New wireless network paradigms will demand higher spectrum use and availability to cope with emerging data-hungry devices. Traditional static spectrum allocation policies cause spectrum scarcity, and new paradigms such as Cognitive Radio (CR) and new protocols and techniques need to be developed in order to have efficient spectrum usage. Medium Access Control (MAC) protocols are accountable for recognizing free spectrum, scheduling available resources and coordinating the coexistence of heterogeneous systems and users. This paper provides an ample review of the state-of-the-art MAC protocols, which mainly focuses on Cognitive Radio Ad Hoc Networks (CRAHN). First, a description of the cognitive radio fundamental functions is presented. Next, MAC protocols are divided into three groups, which are based on their channel access mechanism, namely time-slotted protocol, random access protocol and hybrid protocol. In each group, a detailed and comprehensive explanation of the latest MAC protocols is presented, as well as the pros and cons of each protocol. A discussion on future challenges for CRAHN MAC protocols is included with a comparison of the protocols from a functional perspective.
The allelopathic effect of various concentrations (0, 6.25, 12.5, 50 and 100 g L-1) of Parthenium hysterophorus methanol extract on Cyperus iria was investigated under laboratory and glasshouse conditions. No seed germination was recorded in the laboratory when P. hysterophorus extract was applied at 50 g L-1. In the glasshouse, C. iria was mostly injured by P. hysterophorus extract at 100 g L-1. The phytochemical constituents of the methanol extract of P. hysterophorus were analyzed by LC-ESI-QTOF-MS=MS. The results indicated the presence of phenolic compounds, terpenoids, alkaloids, amino acids, fatty acids, piperazines, benzofuran, indole, amines, azoles, sulfonic acid and other unknown compounds in P. hysterophorus methanol extract. A comparative study was also conducted between P. hysterophorus extract (20, 40 and 80 g L-1) with a synthetic herbicide (glyphosate and glufosinate ammonium at 2 L ha-1) as a positive control and no treatment (negative control) on Ageratumconyzoides, Oryzasativa and C. iria. The growth and biomass of test weeds were remarkably inhibited by P. hysterophorus extract. Nevertheless, no significant difference was obtained when P. hysterophorus extract (80 g L-1) and synthetic herbicides (glyphosate and glufosinate ammonium) were applied on A.conyzoides.
The current study was designed to investigate the effect of Parthenium hysterophorus L. methanol extract on Ageratum conyzoides L., Oryza sativa f. spontanea (weedy rice) and Cyperus iria L. in glasshouse condition. Here, Parthenium hysterophorus methanol extract at 20, 40, and 60 g L-1 concentrations was applied on the test species to examine their physiological and biochemical responses at 6, 24, 48 and 72 h after spraying (HAS). The phytotoxicity of P. hysterophorus was strong on A. conyzoides compared to weedy rice and Cyperus iria at different concentrations and exposure times. There was a reduction in photosynthesis rate, stomatal conductance, transpiration, chlorophyll content and carotenoid content when plants were treated with P. hysterophorus extract concentrations. Exposure to P. hysterophorus (60 g L-1) at 24 HAS increased malondialdehyde (MDA) and proline content by 152% and 130%, respectively, in A. conyzoides compared with control. The activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD)) were also increased in the presence of P. hysterophorus extract. Present findings confirm that the methanol extract of P. hysterophorus can disrupt the physiological and biochemical mechanism of target weeds and could be used as an alternative to chemical herbicides.