Development of LTE (4G) Antenna Design for Highest Efficiency Achievement ^†

Abstract

The quality of RF signal coverage of mobile networks, and for example, the parameters of LTE signals at many places, is not reliable enough for intensive data transfer. This fact causes mobile service customers to look for and to apply additional local devices or systems for amplification of the initially supplied RF signal level to improve the quality of the service. This local improvement covers a small area around the customer’s residence. This paper shares some useful results of design, analyses, and optimization for effective performance of the tranceiving antenna for LTE (4G) signals.

1. Introduction

From the perspective of area coverage, currently the largest territory of mobile communication services is covered by third-generation systems (3G). Although these systems are able to perform just 2 Mbps indoors and 150 kbps in outdoor environments, each local provider’s network succeeds in covering nearly 100% of the country area using 3G, which still has a considerable share in pure telephony services, carrying a large part of this traffic [1]. Of course, the systems with the greatest importance in terms of high data transmission speed and providing mobile communication service to the largest number of people undoubtedly are mainly LTE (4G) networks, which are rapidly enlarging their coverage, but also 5G networks, which cover the cities and some of the bigger towns at the moment. It is obvious that because of their expansion, both LTE (4G) and 5G networks will need new frequency slots. These facts force the process of full decommissioning of 2G networks and a gradual decommissioning of 3G with the respective redistribution of the spectrum resources [2,3,4].

Dynamic Spectrum Sharing (DSS) allows both 4G LTE and 5G NR to use the same frequency band and to share the spectrum resources dynamically between these two technologies, depending on the users’ demand [5].

The RSRP level of LTE coverage in small villages is usually not high enough—it is usually less than −100 dBm, except in very close surroundings of the cell tower. This is the downlink signal from the base station to the mobile device measured by the Network Sell Info application upon receiving. The download data speed does not exceed 3 Mbit/s during the test without any additional amplifying devices.

Some of the previous reports [6,7] state that the telecom providers do not have an economic and business interest to develop the quality of LTE signals concerning the coverage, which guarantees reliable data speed in areas with small populations. Because the village areas covered by a single cell tower are much larger compared to the city coverage area by a single cell tower, the level of the signal fades tremendously far from the base station. This stimulate our exploring team to work on research to achieve an effective and relatively inexpensive solution for the setup of a system of amplifiers and antennas to ensure very good to excellent signal coverage within the yard of a particular real estate property, situated in an area with poor quality of coverage.

Without applying DSS, a mobile service provider has to split a 20 MHz middle band spectrum into two: a 10 MHz of spectrum 4G LTE dedicated to all LTE and the remaining 10 MHz of the band to be used for 5G; however, the real number of 5G users is minimal compared to the number of LTE users. DSS distributes that 20 MHz band dynamically and simultaneously to both LTE and 5G technologies for optimized usage [2,3].

The LTE (4G) technology guarantees a maximum data transfer speed of 100 Mbps through a bandwidth of 20 MHz. LTE architecture is specially adapted for fast data transfer, and data access is considerably faster compared to 3G, so LTE technology provides a lot of new types of interactive services and real-time functionalities for users [1,2].

5G systems are now available—their design and development allow them to provide transfer data rates of 5–250 Mbit/s [4]. Low-band cell towers have a coverage area similar to LTE base stations. The middle band allows traffic speed of 100–900 Mbit/s 5G by microwaves with frequencies of 1.7–4.7 GHz. Each cell tower achieves 5G service coverage of a couple of kilometers around. 5G service has been widely allocated in many city areas since 2020. Some regions are not using the low band and implement the mid-band as the minimum service level. High-band 5G uses frequencies of 24–47 GHz, close to the bottom of the millimeter wave band; however, higher frequencies may be used in the future. It provides download traffic speed in the Gbit/s range, comparable to signal service transmitted through a coaxial cable. But the radio connection using millimeter waves needs too many physically small cells [4], as they have a very limited effective range. At these frequencies, a lot of obstacles impede and even block the signal and tremendously impact the data transfer speed. A network of a large number of 5G devices is much more expensive than the LTE infrastructure, and as a result, from the perspective of mobile service providers’ interests, the middle and high band 5G cells are profitable to be deployed only in dense urban environments and areas where crowds congregate, such as sports stadiums and convention centers [3,4].

In this paper, new exploration is performed to find a definitive correlation between the parameters of the downlink and uplink signals and the real data speed in both channels.

Part of the difficulty comes from the ratio of the RSRP downlink channel to the RSRP of the signal transmitted by the mobile device transmitter towards the base station. This ratio is considerably different in the city and in the village, since the RSRP of the uplink signal is transmitted to the base station antenna from a long distance—at least 4 km.

The main quality parameters of the RF signal in the mobile service network discussed in this study are the maximum and the average power value of the distributed signal, real data traffic speed, and the functional relation between those parameters.

This paper presents some practical results and analyses of the design and exploration of a local RF amplifying system, developed to receive, amplify, transmit, and spread the LTE and 4G signals across a small area of a village home yard [7], where the 4G service signal is very weak. A coordinated and optimized antenna system for both bands has been implemented and studied.

2. Materials and Methods

Based on the previous exploration, it is obvious that the data transfer speed is not linearly correlated to the strength value of the radio signal from the base station. The actual transfer data speed depends critically on the signal level in the uplink channel. This fact pushed the experimental work in a direction of improvements to the antenna system design and parameters. As published previously [7], the antenna system is designed by separate antennas—one receiving antenna, which utilizes the downlink radio signal, and one transmitting antenna for distribution of a radio signal with a level that is able to be recognized by the receiving antenna and input devices of the base station [8,9].

The realized radio frequency amplifying system uses two RF amplifier-antennas, connected separately in both lines, obeying the schematic diagram depicted in Figure 1. The first difference from previous research is that two separate outdoor antennas for the downlink and uplink channels are used. A standard monopole combined antenna optimized for 900 MHz and 1800 MHz with a spherical directivity pattern was used as the receiving antenna. The transmitting antenna of the uplink channel is a 9-element Yagi-Uda design antenna with an elongated directivity pattern towards the base station tower, also optimized for 900 MHz and 1800 MHz bands [10,11].

This approach simplifies the amplifying system from the perspectives of mechanical roof mounting and electromagnetic compatibility of two inevitably closely spaced antennas.

It is connected to the first RF amplifier input by a 50-ohm RJ50 coaxial cable, and the amplifier’s output is connected to a distribution antenna with a spherical directivity pattern. The efficiency of the LTE signal transmission in the uplink channel can be assured by including matching impedance schematics in the output circuits between the respective amplifier, coaxial cable, and the transmitting antenna, optimized for 880–960 MHz and 1720–1890 MHz bands [9,10]. Impedance matching schemes also include the impedances of the coaxial cables and connectors, as well as the connector-transducers where they are applied [9,11].

For ease of exposition, the distribution antennas of the downlink and the receiving antennas of the uplink channel are presented as single two-band antennas and simulated accordingly.

Theoretical Concept

The average reference LTE signal received power (RSRP) can be calculated by the following equation:

$$\mathit{RSRP} = \mathit{RSSI} -\mathsf{10} \mathit{lg} \left(\mathsf{12}\mathit{N}\right),$$

(1)

where N is the number of resource blocks of the carrier RSRP measurement bandwidth. RSRP and RSSI are measured in dBm. The typical range of RSRP is around −44 dBm (good level) to −140 dBm (bad level). RSRP levels for usable signals typically range from about −75 dBm close to an LTE cell site to −120 dBm at the edge of LTE coverage. Reference LTE signal received quality (RSRQ) is defined as the ratio:

$$\mathit{RSRQ} = N {\displaystyle \frac{\mathit{RSRP}}{\mathit{RSSI}}},$$

(2)

where N is the number of resource blocks of the E-UTRA carrier RSSI measurement bandwidth. RSRP and RSSI are measured in dBm. RSSI is a parameter that provides information about total received wide-band power, including all interference and thermal noise, and it represents the entire received power, including the wanted power from the serving cell, as well as all co-channel power and other sources of noise, and it is related to the above parameters through the following formula:

$$\mathit{RSSI} = \mathsf{12} N \mathit{RSRP},$$

(3)

where N is the number of resource blocks across the RSSI, which is measured and depends on the BW. RSSI = wideband power = noise + serving cell power + interference power

RSSI varies with LTE downlink bandwidth, even if the other factors are relatively equal; 10 MHz LTE bandwidth RSSI would be measured 3 dB greater than 5 MHz LTE bandwidth RSSI. But that does not actually correlate to a stronger signal at the user’s end.

RSSI also varies by LTE subcarrier frequencies set—the greater the data transfer speed, the higher the RSSI. And still, that does not actually lead to a stronger signal to the end user.

RSRP practically performs better in measuring the signal power from a specific sector while potentially excluding noise and interference from other sectors. RSRP levels for usable signals typically range from about −75 dBm close to an LTE cell site to −120 dBm at the edge of LTE coverage [2].

Some difficulties for a precise measurement of the above-mentioned parameters and result interpretations could be expected, as the noise and interference impact on the LTE signal is quite different in the 900 MHz band and in the 1800 MHz band.

In order to achieve clear comparisons and to obtain improvements in performance by transmitting the direction-forming, omnidirectional, and one-directional patterns that the latter concept represents, the modern standard antenna design and configuration are also studied. Apart from delivering assessments on the performance of different antenna designs in a cellular network, this study evaluates and optimizes different antenna parameters. It takes into account some network signal power-level aspects instead of being focused on the parameter optimization only [12].

3. Results and Discussion

The measurements of electrical parameters of antennas, coaxial cables with connectors, impedance transforming circuits, and amplifiers are made by a portable vector network analyzer, LiteVNA. Parameters of the LTE signal from the base station are measured by the Network Cell Info Application under Android OS. LiteVNA collects S-matrix data of each block of the amplifying system in order to build a simulation model, which properly represents the real response of the units separately and together.

Table 1. Parameters of the base stations involved in the exploration.

Tower Info	Cell 1 (3)	Cell 53 (51)
Cell Identifier	3473921	3474229
System Subtype	LTE	LTE
PCI	446 (148/2)	448 (149/1)
EARFCN	1300	3678
RSRP max	−91 dBm	−97 dBm
Direction	E (94°)	E (100°)
RSRQ max	−2 dB	−6 dB
SNR max	40 dB	18 dB
Last Check	29.12.2024	29.12.2024
Bandwidth	15 MHz	5 MHz
Uplink Frequency	1720 MHz	902.8 MHz
Downlink Frequency	1815 MHz	947.8 MHz
Frequency Band	DCS (B3FDD)	E-GSM (B8 FDD)

shows all important parameters of the two base stations that provide LTE signal coverage of the area where the private property in question is located and are used as the basic digital data to begin the study.

In Figure 2, both outdoor (Figure 2a) and indoor (Figure 2b) antennas are shown. S-parameter data of both antennas is measured by a portable vector network analyzer, LiteVNA, to convert it into proper one-port block models for simulation.

A Yagi-Uda antenna typically has an elongated and narrow gain pattern, which requires precise pointing towards the signal source. In this particular case, there is about a 36° angle difference between the directions from the mobile service explored spot to each of the cells, which cover that spot.

The final direction of this antenna is fixed concerning the optimal signal level of both bandwidths (900 MHz and 1800 MHz) received simultaneously.

All antennas used in the experiments here are designed to be able to work effectively with SWR close to 1, with high gain for both 900 MHz and 1800 MHz bandwidths.

In Figure 3, signal amplitude and standing wave ratio as a function of frequency, and a Smith chart diagram of an outdoor antenna are depicted. The markers on Figure 3a,b indicate the frequencies where the 9-element Yagi-Uda antenna (Figure 2a) performs best.

In Figure 4, signal amplitude and standing wave ratio as a function of frequency, and a Smith chart diagram of an indoor antenna are shown. The markers in Figure 4a,b indicate the frequencies where the antenna (Figure 2b) performs best.

Two identical RF amplifiers with a gain of at least 40 dB in the range 1770–1880 MHz and at least 25 dB in the range 880–970 MHz are used in the system. Figure 5 represents the behaviour of the RF amplifier over the range 800 MHz–2000 MHz. S-parameter data is collected by LiteVNA.

The experimental schematic for simulation is presented in Figure 6. The outdoor antenna works as a receiving antenna for the LTE signal from the base station in the ranges 935–960 MHz and 1805–1880 MHz. An identical antenna is used as a transmitting antenna towards the base station in the ranges 890–915 MHz and 1710–1785 MHz.

One of the expected problems arises from the inevitable necessity of impedance matching blocks in the input and output circuits of the amplifiers, depending on the length of the coaxial cables. The second problem seems to be the very low level of the radio signal emitted from the mobile device transmitter to the base station.

Some results from the simulation are shown in Figure 7. The blue line represents the full reflection coefficient (S11), and the red line represents the total gain (S21), both over the 800 MHz–2000 MHz band. Maximal gain values are 10 dB achieved at 873 MHz and 15 dB at 1755 MHz. The value of S11 at the lower frequency band is −6.9 dB, and it is −17.9 dB at the higher frequency, where the best performance is evaluated.

The level of the RSRP for the 900 MHz band and for the 1800 MHz band are measured and shown respectively in Figure 8a and in Figure 8b.

The Net Info application screen shots of the real-time measurement of data transfer speed performance on download and upload are depicted in Figure 9. The measurements are made in different conditions: without any amplifiers (Figure 9a), with an amplifier in the downlink loop only (Figure 9b), and with an added amplifier in the uplink loop (Figure 9c), impedance matched to the transmitting antenna for the 900 MHz and 1800 MHz bands.

RSRP of the 900MHz channel is significantly better than the higher range of 1800 MHz. The measurement is performed with amplifiers in both downlink and uplink channels, but without an impedance match between the uplink amplifier and the transmitting antenna. Nevertheless, the successful final result as a transfer data ratio improvement can be seen in Figure 9c.

More detailed analysis requires a more accurate approach, proposing an experimental stage with possibilities for effective measurement of the signal power level emitted by the mobile device antenna. This could improve the schematic and final design of the amplifier, which is responsible for the gain in the uplink channel. As mentioned above, a very important consideration is that the LTE signal power level should be strong enough, but its value must not exceed the optimal value, which the base station receiving devices recognize correctly during any communication session.

4. Conclusions

To achieve sufficiently high data transfer speeds, it is critically important to ensure solid and high-quality LTE signal gain in the uplink channel. In most cases, it is not effective to try to amplify the signal from the phone towards the base station because of the danger of not recognizing the signal, as the base station receiver is set up for weak signals, which is the typical level of signal transmitted by mobile devices. These cases are when the customer is in the city, where the distance between the mobile device and the closest base station is less than 1 km, and where the coverage is guaranteed by at least three or more cells.

In the presented research, there are only two cells at a distance of more than 4 km. The gain is enough to keep the uplink LTE signal recognizable by the receiver of the respective cell.

The next experiments will be performed with a multiband log-periodic tranceiving antenna as an outdoor antenna.