Cognitive subcarrier sharing schemes for cooperative D2D communication frameworks

Abstract

Past few decades have seen a phenomenal growth in wireless multimedia and data applications. Recent forecast (via Cisco) suggested that the overall mobile data traffic could reach 24.3 exabytes per month by 2019. As a consequence, there is a perpetual need for novel spectrum access techniques to alleviate the problem of spectrum scarcity. This dissertation investigates two solutions to enhance the capacity of the wireless/cellular networks: cooperative spectrum sharing (CSS) and device-to-device (D2D) communication.

In a conventional CSS protocol, the secondary system (a cognitive user) acts as an amplify-and-forward (AF) or decode-and-forward (DF) relay for the primary system to achieve the target quality-of-services (QoS) of the primary system in exchange for spectrum access by the secondary system. However, most of the existing CSS protocols are interference limited and performance of the system may degrade due to interference from one system to another. To mitigate the interference, we propose orthogonal frequency division multiplexing (OFDM) based opportunistic spectrum sharing (OSS) for cooperative cognitive radios. According to the scheme, secondary system helps the primary system via two phase DF relaying in exchange of OSS. Both primary and secondary systems employ OFDM modulator and demodulator at transmitter and receiver, respectively. If the primary system is unable to achieve its target rate, then the secondary transmitter (ST) forwards a few subcarriers to the primary receiver (PR) to ful_ll the quality-of-services (QoS) requirement of the primary system and the remaining subcarriers can be used by the secondary system for its own data transmission. Thus, OSS can be achieved by the secondary system without interfering to the primary system, since primary and secondary subcarriers are orthogonal to each other.

To further boost the performance of both primary and secondary systems, we propose an adaptive subcarrier sharing scheme for OFDM-based cooperative cognitive radios. According to the scheme, ST uses adaptive mode of transmission to relay the primary signal with higher throughput while maintaining the bit-error rate (BER) constraint of the primary system. At PR, a BER based selection combining (BER-SC) scheme is employed to combine the signals received in two phases. Closed-form analytical expressions for BER and outage probability of primary and secondary system for a Rayleigh at fading channel have been derived. Results show that the outage probability with the proposed scheme (for dissimilar modulation) outperforms direct transmission and conventional maximal ratio combining scheme (for similar modulation).

Recently, D2D communication has been incorporated as a part of long-term-evolution advanced (LTE-A) Release 12 and 13 to avail the high capacity benefits to the cellular users with minimal constraints on infrastructure maintenance. In a generic D2D framework, two cellular users living in proximity can form a direct link for data transmission without routing it through the evolved node B (eNB) / base station (BS). Utilization of available resource allocation frameworks to facilitate D2D communication while maximizing the throughput of both cellular and D2D users is an open issue. In this dissertation, we propose the rate and outage trade-offs for orthogonal frequency division multiple access (OFDMA) based D2D communication frameworks where multiple D2D users coexist with the cellular users in the same cell. Analytical expressions of outage probability for three D2D frameworks namely underlay, overlay and cooperative D2D (C-D2D) have been derived. Specifically, for underlay framework, a minimum value of the angle (an angle between a cellular link and D2D interference link) is derived for which the target rate and outage probability constraint of both cellular and D2D users are satisfied. For overlay and C-D2D frameworks, an optimal subcarrier sharing scheme is proposed which not only helps the cellular users to achieve the target QoS, but also helps the D2D users to communicate with each other. In addition to above, benefits involved in employing one framework over other have also been investigated. Our results show that for a higher outage probability constraint of the cellular user, the C-D2D framework outperforms the underlay and overlay frameworks.

We augment the development of C-D2D framework by incorporating the best D2D user selection scheme. As per the proposed scheme, among M available D2D pairs, a D2D transmitter (DT) that can achieve the largest QoS improvement for uplink cellular transmission is selected to serve as a user relay for the cellular transmission. In addition, a novel round robin process with priority cube method is proposed to facilitate the fair distribution of available resources among M DTs. In particular, closed-form expressions of the outage probability for two different cases: with and without direct cellular link are derived. Results show that the proposed best D2D user selection improves the QoS of the cellular user as compared to the conventional C-D2D framework.

Another issue related to D2D communication is the lack of measurement results for proof of concept demonstration and performance assessment in a realistic scenario. We try to resolve this issue by designing and developing a software defined radio (SDR) based test-bed implemented on National Instruments (NI) Universal Software Radio Peripheral (USRP) platform. The performance of the testbed has been validated by obtaining received signal to noise ratio (SNR) and symbol error rate (SER) for both cellular and D2D users. The measurement results show that the proposed frameworks significantly improves the performance of cellular network in both line of sight (LOS) and non-line of sight (N-LOS) scenarios.