Madhavan Srinivasan Vajapeyam

A survey on 3GPP heterogeneous networks

As the spectral efficiency of a point-to-point link in cellular networks approaches its theoretical limits, with the forecasted explosion of data traffic, there is a need for an increase in the node density to further improve network capacity. However, in already dense deployments in todays networks, cell splitting gains can be severely limited by high inter-cell interference. Moreover, high capital expenditure cost associated with high power macro nodes further limits viability of such an approach. This article discusses the need for an alternative strategy, where low power nodes are overlaid within a macro network, creating what is referred to as a heterogeneous network. We survey current state of the art in heterogeneous deployments and focus on 3GPP LTE air interface to describe future trends. A high-level overview of the 3GPP LTE air interface, network nodes, and spectrum allocation options is provided, along with the enabling mechanisms for heterogeneous deployments. Interference management techniques that are critical for LTE heterogeneous deployments are discussed in greater detail. Cell range expansion, enabled through cell biasing and adaptive resource partitioning, is seen as an effective method to balance the load among the nodes in the network and improve overall trunking efficiency. An interference cancellation receiver plays a crucial role in ensuring acquisition of weak cells and reliability of control and data reception in the presence of legacy signals.

An overview of 3GPP enhancements on machine to machine communications

The broad connection of devices to the Internet, known as the IoT or M2M, requires lowcost power-efficient global connectivity services. New physical layer solutions, MAC procedures, and network architectures are needed to evolve the current LTE cellular systems to meet the demands of IoT services. Several steps have been taken under the 3GPP to accomplish these objectives and are included in the upcoming 3GPP LTE standards release (3GPP Release 13). In this tutorial article, we present an overview of several features included in 3GPP to accommodate the needs of M2M communications, including changes in the physical layer such as enhanced machine type communications, and new MAC and higher-layer procedures provided by extended discontinuous reception. We also briefly discuss the narrowband IoT, which is in the development stage with a target completion date of June 2016.

Distributed Space-Time Cooperative Schemes for Underwater Acoustic Communications

In resource limited, large scale underwater sensor networks, cooperative communication over multiple hops offers opportunities to save power. Intermediate nodes between source and destination act as cooperative relays. Herein, protocols coupled with space-time block code (STBC) strategies are proposed and analyzed for distributed cooperative communication. Amplify-and-forward type protocols are considered, in which intermediate relays do not attempt to decode the information. The Alamouti-based cooperative scheme for flat-fading channels is modified in order work in the presence of multipath, which is a main characteristic of underwater acoustic channels. A time-reversal distributed space-time block code (TR-DSTBC) is employed, which extends the classical TR-STBC approach to a cooperative communication scenario. We show that, just like in the multi-antenna STBC case, TR along with the orthogonality of the DSTBC essentially allows for decoupling of the vector ISI detection problem into separate scalar problems, and therefore significant complexity reduction.

Performance analysis of distributed space-time coded protocols for wireless multi-hop communications

In resource limited, large scale sensor networks, cooperative communication over multiple hops offers opportunities to save power: intermediate nodes between source and destination act as cooperative relays. In order to exploit spatial diversity, protocols coupled with space-time coding strategies are proposed herein and analyzed for distributed cooperative communication. In contrast to prior work, multi-hop (versus two-hop) schemes are developed and analyzed for amplify-and forward type of communication protocols. First, the Alamouti based two-hop scheme proposed by Hua et al and analyzed by Jing & Hassibi is generalized to an arbitrary number of hops L, and a general approximation for the pairwise error probability (PEP) at high SNR is obtained. This expression is used to provide a close approximation to the achievable diversity gain of the scheme. It is further shown that the diversity decreases with L, for large, but finite signal-to-noise ratio (SNR). This motivates the subsequent development of new distributed multihop protocols to mitigate the diversity losses and, hence, yield improved performance. This work presents two such strategies as well as their diversity characterization, which are analyzed for the specific case of L = 3 hops and shown to exhibit improved performance at high SNR. These schemes are based on the structure of the rate-half codes proposed by Tarokh and the square-matrix embeddable codes of Tirkkonen & Hottinen.

Distance spectrum of space-time block codes: a union bound point of view

Space-time block codes (STBC) have attracted great interest due to their simple structure and ability to exploit the diversity possible in a multiple input multiple output (MIMO) system. A currently popular approach to STBC code design is to find code sets which optimize the worst pairwise error probability (PEP). The worst case PEP is a good performance criterion, but does not guarantee the best overall probability of decoding error. Several works have emphasized the consideration of the overall distance spectrum of the code set; however designing code sets which achieve the optimal symbol error rate (SER) has proven challenging due to the computational complexity associated with determining the exact SER. This paper proposes a single performance index based on union bound analysis on SER at high signal-to-noise ratio (SNR). This union bound approach is extended to the low SNR scenario, where the exact PEP is used in the union bound calculation. Search results are reported and properties of the resultant code sets analyzed.

Tools for Performance Analysis and Design of Space–Time Block Codes

Space-time block codes (STBCs) have attracted recent interest due to their ability to take advantage of both space and time diversity to reliably transmit data over a wireless fading channel. In many cases, their design is based on asymptotically tight performance criteria, such as the worst-case pairwise error probability (PEP) or the union bound. However, these quantities fail to give an accurate performance picture, especially at low signal-to-noise ratio, because the classical union bound is known to be loose in this case. This paper develops tighter performance criteria for STBCs which yield considerably better bounds. First, the union bound is developed as the average of the exact PEPs. By noting that some of the terms in the bound are redundant, a second bound is obtained by expurgation. Since this still yields a loose bound, a tighter bound, denoted as the progressive union bound (PUB), is obtained. Because the PUB cannot be computed in closed form, in its most general case, and to avoid computing a high-dimensional numerical integration, its saddlepoint approximation is developed. In addition to the significant improvement of the PUB analysis over other bounding methods, it is also shown that codes designed to optimize the PUB can perform better than those obtained by the looser criteria

Union bound of space-time block codes and decomposable error patterns

This paper proposes a single performance index based on union bound analysis on SER at high signal-to-noise ratio (SNR) for space-time modulation design. This union bound approach is extended to the low SNR scenario, where the union bound is expurgated via the use of indecomposable error matrices. Properties of the resultant code sets are summarized.

Performance of VoLTE and data traffic in LTE heterogeneous networks

VoLTE (Voice over LTE) is expected to be deployed in the near future to provide voice services in LTE networks. There is also an increasing need for much higher data capacity as a result of smartphone proliferation. Therefore, it is important to investigate and optimize the LTE networks in the presence of voice and data traffic. In this paper, we evaluate the performance of LTE Downlink when both VoLTE and data users are present. Both Macro only (homogenous) and HetNet (heterogeneous with Macro and small cells) scenarios are considered. It is shown that HetNet with Pico cell range expansion significantly improves both the system data performance and VoLTE capacity.

Low SNR design of space-time block codes based on union bound and indecomposable error patterns

Space-time block codes have attracted recent interest due to their ability to take advantage of available diversity to transmit data over a wireless fading channel. Since they were first introduced, several strategies have been proposed for their design. In most cases, these designs were optimized for high SNR through a worst case pairwise error probability analysis and either placed strict constraints on the structure of codewords or were limited to codesets of low cardinality. This work addresses the issue of code design specifically for low SNR environments using union bound-based performance criteria which are SNR dependent. A more accurate union bound-based measure for performance evaluation of codes is developed and a variation of a locally optimum algorithm used to systematically construct good codeword sets is presented. New found codes are compared via simulation with other existing codes and are shown to exhibit improved performance.

CTH15-3: Performance of Distributed Space-Time Cooperative Schemes for Underwater Acoustic Communications

In resource limited, large scale underwater sensor networks, cooperative communication over multiple hops offers opportunities to save power when intermediate nodes between source and destination act as cooperative relays. Herein, protocols coupled with space-time block code (STBC) strategies are analyzed for distributed cooperative communication in underwater channels. Amplify-and-forward type protocols are considered, in which the relays do not attempt to decode the information. The Alamouti-based cooperative scheme proposed by Hua et al (2003) for flat-fading channels is modified in order work in the presence of multipath. A time-reversal distributed space-time block code (TR-DSTBC) is employed, which extends the classical TR-STBC approach from Lindskog and Paulraj (2000) to a cooperative communication scenario. Furthermore, the performance of the scheme employing a DFE equalizer at the destination is analytically investigated in terms of bit error rate (BER) bounds and achievable spatial diversity.