Heterogeneous broadcast channel: Spatial diversity or advanced receiver design

High speed downlink packet access (HSDPA) is specified in the 3GPP to cope with the increasing demand of high-speed Internet-based multimedia services. HSDPA is based on techniques such as base station scheduling, adaptive modulation and coding, hybrid automatic repeat request, and advanced receiver design. This work presents system-level simulation results on the HSDPA performance to reveal the effects of user equipment (UE) capabilities such as the maximum number of multicodes and receive diversity. The results show that a 15- and 10-multi-code UE and the receive diversity significantly improve the cell capacity. Experiments using the HSDPA test-beds are conducted to investigate the true HSDPA performance. Furthermore, TCP/IP-level simulation results show the application-level throughput, i.e. end-user throughput, in the HSDPA system. The results indicate that reduction of the round trip time, a higher uplink data rate, and low uplink block error rate are required to achieve the high TCP-level throughput.

We study a practical version of the broadcast channel (BC) where the channel state information at the transmitter (CSIT) is available only for some of the users, dubbing the system as mixed CSIT. We start with the simplest instance of such a heterogeneous BC where a multi-antenna transmitter base-station (BS) is trying to communicate data to two single-antenna user equipments (UE), having the perfect CSIT about UE1 and no CSIT about UE2. We propose a very simple transmission strategy at the BS for such a system and compare its performance with the interference alignment solution proposed elsewhere which achieves full degrees-of-freedom (DOF) when channel coherence lengths and switching instants abide by certain restrictions. Then with an additional receive antenna granted to UE2 (whose CSIT is not available) and keeping the same CSIT perspective, achievable rates for two users are derived and compared with earlier schemes. Analytical and simulation results show that this additional receive antenna brings significant gains in the system with the same transmission strategy and without any extra requirement of channel training or feedback.

E/W-band millimeter-wave signals are highly promising for long-distance offshore wireless communications. However, the high humidity over the sea surface, together with the continuous fluctuation of sea waves, gives rise to severe near-sea-surface channel impairments, such as strong atmospheric absorption and sea-surface-induced multipath, which significantly hampers long-range E-band transmission. This work proposes a photonic-assisted E-band millimeter-wave 1 × 2 MIMO communication system and conducts a 26 km near-sea-surface transmission experiment in the coastal area of Lianyungang, Jiangsu Province. A 73.5 GHz 5-Gbaud QPSK signal is transmitted, and spatial diversity reception followed by maximal ratio combining (MRC) is applied. Experimental results show that diversity reception improves system performance by about 4 dB, demonstrating that the proposed photonic-assisted E-band spatial diversity system and signal processing method can significantly extend the transmission distance.

Orthogonal frequency division multiplexing (OFDM) technology is presented for use in free-space optical (FSO) communications accompanied by the spatial diversity reception. Using quadrature phase shift keying (QPSK) modulation formats, the OFDM signals show robustness to support high spectral efficiency and compatibility with the spatial diversity reception to improve receiver sensitivity. Compared with the single-carrier QPSK signal, the OFDM-QPSK signal with 64 sub-carriers can reduce the BER from 2.87 × 10−3 to 2.98 × 10−4 at the SNR of 6 dB. Using a two-aperture spatial diversity reception with OFDM, the BER can be reduced from 2.45 × 10−3 of a single receiver to 6.10 × 10−4 under moderate turbulence conditions. Under strong turbulence, the BER of the single receiver is 2.14 × 10−2. It can be improved to 1.16 × 10−3 by using four-aperture receivers, and even 6.87 × 10−4 by using six-aperture receivers. The optimized aperture number should be selected according to channel conditions.

The article focuses on the design, measurement, and simulation of a new Long Term Evolution (LTE) downlink mode in which the polarization diversity replaces the spatial diversity at the user equipment (UE). The proposed system enables the analysis of the circularly polarized (CP) antenna over the multipath SISO channel. The purpose of the article is to design a compact LTE SISO system capable of providing comparable performance to the one with spatial diversity (SIMO 1 × 2). In this respect, the entire process is described from antenna designing and measurements to the final stage of integration and simulation. The results indicate the necessity of taking into account the effect of the polarization diversity to keep up the data rate for the small LTE devices when spatial diversity is not an option. The new system proves valuable to evolve toward new downlink modes suitable especially for tiny devices yet capable of achieving high data rates. The article envisages some new processing stages at the UEs as well as at the base station (eNodeB) for CP usage. © 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 26:547–558, 2016.

Optical wireless links in the solar-blind ultraviolet (SB-UV) band transmit data without the need of a line-of-sight through the use of atmospheric scattering. These links have a host of advantages including security, covertness and ease of deployment, however, they inherently suffer large path loss and delay spread due to the underlying atmospheric scattering process. This paper considers the use of spatial degrees of freedom at the receiver to improve the information rates of SB-UV channels. Classical scattering link models are extended to provide both temporal and spatial arrival information for scattered photons. A spatio-temporal channel model is developed for SB-UV channels using multi-element imaging receivers. Spatial diversity techniques for delay compensation and detector noise suppression are presented and information rates under equally likely signalling are computed to contrast their performance. In the particular cases considered, the information rate using a spatial diversity receiver is improved approximately 50% over single element receiver channels.

Heterogeneous Networks (HetNets) have gained the attraction of the communication industry recently, due to their promising ability to enhance the performance of future broadband Fifth Generation (5G) networks and are integral parts of 5G systems. They can be viewed in multi-dimensional space where, each slice represents a unique tier that has its own Base Station (BS)s and User Equipment (UE)s. Different tiers cooperate with each other for their mutual benefit. Data can be interactively exchanged among the tiers, and UEs have the flexibility to switch between the tiers. The cells in such a heterogeneous cellular networks have variable sizes, shapes, and coverage regions. However, in HetNets with ultra dense BSs, the distance between them gets very small and, they suffer from very high levels of mutual interference. To improve the performance of HetNets, we have done multiple contributions in this dissertation. First, we have developed analytical derivations for optimizing pilot sequence length which is a very crucial factor in acquiring the Channel State Information (CSI) and the channel estimation process in general. Poisson Point Process (PPP) has been widely used to allocate BSs among various tiers so far. However, BS locations obtained using PPP approach may not be optimum to reduce interference. Therefore, in this dissertation, BSs locations are optimized to reduce the interference and improve the coverage and received signal power. Also, we have derived expressions for static UEs coverage probability and network energy efficiency in HetNets. A proper UE association algorithm for HetNets is a great challenge. The classic max-Signal to Interference and Noise Ratio (SINR) or max-received signal strength (RSS) user association algorithms are inappropriate solutions for HetNets as UEs in this context will tend to connect to the Macro BS, which is the one with the highest signal power. A severe load imbalance and significant inefficiency arises and impacts the performance. The aforementioned algorithms tend to associate UEs to BSs with the best received signal power or signal quality. In HetNets, usually Macro BSs are the ones transmitting the strongest signals; hence most UEs tend to associate with the Macro BS leaving Micro BSs with less load. Also, the conventional max-SINR and max-RSS algorithms do not provide adequate results in multi-tier systems. We suggest two centralized algorithms, LSTD and RTLB, for an even UE association to provide fair load distribution. However RTLB outperforms LSTD in real time scenarios as it easily and quickly adapts to rapid network changes. Furthermore, we consider the mobility of nodes. We derive coverage probability for moving UEs considering both handover and no handover scenarios. Proposed algorithms are fast enough to associate the moving users to different Micro and Macro BSs appropriately in real time. Our algorithms are proved to be feasible and provide a path towards attainable future communication systems.

In this paper, an advanced receiver design based on DVB-S2 standard is proposed for satellite downlink transmission over the X-band, which is capable of supporting over one Gbps high data rate transmission on a 375 MHz bandwidth without the need to tackle the rain fade effect. We illustrate its baseband design and implementation details including frame detection, timing and frequency synchronization, and phase recovery. Our fixed point based simulation results show that the proposed design is able to achieve promising performance under realistic frequency offset and phase noise setting.

This work presents an 80 Gbps photonics-aided millimeter-wave (mm Wave) wireless communication system employing 16-Quadrature Amplitude Modulation (16-QAM) and a 1 × 2 single-input multiple-output (SIMO) architecture with maximum ratio combining (MRC) to achieve robust 87.5 GHz transmission over 4.6 km. By utilizing polarization-diverse optical heterodyne generation and spatial diversity reception, the system enhances spectral efficiency while addressing the low signal-to-noise ratio (SNR) and channel distortions inherent in long-haul links. A blind equalization scheme combining the constant modulus algorithm (CMA) and decision-directed least mean squares (DD-LMS) filtering enables rapid convergence and suppresses residual inter-symbol interference, effectively mitigating polarization drift and phase noise. The experimental results demonstrate an SNR gain of approximately 3 dB and a significant bit error rate (BER) reduction with MRC compared to single-antenna reception, along with improved SNR performance in multi-antenna configurations. The synergy of photonic mm Wave generation, adaptive spatial diversity, and pilot-free digital signal processing (DSP) establishes a robust framework for high-capacity wireless fronthaul, overcoming atmospheric attenuation and dynamic impairments. This approach highlights the viability of 16-QAM in next-generation ultra-high-speed networks (6G/7G), balancing high data rates with resilient performance under channel degradation.

This paper suggests a spread-spectrum based receiver architecture that employs both spatial diversity and RAKE reception, in order to achieve reliable communications through multiple-access shallow-water acoustic channels. The key idea is to improve system performance by exploiting the multipath profile and combining, rather than canceling, the energy arriving at the receiver via bottom and surface reflections. The optimization of the receiver parameters is achieved by employing the minimum mean-square error (MMSE) criterion. Results indicate that the suggested receiver algorithm can dramatically reduce the number of receive elements required without any significant loss in performance. However, this is achieved at the cost of increased computational complexity.

Device-to-device (D2D) relay can be used to expand the service coverage of a base station (BS) and improve network throughput. In D2D relay, when a user equipment (UE) has bad channel quality, it solicits a neighbor to relay its data from the BS. In view that UEs are usually owned by self-interested users, token-based incentive methods adopt tokens as virtual currencies to let UEs sell and buy relay services, which encourages UEs to help relay data for other UEs. However, malicious UEs may keep gathering tokens from others but never spend tokens, which we call hoarders. Hence, negotiable tokens would reduce gradually, and many UEs cannot afford relay services due to lack of tokens. In this case, these token-based incentive methods will incur bad performance or even collapse. To this end, we propose an adaptive token circulation (ATC) scheme to tax the UEs that have ample tokens and subsidize the UEs in need of tokens. ATC works well in a multi-cell environment and does not cause the inflation of tokens. Simulation results reveal that the ATC scheme keeps high D2D throughput when there exist hoarders, thereby assuring the efficiency of D2D relay.

Reflecting Surfaces (RSs) are being lately envisioned as an energy efficient solution capable of enhancing the signal coverage in cases where obstacles block the direct communication from Base Stations (BSs), especially at high frequency bands due to attenuation loss increase. In the current literature, wireless communications via RSs are exclusively based on traditional coherent demodulation, which necessitates the estimation of accurate Channel State Information (CSI). However, this requirement results in an increased overhead, especially in time-varying channels, which reduces the resources that can be used for data communication. In this paper, we consider the uplink between a single-antenna user and a multi-antenna BS and present a novel RS-empowered Orthogonal Frequency Division Multiplexing (OFDM) communication system based on the differential phase shift keying, which is suitable for high noise and/or mobility scenarios. As a benchmark, analytical expressions for the Signal-to-Interference and Noise Ratio (SINR) of the proposed system are presented. Our extensive simulation results verify the accuracy of the presented analysis and showcase the performance and superiority of the proposed system over coherent demodulation.

Dynamic time-division duplexing (TDD) is considered a promising solution to deal with fast-varying traffic often found in ultra-densely deployed networks. At the same time, it generates more interference which may degrade the performance of some user equipment (UE). When base station (BS) utilization is low, some BSs may not have an UE to serve. Rather than going into sleep mode, the idle BSs can help nearby UEs using joint transmission. To deal with BS-to-BS interference, we propose using joint transmission with dummy symbols where uplink BSs serving uplink UEs participate in the precoding. Since BSs are not aware of the uplink symbols beforehand, any symbols with zero power can be transmitted instead to null the BS-to-BS interference. Numerical results show significant performance gains for uplink and downlink at low and medium utilization. By varying the number of participating uplink BSs in the precoding, we also show that it is possible to successfully trade performance in the two directions.

Base stations (BSs) are densely deployed in cellular networks to meet the increasing peak demand of mobile data traffics. However, since the distribution and traffic demand of user equipment (UE) fluctuate in time and over space, a number of BSs are underutilized in certain period of time. Therefore, switching on/off BSs dynamically according to the variations of UE distribution is regarded as a promising way to save energy in cellular networks. The existing works on BS switching do not consider the energy cost for switching on/off BSs, and they take the UE distribution and network traffic in a time period as given in advance. On the contrary, we consider the problem of minimizing energy cost of BSs with switching cost included. To solve this problem, we propose an online BS switching algorithm along with a lightweight UE distribution prediction strategy. Simulations are carried out in an example network to show that our online scheme outperforms the existing methods and its performance is close to the offline algorithm, which relies on the accurate network information in the whole time period.

This paper analyzes the problem of optimum user association and sum rate maximization for software defined radio access networks (SD-RANs) with access node diversity for fifth generation (5G) wireless networks. We consider four complementary types of access nodes namely, a massive multiple- input multiple-output (MIMO) base station (BS), MIMO BSs, small cells (SCs), and indoor and outdoor distributed antenna systems (DAS). The SDRAN user association problem is solved through a novel three-stage optimization scheme called the place coverage (TPC). TPC divides the user equipment (UEs) into two sets of indoor and outdoor UEs. Initially, TPC associates indoor UEs with the indoor DAS access nodes. Next, outdoor UEs are associated with the outdoor DAS access nodes, SCs, MIMO BSs, and the massive MIMO BS. Finally, the remaining resources of indoor and outdoor access nodes are used to serve the UEs that have not been served. The scope of TPC is multifold. First, TPC reflects the closest model to a real-world diverse 5G network. Secondly, servicing the indoor UEs with iDAS antennas results in lower power load per user for the access network resulting in better coverage, quality, and data speed. Thirdly, TPC provides reduced radiation levels for indoor UEs. Numerical results show that TPC provides significant sum rate and fairness gains over the received signals strength (RSS) based association schemes [1]. Moreover, TPC provides comparable performance levels as the optimum association scheme based on exhaustive search while bringing significant complexity reduction.