to substantial savings in transmission power, thereby providing another avenue for mobile devices to save on battery power. We propose a low-complexity scheduler that has near optimal performance. We also construct a variable rate QAM based transmission scheme to show the benefits of the proposed formulation in a practical communication system. Power optimal schedulers with absolute packet delay constraints are also studied and their performance is evaluated via simulations.
Dual Problems in Power Control
(2001-10-20) Sabharwal, Ashutosh; Rajan, Dinesh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we study power control to minimize packet loss for delay-bounded transmission of bursty sources over Gaussian channels and transmission of constant rate sources over time-varying block fading channels. In the above two problems, we consider the effect of queuing delay on the achievable packet loss rates. First, we show that small additional delay helps substantially reduce the packet loss probability in fading channels. Next, we show that for transmission of bursty sources over Gaussian channels, a small additional queuing delay also leads to reduction in packet loss probability. Finally, the duality between the two solutions is highlighted for a specific delay case, suggesting that the duality holds for all delays. A special case of our results is the traditional channel based power control, which typically assumes that each packet is served as soon as it arrives without any queuing delay.
Impact of Multiple Access on Uplink scheduling
(2001-09-20) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we consider uplink scheduling for bursty traffic. We characterize the achievable rate region for Gaussian multiple access in terms of minimum required powers, with a constraint on average transmission delay for all users. We show that delay and rate constrained, power minimizing schemes perform scheduling accompanied with power control. Further, for the class of randomized stationary schedulers, it is shown that the achievable region is a convex polytope. We highlight that power requirements of a user can be reduced by either allowing additional delay (time scheduling gain) or increasing the power of another user (multiuser power exchange). Results are presented for two user additive white caussian noise channel and can be extended to finite state fading channels.
New Estimation Techniques for a class of chaotic signals
(2000-06-20) Rajan, Dinesh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a new technique for estimation of chaotic signals in the presence of additive noise. The new method uses statistical measures to restrict the space over which the signals are received. The proposed technique is applicable to a large variety of chaotic signals and has good performance indicated by low estimation error bias and variance. The complexity of the algorithm is shown below.
New Spread Spectrum Techniques for Multiple Antenna Transmit Diversity
(1999-12-20) Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a new scheme for multiple antenna transmission in the context of spread spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple atennas. We show that this new scheme greatly improves the throughout over crrently known multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed feedback rate.
Outage behaviour with delay and CSIT
(2004-06-01) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we demonstrate that the packet outage probability for fading channels can be significantly reduced by exploiting queuing delay and transmitter channel information. Queuing delay gain is conceptually similar to delay diversity, but at a packet time-scale instead of symbol time-scale. First, we compute a lower bound on outage probability assuming full channel state information at the transmitter~(CSIT). We then construct simple outage minimizing transmission policies which adapt the rate and power of the transmitted signal based jointly on buffer occupancy and channel conditions. We demonstrate that the rate of decrease of outage with increasing transmitter channel information is higher for larger delays. We also address the closely coupled problem of designing a practical feedback channel which supplies the CSIT.
Power efficient broadcast scheduling with delay deadlines
(2004-10-01) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we present a framework for the design of minimal power schedulers that satisfy average packet delay bounds for multiple users in a Gaussian wireless broadcast channel. We completely characterize the achievable region in the multidimensional delay-power space, and present schedulers that achieve the boundary regions. The optimal schedulers minimize the transmission power by jointly allocating rate and power to the different users based on various buffer and channel conditions. Finally, we also present low complexity scheduler designs that have near optimal performance.
Power efficient transmission policies for multimedia traffic over wireless channels
(2002) Rajan, Dinesh; Aazhang, Behnaam
The current and future wireless systems need to support a multitude of services with a wide range of data rates and reliability requirements. The limited battery resource at a mobile terminal coupled with the hostile multipath fading channel makes the problem of providing reliable high data rate services challenging. In this thesis we develop a new framework for the design of power efficient transmission mechanisms that support multimedia traffic having different delay constraints. First, we present optimal design techniques that substantially reduce the transmission power (exceeding 60% in certain cases) for small increases in average communication delays for a single user scenario thereby providing another avenue for mobile devices to save on battery power. The power minimizing schedulers adapt the transmission rate and power based on the queue and channel state. We also construct a variable rate modulation scheme to show the benefits of the proposed formulation in a practical system. Power optimal schedulers with absolute packet delay constraints are also studied and their performance is evaluated via simulations. Second, we design power efficient schedulers that satisfy average delay bounds for multiple users in a broadcast channel. Optimal schedulers that jointly allocate rate and power to the different users based on the different buffer and channel conditions are presented in code-division multiple access (CDMA) and time-division multiple access (TDMA) regimes used commonly in current cellular standards. We show that joint scheduling gains over non-cooperative single user schedulers are largest when the users belong to different delay classes. We also show the superiority of CDMA over TDMA in terms of a larger set of achievable delays and at consistently lower powers. Near optimal low complexity schedulers are introduced in which computational complexity increases gracefully with increasing number of users. We also compute achievable power regions under average delay constraints in a multiple access channel with global and local queue state information. Finally, we develop a framework for designing power limited delay bounded transmission schemes that minimize the outage probability in fading channels with feedback. We explicitly construct schemes that quantitatively demonstrates the outage reduction with increasing delays and amounts of feedback information.
Power efficient transmission policies for multimedia traffic over wireless channels
(2002-04-20) Rajan, Dinesh; Center for Multimedia Communications (http://cmc.rice.edu/)
The current and future wireless systems need to support a multitude of services with a wide range of data rates and reliability requirements. The limited battery resource at a mobile terminal coupled with the hostile multipath fading channel makes the problem of providing reliable high data rate services challenging. In this thesis we develop a new framework for the design of power efficient transmission mechanisms that support multimedia traffic having different delay constraints. First, we present optimal design techniques that substantially reduce the transmission power (exceeding 60% in certain cases) for small increases in average communication delays for a single user scenario thereby providing another avenue for mobile devices to save on battery power. The power minimizing schedulers adapt the transmission rate and power based on the queue and channel state. We also construct a variable rate modulation scheme to show the benefits of the proposed formulation in a practical system. Power optimal schedulers with absolute packet delay constraints are also studied and their performance is evaluated via simulations. Second, we design power efficient schedulers that satisfy average delay bounds for multiple users in a broadcast channel. Optimal schedulers that jointly allocate rate and power to the different users based on the different buffer and channel conditions are presented in code-division multiple access (CDMA) and time-division multiple access (TDMA) regimes used commonly in current cellular standards. We show that joint scheduling gains over non-cooperative single user schedulers are largest when the users belong to different delay classes. We also show the superiority of CDMA over TDMA in terms of a larger set of achievable delays and at consistently lower powers. Near optimal low complexity schedulers are introduced in which computational complexity increases gracefully with increasing number of users. We also compute achievable power regions under average delay constraints in a multiple access channel with global and local queue state information. Finally, we develop a framework for designing power limited delay bounded transmission schemes that minimize the outage probability in fading channels with feedback. We explicitly construct schemes that quantitatively demonstrates the outage reduction with increasing delays and amounts of feedback information.
Soft weighted STTD for W-CDMA
(1999-09-20) Hottinen, Ari; Wichman, Risto; Rajan, Dinesh; Center for Multimedia Communications (http://cmc.rice.edu/)
The transmit diversity concept adopted for teh FDD mode of the third generation WCDMA system in 3G standardization has an open-loop and a closed-loop mode. The open-loop mode applies a space-time block for two transmit antennas. The closed-loop mode has two sub-modes, which both utilize downloink measurements at terminal and subsequent feedback in controlling the phase and/or gain transmit weights in the transmit antennas in order to approximate matched beamforming. These concepts are summarized in this paper for two transmit antennas and they are compared to a soft-weighted transmit diversity concept, proposed here, in which the relative transmit powers of the space-time encoded signals are controlled by feedback signaling. This leasds to a robust feeback mode in which the terminal is simplified in the sense that it does not require dedicated reference/pilot symbols, or weight verification, as opposed to the current feedback modes.
Spreading and Power Allocation for Multiple Antenna Transmission using Decorrelating receivers
(2003-05-20) Rajan, Dinesh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
We propose a new scheme for multiple antenna transmission in the context of spread spectrum signaling. The new scheme consists of using shifted Gold sequences to modulate independent information on the multiple antennas. We show that this strategy of using Multiphase Spreading (MPS) on different antennas greatly improves the throughput over currently known spread-spectrum multiple-antenna methods. We also find the optimal power allocation strategy among multiple transmit antennas for a fixed rate of channel state information, which might be provided via a feedback link, at the transmitter. We demonstrate the differences in optimal power distribution for maximizing capacity and minimizing probability of outage. When the transmission from the two antennas uses orthogonal spreading, we find that optimizing the power does not give much gain over the equal power transmission. However, when the transmissions are not orthogonal as in the case of MPS, then allocating power to maximize throughput gives considerable gain over equal power transmission. We also consider the effect of imperfections in the feedback channel on the optimal power allocation and show that our power allocation scheme is robust to feedback errors.
Spreading and Power Control for Multiple Antenna Transmit Diversity
(1999-04-20) Rajan, Dinesh; Center for Multimedia Communications (http://cmc.rice.edu/)
This thesis proposes a new transmission scheme for multiple antenna transmission at the base station of a wireless cellular system. We develop our new scheme in a wideband CDMA framework using multiple phases of Gold codes for spreading on different antennas. We evaluate the performance of this scheme and show that it performs well for a wide range of SNR values. We illustrate a strategy for using channel coding along with the new scheme. Unlike most other schemes, we motivate the need to use feedback information from the mobile to do joint power control on the multiple antennas. Our throughput analysis reveals a surprising result; in the context of our problem setting, one bit feedback, indicating which antenna is better, does not give large gains.
Spreading and power control for multiple antenna transmit diversity
(1999) Rajan, Dinesh; Aazhang, Behnaam
This thesis proposes a new transmission scheme for multiple antenna transmission at the base station of a wireless cellular system. We develop our new scheme in a wideband CDMA framework using multiple phases of Gold codes for spreading on different antennas. We evaluate the performance of this scheme and show that it performs well for a wide range of SNR values. We illustrate a strategy for using channel coding along with the new scheme. Unlike most other schemes, we motivate the need to use feedback information from the mobile to do joint power control on the multiple antennas. Our throughput analysis reveals a surprising result; in the context of our problem setting, one bit feedback, indicating which antenna is better, does not give large gains.
Transmission Policies For Bursty Traffic Sources on Wireless Channels
(2001-03-20) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
In this paper, we present near-optimal transmission policies for bursty traffic sources on noisy wireless channels. We propose to minimize the packet outage probability subject to average power and average delay constraints over a class of deterministic policies. The proposed class of policies are a combination of a leaky-bucket variant, power control and variable rate coding. A lower bound for the achievable performance of the proposed policies is computed, and the near-optimality of the proposed methods is shown for the AWGN and Gilbert-Elliot fading channels. We show that even for a Gaussian channel, the optimal policy involves variable rate coding and power control due to traffic burstiness.
Transmit Diversity for CDMA-2000
(1999-09-20) Rajan, Dinesh; Gray, Steven D.; Center for Multimedia Communications (http://cmc.rice.edu/)
Transmit diversity offers an advantage in the forward link of the cdma2000 system by balancing the spectrum efficiency in the uplink and downlink. Three schemes for performing transmit diversity are examined in this paper - orthogonal transmit diversity, time switched transmit diversity, and selection transmit diversity for vehicular, indoor-outdoor pedestrian and indoor office environments. Also considered are som issues related to implementation complexity in the mobile handset. Under certain channel conditions, we show that low rate feedback of antenna selection and no forward link power control offers performance advantages over transmit diversity schemes with forward link power control and no antenna selection.

Browsing by Author "Rajan, Dinesh"

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