Antenna Combining for the MIMO Downlink Channel

A multiple antenna downlink channel where limited channel feedback is available to the transmitter is considered. In a vector downlink channel (single antenna at each receiver), the transmit antenna array can be used to transmit separate data streams to multiple receivers only if the transmitter has very accurate channel knowledge, i.e., if there is high-rate channel feedback from each receiver. In this work it is shown that channel feedback requirements can be significantly reduced if each receiver has a small number of antennas and appropriately combines its antenna outputs. A combining method that minimizes channel quantization error at each receiver, and thereby minimizes multi-user interference, is proposed and analyzed. This technique is shown to outperform traditional techniques such as maximum-ratio combining because minimization of interference power is more critical than maximization of signal power in the multiple antenna downlink. Analysis is provided to quantify the feedback savings, and the technique is seen to work well with user selection and is also robust to receiver estimation error.Comment: Submitted to IEEE Trans. Wireless Communications April 2007. Revised August 200

Process Matters: Specialization in Federal Appellate Review of Noncapital Section 2254 Cases

This Note assesses the need for specialized review in the federal circuit courts of noncapital habeas cases brought by state prisoners under 28 U.S.C. § 2254. It first argues that the complexity of federal habeas law, the substantial disuniformity between circuits, the conflicting visions proffered by the Warren Court’s habeas jurisprudence and Congress’s recent statutory enactments—together with the greatest stakes possible at issue, liberty—are all factors warranting the creation of a national court of appeals that would hear only habeas cases. Recognizing, however, that creating such a court is a low priority for Congress at best and simply unfeasible at worst, this Note also makes another recommendation for injecting specialized review into appellate adjudication. Specifically, the circuit courts’ use of line staff attorneys to screen petitions can be much improved by creating a career staff attorney position dedicated solely to review of noncapital § 2254 cases. A formal position will attract better candidates, have lower rates of turnover, and concentrate experience and expertise to the benefit of judges and litigants

Exploring demand for forestry in Lake Victoria Basin (Western Kenya): An econometric approach

This paper determines the demand for a forestry program amongst rural households in western Kenya. It is based on a field survey with 277 households, using a stratified random sampling approach. The study follows attribute based method to elicit farmers’ preferences. Demand is measured in terms of additional number of trees that a household is willing to plant under different price schedules, including direct economic incentive to plant new seedlings. The mean willingness to plant new trees per household increases from 44 trees when farmers have to pay 10ksh/seedling, to 244 trees when farmers receive a payment of 10ksh/seedling. The paper uses fixed effects, random effects and random effect tobit models to estimate relevant parameters. Hausman specification test is returned insignificant, while implies that random effects specification is not incorrect. Price of seedlings (negative effect), availability of timber species (positive effect), gender of the respondent (men likely to plant more trees than women), and availability of agricultural labor at the household (positive) were all found to be significant. Increase in price of a seedling by 1Ksh reduced demand by nine seedlings, while addition of an adult who works full-time on the family farm will raise the demand for seedlings by 18. Furthermore, farmers in Yala River basin were likely to plant more trees than those in the Nyando River basin.Kenya, Lake Victoria, demand, tree seedlings, attribute based method, Demand and Price Analysis, Resource /Energy Economics and Policy, C23, Q23, Q57,

Limited Feedback-based Block Diagonalization for the MIMO Broadcast Channel

Block diagonalization is a linear precoding technique for the multiple antenna broadcast (downlink) channel that involves transmission of multiple data streams to each receiver such that no multi-user interference is experienced at any of the receivers. This low-complexity scheme operates only a few dB away from capacity but requires very accurate channel knowledge at the transmitter. We consider a limited feedback system where each receiver knows its channel perfectly, but the transmitter is only provided with a finite number of channel feedback bits from each receiver. Using a random quantization argument, we quantify the throughput loss due to imperfect channel knowledge as a function of the feedback level. The quality of channel knowledge must improve proportional to the SNR in order to prevent interference-limitations, and we show that scaling the number of feedback bits linearly with the system SNR is sufficient to maintain a bounded rate loss. Finally, we compare our quantization strategy to an analog feedback scheme and show the superiority of quantized feedback.Comment: 20 pages, 4 figures, submitted to IEEE JSAC November 200

Multi-User Diversity vs. Accurate Channel State Information in MIMO Downlink Channels

In a multiple transmit antenna, single antenna per receiver downlink channel with limited channel state feedback, we consider the following question: given a constraint on the total system-wide feedback load, is it preferable to get low-rate/coarse channel feedback from a large number of receivers or high-rate/high-quality feedback from a smaller number of receivers? Acquiring feedback from many receivers allows multi-user diversity to be exploited, while high-rate feedback allows for very precise selection of beamforming directions. We show that there is a strong preference for obtaining high-quality feedback, and that obtaining near-perfect channel information from as many receivers as possible provides a significantly larger sum rate than collecting a few feedback bits from a large number of users.Comment: Submitted to IEEE Transactions on Communications, July 200