Summary
Today, smartphones are an integral part of our daily lives, and just as important is WiFi connectivity. WiFi performance varies among mobile phones of different brands as implementation differences can impact speed, signal range, and latency. With new virtual reality/ augmented reality (VR/AR) applications, real-time gaming, and increased use of smartphones as mobile hot spots, WiFi performance testing is as important as ever.
In this article, we seek to examine some common indicators used for assessing mobile phone WiFi performance, and the causes of certain issues. Then, from the perspective of testing, we discuss how mobile phone manufacturers avoid quality issues with WiFi performance throughout the manufacturing and testing processes.
Introduction
Currently, cellular technology and IEEE802.11 wireless LAN technology (or more popularly known as “WiFi”) are two key technologies used within mobile phones to connect to the internet. WiFi works in bands that do not require licensing, is relatively low-cost, fast, easily deployable, and widely applied. WiFi is the connection method of choice for users at home, in shopping malls, hotels, offices, public facilities, etc.
Available information tells us the amount of data received and transmitted by mobile phones via WiFi connections is far greater than the amount received and transmitted via cellular networks.
Figure 1: Data received/transmitted via mobile networks and through WiFi connections. (Image Source: Cisco)
Thus, the performance and quality of WiFi components will have a significant impact on smartphone user experience, making it important that all mobile brands have adequate testing procedures in place to ensure the quality of WiFi implementation. Therefore, determining how to enhance mobile WiFi performance is critical for mobile phone makers. Some of the fundamental WiFi issues include:
1. Poor Signal Reception
Sometimes, two phones from different makers can be at the same location, alongside each other, but show differing signal strengths. The user holding the phone showing a weaker signal will then complain their phone has poor signal reception. This is related to the performance of the WiFi receiver inside the phone. When the WiFi component is connected to the router, the RF receiver will measure the data packets broadcasted by the AP and use these measurements to calculate the RSSI (received signal strength indicator) value. The signal strength is then indicated through the operating system based on the RSSI value. Under normal circumstances, the closer the mobile phone is to the router, the higher the RSSI value (or the stronger the WiFi signal).
Conversely, the farther the phone is from the router, the lower the RSSI value and thus the weaker the WiFi signal. RSSI and signal strength values measured by calibrated and tested phones are consistent with actual received values. These types of phones also perform better in terms of the reception of downlink signals. On the other hand, RSSI and signal strength values measured by uncalibrated and untested phones are inconsistent with actual received values. Such phones also perform poorer in terms of the reception of downlink signals. Thus, uncalibrated and untested phones will not only show a weaker WiFi signal, but also perform worse in terms of internet speeds and signal range compared to phones that have been calibrated and tested.
Internet speeds might also be low despite the indication of a strong WiFi signal strength (more than one “bar”) on the user’s phone. Sometimes, there is a virtual disconnection. This has a grave impact on user experience. If the phone indicates that there is a signal, it means the receiver is working. The issue here is with the transmitter’s power level inside the phone. Transmitter power level is critical for mobile phone performance. If the transmitting power is too high, the phone battery will drain very quickly and thus require the user to charge the phone more often. If the transmitting power is too low, uplink signals will not be able to reach the router’s receiving antenna after path attenuation. This will, in practice, limit the working area of the mobile phone. Thus, stringent WiFi calibration and testing on transmitting power are needed during phone manufacturing to ensure the accuracy of the transmission power.
Wireless data flows in two directions, stringent calibration and testing of WiFi transmitting power and receiver sensitivity during the phone manufacturing process can help ensure that the performance of the uplinks and downlinks of the phone are compatible to the greatest extent.
Figure 2: The impact of the calibration and testing of mobile phone transmitters and receivers.
Consider a third signal strength scenario: Since WiFi typically operates in free-to-use frequency bands, they can become more congested. Hence, experienced users can switch their routers to frequencies with less interference. However, when the user connects to the router using a different frequency, they can sometimes find that signal strength and internet speeds are compromised as a result. This is because WiFi signals mainly operate at 2.4 GHz and 5 GHz frequency bands. Each band can be further divided into smaller sub-bands. For instance, the 2.4 GHz band can be divided into 14 smaller sub-bands with frequencies ranging from 2.412 MHz to 2.484 MHz, while the 5 GHz band can be further divided into as many as 24 smaller sub-bands with frequencies ranging from 5.180 MHz to 5.825 MHz. Mobile WiFi component performance can vary dramatically from sub-band to sub-band. Therefore, over the course of in-production test, calibration and testing should be performed for multiple sub-bands in these two frequency bands.
2. Slow Internet Speeds
WiFi is a wireless local area network (LAN) technology with coverage area dramatically smaller than that of the cellular network. Device locations are typically fairly fixed. Thus, the frequency selection and spatial orthogonality for WiFi in the wireless channel are easier to handle compared to cellular networks. As such, WiFi product designers can make better use of the high-bandwidth and multi-antenna technologies to significantly raise internet speeds. Future mobile platforms will be capable of using channels with even greater bandwidth and an even more MIMO antennas.
However, if these new features are neglected during in-production test, there is the possibility internet speeds will remain low in actual use. For instance: many early test instrument models only supported signal generation and demodulation below 80 MHz or 40 MHz levels. As a result, certain high-speed modes could not be tested during in-production test, and product designers were unable to know if there were any product performance issues in high-speed mode. This presented the risk of poor quality following product launch. Some manufacturers have resorted to frequency spectrum splicing in order to obtain transmitter test results for mobile phone RF modules. However, as the sampling is not in real time (i.e., taken at different points in time), the results obtained are not an accurate reflection of signal characteristics and will contain errors and deviation to a certain degree.
Figure 3: Various bandwidth-and-MIMO-antenna-number combos.
Similar issues exist with MIMO testing. Early test instruments typically contained only one VSA/VSG combo and were thus able to test only one RF chain at one time. With MIMO-enabled mobile phones, time-division switching had to be used to test each and every RF chain in sequence. Manufacturers using this method were unable to determine the actual DUT performance in MIMO mode.
3. High Latency
With certain hot new internet applications, such as online games or AR/VR, internet response time is critical due to the real-time and interactive nature of these applications. The lower the latency, the better the gaming experience; Sometimes, high latency can even make games unplayable. Hence, latency is a key indicator in assessments of internet performance in mobile phones. There are many factors that can cause latency. IEEE 802.11 works on the MAC and physical layers, under the TCP/IP protocol that makes use of the collision avoidance mechanism in CSMA/CD. Thus, latency in WiFi networks is mainly due to poor mobile phone signal quality or multiple retransmissions caused by interference from other WiFi devices. Poor phone transmitter performance can also lead to an increase in downlink retransmissions.
The main indicator used for evaluating the quality of signals from mobile phone WiFi transmitters is the error vector magnitude or EVM. EVM is a reflection of the degree to which transmitter signal IQ modulation has deviated from the ideal value. The higher the value of EVM, the poorer the quality of the signal. As a result of wireless channel attenuation, it is very challenging for the router receiver to modulate such signals. This leads to packet loss and retransmission that in turn leads to higher latency. Therefore, we expect the signal from mobile phone transmitters to be as good as possible to ensure the stability of transmissions and low latency. Over the course of the mobile phone manufacturing process we use the vector signal analyzer (VSA) within the instrument to demodulate and analyze the RF signals transmitted by the mobile phone in order to obtain the EVM value.
The EVM value is a reflection of the noise performance of the RF chain. If an instrument’s baseline noise is high, this will add to the detected signals and thus the accuracy of the EVM reading will be affected. Generally speaking, the baseline noise of the instrument should be lower than approximately 10 dB as specified for EVM. For instance, 11ac 256QAM specifications set the EVM threshold to be -32 dB.
For the VSA, baseline noise has to be lower than -42 dB for relatively accurate EVM readings to be taken. Specifications for the next-generation 802.11ax standard require support for 1024QAM modulation and an EVM threshold of -35 dB. Apart from EVM, frequency error and symbol clock error are another two key indicators for mobile phone WiFi signal quality. Test instruments are needed to measure these two indicators.
Figure 4: EVM specifications for different modulation orders.
One key cause of WiFi interference between mobile phones is interference between adjacent channels typically caused by channel leakage between adjacent channels. The adjacent channel leakage ratio (ACLR) indicates the ratio of the transmitted power to the power in the adjacent radio channel. If the phone’s adjacent channel suppression capabilities are poor, there will be significant interference caused to the surrounding environment, thereby increasing retransmission across the entire network and in turn raising latency and lowering throughput rates. In the 802.11 specifications, the key mode of assessing the spectrum performance of mobile phone signals is the transmit spectral mask indicator. The transmit spectral mask test function of test instruments is needed to measure this indicator for the mobile phones. Doing so will ensure that signal interference by products in adjacent channels during actual use is minimized.
The performance of the mobile phone receiver is also a primary determinant of latency, as a well-performing receiver can correctly receive the data packets from the opposite party. If receiving fails, retransmission will occur; and data transmission speed will decrease, thus causing higher latency. The key way of evaluating receiver performance is to use the test instrument vector signal generator (VSG) to generate the standard WiFi signals for various modulation and coding schemes (MCS) and then to determine how well the receiver is performing by measuring the subsequent packet error rate (PER) with the mobile phone receiver. With VSGs, there is a need for a wider dynamic range and more stringent requirements with regard to baseline noise.
Conclusion
Today, mobile phones are an indispensable part of daily life. Various major smartphone manufacturers are in a continuous process of technological development and innovation. However, regardless of what new features or functions are added to smartphones, the WiFi components within continue to handle most of the internet data received/transmitted. As such, the performance and quality of WiFi components can influence the user experience and the maker’s branding. In this article, we have taken a technical approach in looking at the causes of certain issues associated with some common indicators used for assessing mobile phone WiFi performance such as signal strength, speed and latency. We also approached the issue from the testing angle and comprehensively discussed how mobile phone makers can perform complete and accurate testing in the course of production to minimize the occurrence of these quality issues.
