Showing papers by "Thomas L. Marzetta published in 2006"

How Much Training is Required for Multiuser Mimo

Abstract: An M-element antenna array (the base station) transmits, on the downlink, K les M sequences of QAM symbols selectively and simultaneously to K autonomous single-antenna terminals through a linear pre-coder that is the pseudo-inverse of an estimate of the forward channel matrix. We assume time-division duplex (TDD) operation, so the base station derives its channel estimate from taurp pilot symbols which the terminals transmit on the reverse link. A coherence interval of T symbols is expended as follows: taurp reverse pilot symbols, one symbol for computations, and (T-l-taurp) forward QAM symbols for each terminal For a given coherence interval, number of base station antennas, and forward- and reverse-SINR's we determine the optimum number of terminals to serve simultaneously and the optimum number of reverse pilot symbols to employ by choosing these parameters to maximize a lower bound on the net sum-throughput. The lower bound rigorously accounts for channel estimation error, and is valid for all SINR's. Surprisingly it is always advantageous to increase the number of base station antennas, even when the reverse SINR is low and the channel estimate poor: greater numbers of antennas enable us to climb out of the noise and to serve more terminals. Even within short coherence intervals (T= 10 symbols) and with low SINR's (-10.0 dB reverse, 0.0 dB forward) given large numbers of base station antennas (M ges 16 ) it is both feasible and advantageous to learn the channel and to serve a multiplicity of terminals simultaneously as well.

Fast transfer of channel state information in wireless systems

Abstract: Knowledge of accurate and timely channel state information (CSI) at the transmitter is becoming increasingly important in wireless communication systems. While it is often assumed that the receiver (whether base station or mobile) needs to know the channel for accurate power control, scheduling, and data demodulation, it is now known that the transmitter (especially the base station) can also benefit greatly from this information. For example, recent results in multiantenna multiuser systems show that large throughput gains are possible when the base station uses multiple antennas and a known channel to transmit distinct messages simultaneously and selectively to many single-antenna users. In time-division duplex systems, where the base station and mobiles share the same frequency band for transmission, the base station can exploit reciprocity to obtain the forward channel from pilots received over the reverse channel. Frequency-division duplex systems are more difficult because the base station transmits and receives on different frequencies and therefore cannot use the received pilot to infer anything about the multiantenna transmit channel. Nevertheless, we show that the time occupied in frequency-duplex CSI transfer is generally less than one might expect and falls as the number of antennas increases. Thus, although the total amount of channel information increases with the number of antennas at the base station, the burden of learning this information at the base station paradoxically decreases. Thus, the advantages of having more antennas at the base station extend from having network gains to learning the channel information. We quantify our gains using linear analog modulation which avoids digitizing and coding the CSI and therefore can convey information very rapidly and can be readily analyzed. The old paradigm that it is not worth the effort to learn channel information at the transmitter should be revisited since the effort decreases and the gain increases with the number of antennas.

Capacity of Differential Versus Nondifferential Unitary Space–Time Modulation for MIMO Channels

Abstract: Differential unitary space-time modulation (DUSTM) and its earlier nondifferential counterpart, USTM, permit high-throughput multiple-input multiple-output (MIMO) communication entirely without the possession of channel state information by either the transmitter or the receiver. For an isotropically random unitary input we obtain the exact closed-form expression for the probability density of the DUSTM received signal, permitting the straightforward Monte Carlo evaluation of its mutual information. We compare the performance of DUSTM and USTM through both numerical computations of mutual information and through the analysis of low- and high-signal-to-noise ratio (SNR) asymptotic expressions. In our comparisons the symbol durations of the equivalent unitary space-time signals are equal to T. For DUSTM the number of transmit antennas is constrained by the scheme to be M=T/2, while USTM has no such constraint. If DUSTM and USTM utilize the same number of transmit antennas at high SNRs the normalized mutual information of the two schemes expressed in bits/s/Hz are asymptotically equal, with the differential scheme performing somewhat better. At low SNRs the normalized mutual information of DUSTM is asymptotically twice the normalized mutual information of USTM. If, instead, USTM utilizes the optimum number of transmit antennas then USTM can outperform DUSTM at sufficiently low SNRs

Channel-adaptive waveform modulation

Abstract: A method for transmitting a sequence of data blocks of equal length includes obtaining part of a matrix for the impulse response function of a communication channel between a transmitter and a receiver. The part relating to channel-induced interference between sampling intervals of adjacent ones of the data blocks. The method includes designing a set of one or more linearly independent waveforms based on the obtained part of the matrix for the impulse response function and transmitting a sequence of the data blocks over the channel from the transmitter to the receiver. Each data block of the sequence is a weighted linear superposition of the one or more waveforms of the designed set.

Synthetic aperture for locating mobile transmitters

Abstract: The present invention provides a module for locating a mobile device. In one embodiment, the module is configured to determine a location of a mobile device based on phase-sensitive measurements of wireless signals transmitted by the mobile device. Correspondingly, the module is configured to determine the location based on the phase-sensitive measurements of the wireless signals made at multiple measurement sites.