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Top1. Introduction
A wireless cellular network is made up of a number of radio cells, each served by at least one fixed-location transceiver known as a cell site or base station. Hundreds of cell sites are deployed in a typical large city to create coverage over 95% of the targeted area, so that a variable number of portable transceivers can be served in any cell.
According to Li (2014), coverage hole is an area within the radio coverage footprint of a wireless system in which the Radio Frequency (RF) signal level is below the designed threshold, which is usually caused by physical obstructions such as buildings, trees, hills. It also occurs in places where there are no nearby base stations. In these areas, service quality may drop off sharply, resulting in slow data rates and poor voice quality. Improvements in the quality of service are always pursued by wireless service providers in the competitive markets, so a solution which offers uninterrupted access to wireless services can be very compelling.
Poor indoor wireless coverage is now recognized globally as one of the biggest communication obstacles for mobile subscribers today. This is particularly prominent with data service and with LTE licenses being issued in frequencies as high as 2.6 GHz with a promise of 100 Mbps download and 50 Mbps upload speeds per cell.
There are many solutions to solve the poor indoor coverage issues, such as deploying relay stations according to Nourizadeh (2006), the repeaters, the passive relays and so on. Relay stations are introduced in IEEE 802.16 and 3GPP LTE-A, other solutions such as picocell and femto cell can also be deployed in existing network for coverage extension and throughput enhancement. However, the adoption of those solutions means more power consumption due to new elements introduced into the network, which also results in a complicated network topology, a great deal of signaling exchange and higher requirement for backhaul link, etc.
Another traditional solution to coverage problems is the repeater according to Oh (2014). As illustrated in Figure 1, a repeater is typically a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher-level power, so that the signal can reach longer distances or fill in coverage holes without degradation. Although simpler and low cost compared to the relay station, it has the following drawbacks: (1) power consumption by low noise amplifier and power amplifier; (2) amplification and forwarding of heat noise; (3) restricted to electric power supply.
Figure 1. Typical structure of a repeater
For the purpose of further reduction of energy consumption and deployment cost, a kind of passive relay technique for coverage enhancement is shown in Figure 2. It is basically made up of a pair of directional antennas (donor antenna and service antenna), basic antenna feed system and coaxial line for connection of receiving and transmitting parts. Consider the case of downlink, donor antenna is deployed in a favorable position to harvest radio energy from donor eNB and forwards it through coaxial line to service antenna. The transferred energy is served as the excitation to drive service antenna. Service antenna is typically deployed to cover coverage hole or into buildings for indoor coverage enhancement. For better indoor coverage, the service antenna should be designed with high gain and wide beamwidth. However, unless a very large-size antenna is configured, it would be difficult to achieve the high-gain and widebeam antenna for passive relay system.
Figure 2. Structure of passive relay system (downlink)