1. general introduction

Wi Fi (IEEE 802.11) has been continuously developing since its inception more than 20 years ago to meet people’s growing demand for higher data transmission rates and more places and indoor coverage. Recently launched Wi Fi 6 and 6e – Wi Fi 6 (E) – in addition to improving data transmission rate and coverage, and complementing 5g to achieve comprehensive indoor and outdoor connections, they also focus on improving data throughput, expanding channel capacity and reducing interference, as shown in Figure 1.

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Figure 1 Wi Fi 6 (E) and 5g for full connectivity

The data throughput is differentiated according to the types of data transmitted. The data rate considers all data, including management and control data. Data throughput refers only to user data – useful payloads. Wi Fi 6 (E) optimizes the packet structure of transmission, so it contains a higher percentage of payload than previous Wi Fi standards.

With a large number of connected “objects” in homes and places around the world, from baby monitors to intelligent ultra high definition (UHD) displays, the development of Wi Fi standards needs to be addressed to address the explosive growth in the number of interconnected devices. According to [1], by 2023, the number of public Wi Fi hotspots will quadruple to nearly 628million, of which 11% will be Wi Fi 6 (E).

This white paper is the second part of the series. Compared with the previous Wi Fi standards, it will introduce some of the most innovative new features of Wi Fi 6 (E) and how they promote the development of RF front-end. Then, for how to help solve these challenges, we will provide guidance on the use of Soitec’s rf-soi substrate, and explain our views on how to continue to innovate to better meet the needs of current and Future Wi Fi applications.

2. Wi Fi 6 (E) for next generation connection

Wi Fi 6 (E) enables challenging home application scenarios, such as game UHD virtual reality (VR) streaming and its coexistence with a large number of other connected devices, such as the increasingly popular intelligent assistant with voice recognition. In offices and large locations, Wi Fi 6 (E) provides a seamless experience through reliable network connectivity, while improving battery consumption of mobile and distributed devices such as smartphones and laptops – as well as power consumption of plug-in powered devices.

To deliver on its promise, Wi Fi 6 (E) relies on the following processes and technologies, such as:

Spatial multiplexing and frequency coexistence

High order data modulation over wide bandwidth

Uplink (UL) / downlink (DL) orthogonal frequency division multiplexing access (OFDMA) and multiuser – multiple input multiple output (MU-MIMO)

Scheduling and target wakeup time (TWT)

New 6GHz spectrum

From our point of view, this list is not complete, but represents some of the most innovative features implemented in Wi Fi 6 (E).

2.1 spatial multiplexing and frequency coexistence

Wi Fi is a half duplex standard. At any given time, only one radio signal can be transmitted on the channel. If it is detected that the specified channel is used by another radio signal, the radio signal to be transmitted is added to the waiting time. Due to the limited available spectrum, many radio signals may be allocated to the same channel, so the waiting time may be quite long. Given the increasing number of mobile and distributed devices in Wi Fi networks, this may lead to low spectrum efficiency and poor quality of service. In the example of Fig. 2, the distributed users connected to the hotspot AP-1 will impose a waiting time on other users who wish to use the channel 100 through the hotspot AP-4.

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Figure 2 Frequency coexistence conflict

In order to make better use of the spectrum, Wi Fi 6 (E) implements an adaptive signal strength threshold. In the example of Fig. 3, when device a detects that the signal strength is -96dbm, it will reserve channel 100 to communicate with device B sending the signal – both will become a blue group. Devices a and B will increase the required detection signal strength level and add a waiting time to any device that attempts to use channel 100. In this example, increasing this signal strength to a level between -96dbm and -83dbm will allow device C to use channel 100 again without conflicting with the communication between the blue pair of devices.

Figure 3 Wi Fi 6 (E) adaptive signal strength threshold for frequency coexistence

This technology strengthens the existing strict requirements for signal detection and distinguishing from background noise – any unwanted signal in the system is defined as noise here. In the example of Fig. 4, a typical usage of a Wi Fi channel in a residential area with several apartment units is shown; Each location has 31 2.4 GHz and 49 5 GHz hot spots (APS) [2].

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Figure 4 Utilization of 2.4GHz and 5GHz Wi Fi (residential Prague, Czech Republic)

Figure 5 shows the proximity between 5g N7 (FDD), N40 (TDD) and n41 (TDD) frequency bands and Wi Fi 2.4GHz and 5g N79 (TDD) and Wi Fi 5.8GHz frequency bands, which makes it very challenging to have high enough sensitivity when receiving without interference during transmission, which increases the complexity of Wi Fi radio front end (RFFE) design.

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Figure 5 Proximity of 5g and Wi Fi bands

Figure 6 (a) shows how the cellular radio frequency front end (RFFE) interferes with adjacent Wi Fi bands. Similarly, the Wi Fi radio frequency front end (RFFE) also interferes with adjacent cellular frequency bands, as shown in Figure 6 (b). Reducing interference sources to harmless levels requires careful design and process selection.

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Figure 6 Rf-soi minimizes (a) the total noise figure (NF) of the receiving path and (b) interference sources due to the high linearity of the RF front end

2.2 high order data modulation over wide bandwidth

In Wi Fi 6 (E), in order to improve the data throughput, 1024qam modulation scheme is adopted for the first time. At the same time, the same channel bandwidth as previous generations is used and the new 6GHz bandwidth is effectively used. This scheme adds new limits to the dynamic error vector amplitude (D – EVM) of the transmitting circuit, which requires the transmitting circuit to have high linearity and anti-interference.  

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Figure 7 Crosstalk of different rf-soi substrates

Isolation between signal paths (anti crosstalk) is very important to prevent harmful signals from leaking from one part of the system to another. Similarly, linearity is very important to prevent any interference source from affecting the normal function of the RF front end (RFFE). Figure 7 shows the crosstalk immunity of hr-soi (non trap rich SOI) and ifem SOI (trap rich SOI) substrates. Both materials are widely used in traditional and modern Wi Fi RF front ends (RFFE). We will discuss the linearity in Section 2.3.

2.3dl/ul OFDMA and MU-MIMO

In Wi Fi 6 (E), the hotspot (AP) determines which devices are used for sending / receiving, when to use what resources and how much data to send, which is contrary to the decentralization in the previous Wi Fi version. As shown in Fig. 8, this can make more effective use of the spectrum for packets of various sizes generated by different types of devices. OFDMA can allocate user packets according to time and frequency, while OFDM only allocates user packets in the time domain.  

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Figure 8 Comparison of OFDM and OFDMA and traditional Wi Fi and wi-fi6 (E)

To achieve Gbps data rate, Wi Fi 6 (E) can associate OFDMA with higher-order data modulation over a wide bandwidth, as described in Section 2.2. This leads to very complex signal waveforms, which makes the recording of the Wi Fi radio frequency front end (RFFE) linear. Figure 9 shows the linearity that can be achieved using Soitec’s ifem SOI and rfesitm substrates (both trap rich SOI) and compares them with the performance of hr-soi.

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Figure 9 (a) second order harmonics (b) third order harmonics (c) third order intermodulation (IMD3) on different rf-soi substrates

MU-MIMO uses multiple RF front ends and antennas of hotspot (AP) to allow multiple user equipment to communicate at the same time. By using the transmission time more efficiently, multiple data frames with unique modulation can be transmitted simultaneously. Figure 10 shows the BOM of radio frequency front end (RFFE) added to realize MU-MIMO function.

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Figure 10 MU-MIMO and Wi Fi RF front end (RFFE) BOM increase

Figure 11 shows the advantages of integrating LNA in the RF front end (RFFE) (increased by 0.9db in this case) compared with the integrated low noise amplifier (LNA) in the transceiver, which is usually the case in the traditional Wi Fi chip design. Rf-soi is a standard technology to realize RF switch. It is fully compatible with CMOS process and can be directly used for LNA implementation of noise figure (NF) optimization.

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Figure 11 Advantages of LNA integration in RF front end (RFFE)

2.4 scheduling and target wake-up time (TWT)

Target wake-up time (TWT) is an energy-saving feature introduced in Wi Fi 6 (E), which can help accommodate a greater number of device connections (such as IOT devices), while the power consumption is similar to or lower than that of previous generations of Wi Fi. Sleep and wake-up times are driven by applications (such as video streaming). Each connected device can “negotiate” with AP specific requests; The last decision is whether to approve or reject such requests. In this way, not only power consumption but also access to spectrum resources can be optimized.

Vendors can implement proprietary algorithms to optimize power consumption and spectrum access (as well as final data rates) to maintain an advantage in the highly competitive Wi Fi market.

In order to achieve more intelligent Wi Fi 6 (E), digital content, control, memory and other support functions are also required. CMOS rf-soi can effectively integrate these support functions to minimize interference. Figure 12 shows the digital noise in different types of substrates [3].

2.5 new 6GHz spectrum

Previous generations of Wi Fi have been operating on a spectrum of about 400MHz divided between 2.4 and 5.8GHz. Since 2020, the Federal Communications Commission of the United States has approved the additional use of 1200MHz in the 6GHz frequency band of the United States, and it is reported that other countries will adopt similar schemes in the short term [4] [5].

Based on the new available spectrum, several channels up to 160MHz can be used for multi Gbps data rate transmission. In addition, this additional spectrum resource will help minimize signal interference.

Due to the compatibility with CMOS process, rf-soi technology provides a good platform to realize a higher integration of multi-user and multi band Wi Fi RF front end (RFFE). The fully integrated Wi Fi radio frequency front end (RFFE) [6] simplifies the implementation of 2×2, 4×4 and even 8×8 MIMO radio frequency front ends (RFFE), thereby reducing complexity and improving chip area, because connected devices, whether mobile devices, distributed devices or fixed devices, are becoming more and more limited by size. See figure 10. In addition, in some cases, this integration can also reduce power consumption because more energy-saving technologies can be integrated into the RF front end (RFFE).

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Figure 12 Digital noise reduction of rf-soi substrate

3. rf-soi substrate innovation for Wi Fi

Due to the significant increase of wireless applications that rely on Wi Fi connections every year, it is not difficult to see the pressure from the RF front end (RFFE) in terms of power consumption, performance and integration. The total cost of ownership (TCO) is also a non negligible restriction on the RF front end (RFFE), because most of the final applications of Wi Fi are all kinds of consumer applications. Soitec has introduced an innovative substrate to achieve the best tradeoff between the linear performance and cost of the RF front end (RFFE). Similar to rfesitm, Soitec’s ifem SOI is a trap rich substrate. It has the same inherent advantages as trap rich SOI technology in isolation and anti noise, and has appropriate linear performance at optimized cost.

Figures 7, 9 and 12 show that Soitec’s ifem SOI has obvious performance advantages over traditional non trap rich hr-soi substrates. This is consistent with the expected performance of trap rich rf-soi substrates such as rfesitm substrates. It is an excellent substitute for high-performance low-cost Wi Fi applications.

4. conclusion

With the continuous development of Wi Fi, in order to meet the requirements for larger capacity, higher data throughput and minimum delay in current applications, CMOS or SiGe semiconductor processes on wafers used in previous generations of Wi Fi RF front ends (RFFE) are reaching their performance limits.

Wi Fi 6 (E) can rely on advanced RF front-end (RFFE) semiconductor technology and technology that has been continuously improved for decades. These processes are the result of years of close cooperation between R & D personnel, material suppliers, wafer foundries, design companies, packaging and testing manufacturers, smartphone manufacturers, operators and many other institutions. CMOS on rf-soi optimized substrate is one of the technologies. Soitec’s hr-soi, ifem-soi and rfesitm series products have made great contributions in the process of rf-soi optimized substrate becoming the industry standard of advanced RF front end (RFFE).

Soitec’s ifem SOI is an excellent complement to the rfesitm series of products for the Wi Fi RF front end (RFFE). These choices enable RF front end (RFFE) designers and integrators to strike a balance between appropriate RF performance and cost.

5. bibliography

[1] Cisco, “Cisco Annual Report (2018-2023),” February 2020

[2] Axiros, “The case for WiFi opTImizaTIon,” https://www.axiros.com/the-case-for-wifi-opTImizaTIon available online as June 02, 2020

[3] K. Ben Ali, C. Roda Neve, A. Gharsallah and J. P. Raskin, “RF SOI CMOS technology on commercial trap-rich high resistivity SOI wafer,” 2012 IEEE International SOI Conference (SOI), NAPA, CA, 2012, pp. 1-2.

[4] Claus Hetting, “Europe’s process to release 6 GHz spectrum to Wi-Fi on track, expert says,” Wi-Fi Now, available online as June, 2020

[5] Claus Hetting, “South Korea could become Asia’s first 6 GHz Wi-Fi nation,” Wi-Fi Now, available online as June, 2020

[6] pSemi, PE561221 monolithic SOI Wi-Fi Front End Module https://www.psemi.com/products/wi-fi-front-end-modules/pe561221 , available online as July, 2020 

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