# Crescent Microstrip Antenna for LTE-U and 5G Systems
## 3 Characteristics of the LTE-U System  
The LTE-U (LTE-Unlicensed) is a proposal, originally developed by Qualcomm, to use 4G LTE radio technology in an unlicensed spectrum such as the 5 GHz band used by 802.11a compliant Wi-Fi equipment. It will be an alternative to the operator's Wi-Fi hotspots. LTE networks carry more and more data. Cells can be made smaller to do this, but this is not a complete solution as more spectrum is needed [12; 13]. One approach is to use an unlicensed spectrum along with licensed bands. Known in 3 GPP as LTE-LAA (LTE license-assisted access) or more generally as LTE-U (LTE-Unlicensed), it allows access to an unlicensed spectrum, particularly in the 5 GHz ISM band. A significant amount of unlicensed spectrum is available worldwide. These bands are used worldwide to provide unlicensed access for short-range radio transmissions. These bands, called ISM (industrial, scientific, and medical) bands, are allocated in different parts of the spectrum and are used for many different applications, including microwave ovens, Wi-Fi, Bluetooth, and many more [14; 15]. The most interesting frequency band for LTE-U/LTE-LAA (license-assisted access) is the 5 GHz band. In this case, there are several hundred MHz of spectrum bandwidth available, although the exact bandwidth availability varies from country to country [12]. The division of the 5 GHz band used for LTE-U is shown in Figure 1.  
In addition to the basic frequency limits, the use of the 5 GHz bands for applications such as LTE-U or LTE-LAA has some regulatory requirements. One of the main requirements for accessing these frequencies is the ability to coexist with other users of the band--the CCA (clear channel assessment) or LBT (listen-before-talk) method is required. This often means that instant access may not always be available when LTE-U is deployed. Another important limit is that different power levels are allowed depending on the country and area of the frequency band used. Typically, between 5150 and 5350 MHz, the maximum power limit is 200 mW and operation is limited to indoor use only, and higher frequencies often allow power levels up to 1 W [14; 15]. The use of LTE-U/LTE-LAA was first introduced in the Rel13 3 GPP standard. Basically, LTE-U is based on the LTE-Advanced carrier aggregation capability, which has been  
Figure 1: LTE-U frequency band. implemented since around 2013. Carrier aggregation aims to increase the overall capacity available to the user equipment by allowing it to use more than one channel on the same frequency band or on a different band [12]. There are several ways to implement LTE-U:  
Downlink only: This is the most basic form of LTE-U and is similar to some of the first LTE carrier aggregation implementations. In this case, the primary cell link is always within the licensed spectrum bands (Figure 2). Uplink and downlink: Full operation of TDD LTE-U with user equipment having an uplink and downlink connection in an unlicensed spectrum requires more functions to be enabled (Figure 3). FDD/TDD Aggregation: LTE-CA allows a combination of carrier aggregation between FDD and TDD. This provides much more flexibility in selecting the band to be used in the unlicensed spectrum for LTE-LAA operation [16; 17]. The LTE-U builds on the existing backbone for backhaul and other capabilities such as security and authentication. Therefore, no changes to the backbone are needed. Some changes to the base station are needed to adapt to the new frequencies, and to take into account the capabilities required to ensure proper sharing of unlicensed frequencies. In addition, phones or devices in the EU will need to have the new LTE-U/LTE-LAA feature built in to be able to access LTE on these additional frequencies [12]. The LTE-U would enable mobile operators to increase the coverage of their mobile networks, using the unlicensed 5 GHz band, in which Wi-Fi devices already operate. T-Mobile and Verizon Wireless showed interest in implementing such a system as early as 2016.