# Your search: "author:"Jafarkhani, Hamid""

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## Scholarly Works (35 results)

We design and analyze noncoherent detection schemes for multi-user/multi-node communication systems where neither the transmitters nor the receiver knows the channel. First, we propose differential decoding schemes for two-user MIMO systems based on orthogonal space-time block codes (OSTBCs). We derive low complexity differential decoders for users with two transmit antennas. We also present differential decoding schemes that achieve full diversity, perform significantly better than the existing schemes, and work for any square OSTBC, but need higher decoding complexity compared to our low complexity decoders. Moreover, we analyze the diversity of the proposed schemes. To the best of our knowledge, our low complexity schemes are the first low complexity differential schemes for multi-user systems.

We then propose differential decoding schemes for asynchronous multi-user MIMO systems based on OSTBCs. We derive novel low complexity differential decoders by performing interference cancelation in time. The decoding complexity of these schemes grows linearly with the number of users. We also present differential decoding schemes that perform significantly better than our low complexity decoders and outperform the existing synchronous differential schemes but require higher decoding complexity compared to our low complexity decoders. The proposed schemes work for any square OSTBC, any number of users, and any number of receive antennas. Furthermore, we analyze the diversity of the proposed schemes and derive conditions under which our schemes provide full diversity. To the best of our knowledge, the proposed differential detection schemes are the first differential schemes for asynchronous multi-user systems.

Finally, we present novel distributed beamforming (DBF) algorithms using feedback control based on Tree-Structured Vector Quantization (TSVQ). We develop TSVQ-based DBF algorithms for static channels. To the best of our knowledge, the proposed algorithms are the first deterministic DBF methods that can feed back more than 1 bit per time slot for faster phase synchronization. We analytically prove that our TSVQ-based DBF algorithms attain phase synchronization in probabilistic senses. Moreover, we modify our TSVQ-based DBF algorithms to enable them to track time-varying channels without the knowledge of the channel. Simulation results demonstrate that our algorithms significantly outperform the existing adaptive DBF algorithms for static and time-varying channels.

We study quantized beamforming in wireless amplify-and-forward relay-interference networks with any number of transmitters, relays, and receivers. We design the quantizer of the channel state information to minimize the probability that at least one receiver incorrectly decodes its desired symbol(s). Correspondingly, we introduce a generalized diversity measure that encapsulates the conventional one as the first-order diversity. Additionally, it incorporates the second-order diversity, which is concerned with the transmitter power dependent logarithmic terms that appear in the error rate expression. First, we show that, regardless of the quantizer and the amount of feedback that is used, the relay-interference network suffers a second-order diversity loss compared to interference-free networks. Then, two different quantization schemes are studied: First, using a global quantizer, we show that a simple relay selection scheme can achieve maximal diversity. Then, using the localization method, we construct both fixed-length and variable-length local (distributed) quantizers (fLQs and vLQs). Our fLQs achieve maximal first-order diversity, whereas our vLQs achieve maximal diversity. Moreover, we show that all the promised diversity and array gains can be obtained with arbitrarily low feedback rates when the transmitter powers are sufficiently large. Finally, we confirm our analytical findings through simulations.

We determine necessary conditions on the structure of symbol error rate (SER) optimal quantizers for limited feedback beamforming in wireless networks with one transmitter-receiver pair and R parallel amplify-and-forward relays. We call a quantizer codebook "small" if its cardinality is less than R, and "large" otherwise. A "d-codebook" depends on the power constraints and can be optimized accordingly, while an "i-codebook" remains fixed. It was previously shown that any i-codebook that contains the single-relay selection (SRS) codebook achieves the full-diversity order, R. We prove the following: Every full-diversity i-codebook contains the SRS codebook, and thus is necessarily large. In general, as the power constraints grow to infinity, the limit of an optimal large d-codebook contains an SRS codebook, provided that it exists. For small codebooks, the maximal diversity is equal to the codebook cardinality. Every diversity-optimal small i-codebook is an orthogonal multiple-relay selection (OMRS) codebook. Moreover, the limit of an optimal small d-codebook is an OMRS codebook. We observe that SRS is nothing but a special case of OMRS for codebooks with cardinality equal to R. As a result, we call OMRS as "the universal necessary condition" for codebook optimality. Finally, we confirm our analytical findings through simulations.

The fundamental performance limits of space-time block code (STBC) designs when perfect channel information is available at the transmitter (CSIT) are studied in this report. With CSIT, the transmitter can perform various techniques such as rate adaption, power allocation, or beamforming. Previously, the exploration of these fundamental results assumed long-term constraints, for example, channel codes can have infinite decoding delay, and power or rate is normalized over infinite channel-uses. With long-term constraints, the transmitter can operate at the rate lower than the instantaneous mutual information and error-free transmission can be supported. In this report, we focus on the performance limits of short-term behavior for STBC systems. We assume that the system has block power constraint, block rate constraint, and finite decoding delay. With these constraints, although the transmitter can perform rate adaption, power control, or beamforming, we show that decoding-error is unavoidable. In the high SNR regime, the diversity gain is upperbounded by the product of the number of transmit antennas, receive antennas, and independent fading block channels that messages spread over. In other words, fading cannot be completely combatted with short-term constraints. The proof is based on a sphere-packing argument.

In this paper, we propose a novel class of Space Frequency and Space-Time-Frequency block codes based on Quasi-Orthogonal designs, over a frequency selective Rayleigh fading channel. The proposed Space-Frequency code is able to achieve rate-one and full space and multipath diversity gains available in the MIMO-OFDM channel. As simulation results demonstrate, the code outperforms the existing Space-Frequency block codes in terms of bit error rate performance. By coding across the three dimension of space, time and frequency, we propose a Quasi-Orthogonal Space-Time-Frequency code that is capable of achieving rate-one and exploiting all of the spatial, multipath and temporal diversity gains offered by the channel. In case of a channel which is quasi-static over adjacent OFDM symbol durations, we propose a Space-Time-Frequency code that benefits from a reduced maximum likelihood decoding complexity.

In this dissertation, the potential of limited feedback in multiuser/multinode networks is explored, and our goal is to design efficient and practical quantizers to mitigate the perfor- mance loss brought by limited feedback. For the multiple amplify-and-forward relay network, we propose variable-length quantizers (VLQs) with random infinite-cardinality codebooks in contrast to the fixed-length quantizers (FLQs) with finite-cardinality codebooks that cannot attain the full-channel-state-information (full-CSI) performance. We validate through both theoretical proofs and numerical simulations that the proposed VLQs can achieve the full- CSI outage probabilities with finite average feedback rates. We also apply the idea of VLQ to the multicast network, and show that the global VLQ can achieve the minimum full-CSI outage probability with a low average feedback rate. For the two-user interference network where interferences are treated as noise, we introduce the idea of cooperative quantization to allow multiple rounds of feedback communication in the form of conferencing between receivers. For both time-sharing and concurrent transmission strategies, the proposed co- operative quantizers are able to achieve the full-CSI network outage probability of sum-rate and the full-CSI network outage probability of minimum rate, respectively, with only finite average feedback rates. For non-orthogonal multiple access (NOMA) which is recognized as a key technique for 5G, we propose efficient quantizers using variable-length encoding, and prove that in the typical application with two receivers, the losses in the minimum rate and outage probability decay at least exponentially with the minimum feedback rate. In addition, a sufficient condition for the quantizers to achieve the maximum diversity order is provided. For NOMA with K receivers where K>2, the minimum rate maximization problem is solved within an accuracy of ε in time complexity of O(K*log(1/ε)).

We consider progressive transmission over a hybrid channel introducing bit errors and packet erasures. The existing solutions are analyzed and extended to the case of a channel that exhibits memory on both bit errors and packet erasures. We then propose a simple, low-complexity coding scheme that transforms the hybrid channel into a channel with a single impairment for which various optimization techniques exist. Both rate-based and distortion-based optimization problems are investigated. It is shown that our proposed solution has lower channel coding and rate-distortion optimization complexities compared to the known solutions. Simulation results for channels with and without memory show the effectiveness of our proposed solution over a wide range of operating conditions. Numerical results also indicate that the rate-based solution of our proposed algorithm is very close to the corresponding distortion-based solution.

When two users transmit signals to a common receiver, one can design precoders to cancel the interference for each user, if each user knows all the channel information perfectly. Also the diversity for each user is full. However, in practice, perfect channel information is not available. In this paper, we design precoders for two users with two transmit antennas and one receiver with two receive antennas using quantized feedback. We propose to construct codebook using Grassmannian line packing. By choosing precoders from the codebook properly, our proposed scheme can cancel the interference for each user. Also we analytically prove that our system can achieve full diversity for each user. Then we extend our scheme to any number of transmit and receive antennas. Simulation results confirm our analytical proof and show that our scheme can serve as a bridge between a system with no feedback and a system with perfect feedback.