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Abstract-Base station (BS) sleeping has emerged as a viable solution to enhance the overall network energy efficiency by inactivating the underutilized BSs. However, it affects the performance of users in sleeping cells depending on their BS association criteria, their channel conditions towards the active BSs, and scheduling criteria and traffic loads at the active BSs. This paper characterizes the performance of cellular systems with BS sleeping by developing a systematic framework to derive the spectral efficiency and outage probability of downlink transmission to the sleeping cell users taking into account the aforementioned factors. In this context, we develop a user association scheme in which a typical user in a sleeping cell selects a BS with Maximum best-case Mean channel Access Probability (MMAP) which is calculated by all active BSs based on their existing traffic loads. We consider both greedy and round-robin schemes at active BSs for scheduling users in a channel. Once the association is performed, the exact access probability for a typical sleeping cell user and the statistics of its received signal and interference powers are derived to evaluate the spectral and energy efficiencies of transmission.

The diverse transmit powers of the base stations (BSs) in a multi-tier cellular network, on one hand, lead to uneven distribution of the traffic loads among different BSs when received signal power (RSP)-based user association is used. This causes under utilization of the resources at low-power BSs. On the other hand, strong interference from high-power BSs affects the downlink transmissions to the users associated with low-power BSs. In this context, this paper proposes a channel access-aware (CAA) user association scheme that can simultaneously enhance the spectral efficiency (SE) of downlink transmission and achieve traffic load balancing among different BSs. The CAA scheme is a network-assisted user association scheme that requires traffic load information from different BSs in addition to the channel quality indicators. We develop a tractable analytical framework to derive the SE of downlink transmission to a user who associates with a BS using the proposed CAA scheme. To mitigate the strong interference, the almost blank subframe (ABS)-based interference coordination is exploited first in macrocell-tier and then in smallcell-tier. The performance of the proposed CAA scheme is analyzed in presence of these two interference coordination methods. The derived expressions provide approximate solutions of reasonable accuracy compared to the results obtained from Monte-Carlo simulations. Numerical results comparatively analyze the gains of CAA scheme over conventional RSP-based association and biased RSP-based association with and without the interference coordination method. Also, the results reveal insights regarding the selection of the proportion of ABS in macrocell/smallcell-tiers for various network scenarios. Index Terms-Two-tier cellular network, downlink user association, interference coordination, channel access probability, interference statistics, universal channel reuse, almost blank subframe (ABS), spectral efficiency (SE).

The microwave attenuation due to rainfall in tropical regions has not been very widely studied yet. In Pakistan's tropical environment, line-of-sight microwave communication links were set up and have been operated for several years to study the microwave attenuation characteristics due to tropical rainfall. In this paper the experimental results are presented, including the cumulative distributions of microwave attenuation and the relationship between specific attenuation and rainfall rate. In addition, a rain outage prediction model is proposed which not only predicts microwave radio link performance but will also be useful in calculating the link degradations due to interference issues. The main focus of this research work has been done keeping in mind the wireless networks of Pakistan. The results show that the rainfall rate, the microwave propagation characteristics, and outage predictions in Pakistan differ from the International Radio Consultative Committee predictions and ITU recommendation P530.7/8, respectively.