**1. Introduction**

Mobile communication has been updated greatly from the first-generation (1G) to recent fifth-generation (5G) and future promising sixth-generation (6G) mobile communication systems to meet the requirement of high data rate, large capacity, low latency, *et al.* for consumers [1]. As one of the key techniques for 5G and 6G communication system, millimeter-wave (mm-wave) frequency band with large bandwidth is adopted [2, 3]. Comparison with the centimeter-wave or decimeter-wave, the frequency band of mm-wave is much higher and thus shorter wavelength. However, due to its high frequency with short wavelength, the free-space loss of mm-wave is higher than that of lower frequency bands, and the mm-wave beam is usually blocked [4]. To mitigate the path loss and beam blockage, a high-gain antenna with wide-angle beam scanning is usually adopted to catch the strongest signal and ensure effective radiation in a 5G mm-wave system [5].

To ensure the wireless connection using mm-wave frequency band, the mm-wave array antenna should be applied in mobile phones. Besides the requirements of frequency bands, maximum output power, additional spectrum emission mask,

and spurious emission from the 3GPP [6], four additional key difficulties should be considered for mm-wave array in mobile phones:


#### **Figure 1.**

*Conceptual diagram of the mm-wave array with desired spherical coverage for mobile phones. (a) Front view. (b) Side view [11].*

*Recent Advances in the mm-Wave Array for Mobile Phones DOI: http://dx.doi.org/10.5772/intechopen.112043*

Since the first mm-wave array was designed for mobile phones in 2014 [9] and the first commercial mm-wave array module was adopted in Samsung Galaxy S20 in 2020 [10], significant progress has been achieved in addressing the above difficulties in recent years. The desired beam coverage as shown in **Figure 1** should be achieved with mm-wave arrays.

In this chapter, a comprehensive summary of the recent advances in the mm-wave array for mobile phone, such as broadside mm-wave array, endfire mm-wave array, co-design of the mm-wave array with metal-bezel and lower frequency band antenna, and user influence is conducted. **Figure 2** illustrates the block diagram of the mmwave array for mobile phones. For the broadside mm-wave array, we focus on the single-band, dual-band, and reconfigurable designs. For the endfire mm-wave array, single-polarization and dual-polarization designs are summarized. For the co-design of the mm-wave array, integrating metal-bezel with mobile phone design and sharedaperture with lower band antenna design are summarized.

This chapter is organized as follows. In Section 2, the common antenna element types of broadside radiation are introduced, and the design challenges of mm-wave broadside arrays are analyzed. Then, the broadside arrays are divided into three parts: single-band design, dual-band design, and reconfigurable design, to be summarized respectively. In Section 3, the challenges of endfire mm-wave array design are first analyzed. Then, the typical horizontal polarized, vertical polarized, and dual-polarized mm-wave endfire arrays are summarized. In Section 4, the co-design of the mm-wave array in the mobile phone with a lower frequency band antenna is introduced, including an integrated design with metal-bezel and shared-aperture design, respectively. In Section 5, the user influence on the mm-wave array in the mobile phone is illustrated. Finally, conclusions are drawn in Section 6.

### **2. Broadside mm-wave array for mobile phone**

Broadside array antenna is the array with the direction of maximum radiation, which is vertical to the array. As shown in **Figure 3**, the broadside array is achieved

**Figure 3.**

*Conceptual diagram of the antenna array with broadside radiation pattern.*

**Figure 4.**

*Conceptual diagram of the broadside mm-wave array and desired beam directions. (a) Array placed horizontally with the mobile phone. (b) Array placed vertically with the mobile phone.*

with the array element in the x*y*-plane and maximum radiation direction along the *z*-axis. As shown in **Figure 4(a)**, when the array is placed horizontally with the mobile phone, the desired beam directions 5 and 6 in **Figure 1** can be achieved. In **Figure 4(b)**, when the array is placed vertically with the mobile phone, the desired beam directions 1, 2, 3, and 4 in **Figure 1** can be achieved. Usually, due to the thickness limitation of the mobile phone, beam directions 1, 2, 3, and 4 are mainly achieved by endfire arrays, which will be explained in detail in Section 3. The broadside arrays are mainly used to achieve beam directions 5 and 6.

For broadside array, the typical antenna elements are patch antenna [12], slot antenna [13], printed dipole antenna [14], dielectric resonant antenna [15], substrate integrated waveguide (SIW) cavity antenna [16], and so on. For the above antenna types, dual polarization can be achieved simply by quadrature feeding or by placing a pair of quadrature elements. The main challenge of broadside array design is how to achieve the array with the superior performance of small size, wide bandwidth, multiband, and wide beam coverage. In this section, the broadside arrays are classified into three parts: single-band, dual-band, and reconfigurable to summarize. At the same time, several solutions to the above challenges are also summarized.

#### **2.1 Single-band broadside mm-wave array**

The patch antenna is one of the most commonly used antenna elements in the broadside mm-wave wave array. The bandwidth of patch antennas is usually narrow, covering only a portion of the commercial mm-wave band. For example, as shown in **Figure 5(a)**, an optically invisible common patch antenna on display only has a bandwidth of 9% (27.1–29.7 GHz) [17]. As shown in **Figure 5(b)**-**(d)**, in order to improve the bandwidth of *Recent Advances in the mm-Wave Array for Mobile Phones DOI: http://dx.doi.org/10.5772/intechopen.112043*

#### **Figure 5.**

*Typical design schematics of single-band broadside mm-wave array. (a) Common patch antenna element. (b) Slotting on the patches. (c) Using parasitic patches. (d) Using parasitic branches. (e) Printed dipole antenna, dual-polarized realized by orthogonal placement. (f) Dual-polarized. Realized by quadrature feeding.*

the patch antenna element, slotting on the patches [18], using parasitic patches [19], and using parasitic branches [20] are used to obtain more than 20% impedance bandwidth. Although the optimized patch antenna in [19] can cover the mm-wave band from 23 to 30.5 GHz (N257/258) band, it cannot cover the N259/N260 near 40GHz. In contrast, printed dipole antennas have a wider bandwidth. For example, the printed dipole antenna in [21] achieves a 50% (24–40 GHz) impedance bandwidth. In addition, as shown in **Figure 5(f)** and **(g)**, dual polarization can be easily achieved by placing a pair of antenna elements orthogonally [22] or by quadrature feeding [23].

The patch antenna is one of the most commonly used antenna elements in the broadside mm-wave wave array. The bandwidth of patch antennas is usually narrow, covering only a portion of the commercial mm-wave band. For example, similar to the common patch antenna element shown in **Figure 5(a)**, an optically invisible common patch antenna on display only has a bandwidth of 9% (27.1–29.7 GHz) [17]. As shown in **Figure 5(b)**-**(d)**, in order to improve the bandwidth of the patch antenna element, slotting on the patches [18], using parasitic patches [19], and using parasitic branches [20] are used to obtain more than 20% impedance bandwidth. Although the optimized patch antenna in [19] can cover the mm-wave band from 23 to 30.5 GHz (N257/258) band, it cannot cover the N259/N260 near 40GHz. In contrast, printed dipole antennas have a wider bandwidth. For example, the printed dipole antenna in [21] achieves a 50% (24–40 GHz) impedance bandwidth. In addition, as shown in **Figure 5(e)** and **(f )**, dual polarization can be easily achieved by placing a pair of antenna elements orthogonally [22] or by quadrature feeding [23].

#### **2.2 Dual-band broadside mm-wave array**

With several mm-wave bands around 28, 38, 45, and 60 GHz have been assigned for 5G development [24], mm-wave ultra-wideband antennas or multiband antennas are widely investigated to cover two or more frequency bands simultaneously to expand the available spectrum, improve antenna space utilization, save fabricated cost, and achieve high integration. And this part mainly focuses on dual-band broadside mm-wave array.

There are usually two ways to achieve dual-band antennas. One general way to achieve dual-band antennas is to combine two different structures operating at different frequency bands together [15, 25, 26]. For example, a hybrid antenna consisting of three resonators of strip, slot, and dielectric resonant antenna is proposed [15]. The strip and slot modes are used to cover the lower frequency band of 26.41–30.42 GHz, while the TE111 and TE131 modes of the DRA are employed to cover the upper-frequency band of 36.05–40.88 GHz. Two pairs of dipole antennas are proposed in [25]; the low-band radiation is generated by the pair of dipole arms along co-polarized direction, while the high-band radiation is realized by the dipole arms along crosspolarized direction.

Another way to achieve dual-band antennas is to adjust different modes of the same antenna structure to achieve dual resonance [14, 27–29]. For example, a compact dual-wideband magnetoelectric dipole is proposed in [14], the lower band of 24–29.3 GHz is achieved by 0.5λ mode, and the higher band of 35.5–43.5 GHz is achieved by 1λ mode. Also, the TM10 mode and TM20 mode of the gridded patches antenna are used to achieve dual-band coverage [27] .

#### **2.3 Reconfigurable broadside mm-wave array**

Considering the massive production of consumer electronics, the cost of each mobile antenna should be as low as possible. The reconfigurable design enables multiple operating modes of the mm-wave array through simple p-i-n diodes control and switching, which is one of the effective ways to save cost.

The reconfigurable design can be divided into two categories: direct control of the pattern reconfiguration, and control of the phased array excitation phase difference reconfiguration. As direct control of the pattern reconfiguration usually requires a large antenna design space and is not applicable for mobile phones [30], this part focuses on the reconfigurable design of the phase shifter. For the mm-wave arrays, the phase shifter is usually designed with the feeding network, and the excitation phase difference between elements is set by switching p-i-n diodes to achieve different

**Figure 6.** *1-bit reconfigurable broadside mm-wave design [31].*

directions. As shown in **Figure 6**, in [31], a low-cost reconfigurable 1-bit patch antenna is designed with moderate performance. What is more, a series-fed beamsteerable 2-bit reconfigurable design is proposed in [32].
