Jeff Baer and Sachin Gupta, Cypress Semiconductor, explain the role of Real Simultaneous Dual Band (RSDB).
Remember the days when GPS did not exist and maps were the essential tool for navigation? Back then, the only source of in-car entertainment was AM/FM radio or physical media like cassettes and CDs. This was a time when cars were primarily differentiated by their drivability. Then came the era when Bluetooth began to be integrated into vehicles for hands-free calling and basic music streaming, and car buyers begin to focus more on in-car functionality when making purchase decisions.
Today, many vehicles offer at least this level of in-car connectivity. However, the rising number of cell phones, tablets, and other smart devices has increased the average consumer’s technological awareness. As a consequence, their expectations are stretching the limits of in-car connectivity requirements.
Infotainment systems have now become a key differentiating factor in automobiles. These systems are highly complex and powerful, including both driver assist systems as well as entertainment support for the driver and passengers. Cars are being equipped with rear seat high definition displays for video. Apps like Apple CarPlay and Android Auto allow each user’s phone to be seamlessly connected to the vehicle’s infotainment system.
As all of these devices are connecting wirelessly in the constrained environment of a vehicle, the available RF spectrum must be used efficiently to maximum possible usage with available wireless technologies. In this article, we will explore in-car connectivity use cases, their technical requirements, and how these can be addressed effectively to provide the best in-car experience.
Infotainment system features are advantages
The following are a few of the use-cases/features of an infotainment system that are either available in today’s cars or being introduced in upcoming vehicles, as well as their advantages:
Hands-free calling: This feature allows users to attend or make calls using controls available on the steering wheel without touching the phone. This feature not only makes it easy to access the phone’s calling function, it also makes driving safer as a user does not need to handle the phone physically.
Ability to read text messages: This feature allows users to read text messages on an infotainment system’s screen using the controls available on the steering wheel without the need for touching the phone. Similar to hands-free calling, this feature also makes driving safer as users do not need to physically access the phone to read a text message. Text-to-voice is often combined with this capability to provide a distraction-free experience.
Audio-synchronisation to play music: This feature allows a user to synchronise a phone’s audio with the infotainment system. Using this feature, music on a phone can be enjoyed over in-car speakers and can be controlled using various buttons available on the steering wheel or the dashboard controls. With wireless audio synchronisation, there is no need to connect an auxillary cable to the infotainment system and music can be controlled by any passenger.
Support for new apps like Apple CarPlay and Android Auto: A car that supports these apps allows the phone’s display to be replicated on the dashboard LCD display, providing easy access to various phone features and commonly used applications like maps, music etc. For example, with support of these apps, navigation can be displayed on the dashboard display, or a YouTube video can be played solely using the dashboard touchscreen. This feature not only allows access to the phone, it also provides an alternative to generally complicated infotainment system user interface, by allowing the user to retain their familiar smartphone interface.
Video steaming to rear seat displays: This feature allows video streaming to rear seat high-definition displays using a car’s LTE modem or a local media device, or through the smartphone’s WiFi hotspot. It can be used by passengers who do not need to concentrate on the road, or children.
Internet access through a car’s in-built LTE modem: This feature allows passengers to access the internet on phones, tablets or laptops using the car’s in-built LTE modem.
Wireless technology needed by infotainment features
Figure 1 shows a high-level block diagram of in-car connectivity. The actual implementation varies on the system architecture. Also, this diagram does not capture use cases that may extend to body electronics (although wireless connectivity is becoming increasingly popular for these use cases as well).
Bluetooth: Bluetooth in vehicles has been around for more than a decade. Features like hands-free calling, reading text messages and audio synchronisation features use Bluetooth.
WiFi: Features like rear seat video streaming and internet access through a car’s in-built LTE modem use WiFi. Apple CarPlay and Android Auto applications started with wired connectivity to the infotainment system but are now moving to wireless and use WiFi for over-the-air connectivity.
Almost all of these features need to be available simultaneously. For instance, when the driver is on a call, maps must be displayed at the same time and rear-seat entertainment must not be interrupted. However, it is important to understand the constraints imposed by limited spectrum available for over-the-air communication. Bluetooth uses the 2.4GHz ISM band for communications as do most WiFi devices. Until IEEE 802.11n, WiFi used 2.4GHz band for communication. Both Bluetooth and WiFi operating in the 2.4GHz band without coordinated coexistence measures in place can result in choppy audio and severely impacted WiFi throughput.
For a better understanding of this issue, consider how Bluetooth and IEEE 802.11b use the spectrum (see Figure 2). Bluetooth has 79 1MHz channels from 2.402GHz to 2.480GHz. WiFi has 20/22MHz channels within the same frequency range. Figure 2 shows what happens when both Bluetooth (red) and WiFi (blue) try to communicate using channels with common frequencies.
Figure 1. Connectivity with Infotainment/Telematics system
The biggest challenge in constrained environments like a car is enabling both Bluetooth and WiFi into communicate in proximity at the same time with high throughput. Active coexistence measures using a Packet Traffic Arbiter can mediate whether Bluetooth or WiFi gets access to the spectrum to achieve better performance for both Bluetooth and WiFi. It is important to understand the quality of coexistence before selecting an approach for automotive applications as basic time multi-plexing based coexistence is very inefficient and not suitable for Bluetooth synchronous links.
Active coexistence can help Bluetooth and WiFi to coexist in the 2.4GHz band. However, this requires both the Bluetooth and WiFi radios to be collocated. Throughput is limited as the spectrum is not available all the time to either technology as they are still sharing the spectrum on a per-packet basis (rather than a fixed time split manner when the radios are not collocated). WiFi takes a maximum hit for in-car connectivity use cases.
For example, the continuous nature of hands-free calling and music streaming significantly impact WiFi throughput because the coexistence packet traffic arbiter tries to give higher priority to HFP or A2DP packets. Packet drop during a call is unacceptable because calls are live data packets.
Data buffering or resending is not an option as everything happens in real time. This is almost the same case with music. Packet drop will result in degraded audio quality and, in turn, impacts the overall user experience. Moreover, 2.4GHz has its own limitations as it is crowded by the relatively large number of devices using it.
The answer to WiFi throughput issues is moving to the 5GHz band. With 802.11n, the 5GHz band became available for WiFi communications. This has enabled WiFi and Bluetooth to coexist without either compromising on performance. Compare this to the nearly 50% reduction in throughput when 2.4GHz coexistence measures are used.
Figure 2. Collision when Bluetooth and WiFi operate in the 2.4GHz band
With 5GHz band operation in IEEE 802.11n and 802.11ac, WiFi throughput is not impacted when Bluetooth is being used for hands-free calling or music streaming. Also, the 5GHz band is less congested than the 2.4GHz band as several other technologies use the 2.4GHz band, including most WiFi devices in use today. Thus, the 5GHz band provides lower packet drop compared to 11n. Even though both .11ac and .11n support the 5GHz band, .11ac becomes the only choice for in-car connectivity due to very high throughput requirement. .11ac uses 256-QAM modulation whereas .11n uses only 64-QAM modulation. .11ac offers 20MHz, 40MHz, 80MHz and 160MHz channels while .11n allows only 20MHz and 40MHz channels. That makes .11ac more appropriate than .11n for in-car connectivity.
WiFi devices that support 2.4GHz and 5GHz for WiFi connectivity are known as dual-band WiFi devices. As a term, dual band is the most commonly misunderstood as a device that can transmit at both 2.4GHz and 5GHz at the same time. In reality, these dual bands can’t be used at the same time in most devices. Rather, the system multiplexes between 2.4GHz and 5GHz traffic. In this way, a device can switch between 2.4GHz and 5GHz bands to support devices that use 2.4GHz and 5GHz channels respectively. However, this impacts throughput significantly. Due to time multiplexing and the delay of switching from one mode to another, effective throughput is less than 50% of what could be achieved if both bands were active all the time. This kind of implementation is also called virtual simultaneous dual band. For automotive use cases that require several devices to be connected for video steaming, hands-free calling and data access, there is a need for Real Simultaneous Dual Band (RSDB) WiFi.
Real simultaneous dual band: Real simultaneous dual band means that a device can communicate using both 2.4GHz and 5GHz bands at the same time. RSDB ensures 100% bandwidth utilisation based on each band compared to less than 50% in the case of virtual dual band implementations. For example, a 2.4GHz 20MHz channel provides PHY rates of 72Mbps, resulting in TCP throughput of 50Mbps. A 5GHz 80MHz channel supports PHY rates of 433Mbps, resulting in 300Mbps TCP throughput. With both a 2.4GHz 20MHz channel and 5GHz 80MHz channel, a RSDB implementation provides 50Mbps and 300Mbps throughput for these bands with an effective throughout of 100% compared to what can be achieved using separate single band operations.
Support for RSDB is defined by the hardware’s architecture. A true RSDB implementation requires dual MAC, PHY, and radio hardware to allow both bands to operate concurrently. Newer devices that use 5GHz for their operation can make the best use of available spectrum without worrying about Bluetooth that operates at 2.4GHz and takes priority for various activities like phone calls and music. The requirement for RSDB is primarily driven by the fact that many consumer devices still use 2.4GHz for WiFi. For optimal performance, it is required that both the 2.4GHz and 5GHz bands are available at the same time. With RSDB, 2.4GHz can be used to connect to phones, tablets, and laptops that support only 2.4GHz WiFi operation.
Also, internet access for data can be time-multiplexed with Bluetooth since providing priority to Bluetooth for this use case does not impact the user experience significantly while 5GHz band can be used for rear seat displays and apps like Android Auto. Thus, a combination of efficient coexistence and RSDB is the key to providing the best user experience (i.e., always available 5GHz communication with 2.4GHz WiFi communication and Bluetooth to coexist on 2.4GHz band for in-car connectivity). Such solutions are becoming available on the market. For example, the
CYW89359 from Cypress is single-chip RSDB WiFi + Bluetooth solution that provides a parallel coexistence interface for an efficient coexistence solution.
For mid-end and high-end cars that are equipped with all this sophisticated technology to connect everything in their car, reliability, performance, and interoperability are of the utmost important. Thus, it is critical for OEMs to choose a hardware platform that can provide the required throughput with a guarantee to work with other devices like mobile phones, PCs, tablets, and laptops.
Cypress provides 11ac RSDB + Bluetooth combo devices that can support either 2.4GHz and 5GHz SISO (Single Input Single Output) simultaneous operation + Bluetooth, 2.4GHz 2x2 MIMO (Multiple Input Multiple Output) + Bluetooth, or 5GHz 2x2 MIMO + Bluetooth. The most widely deployed WiFi and Bluetooth stacks are supported. 2.4GHz and 5GHz SISO with Bluetooth is the most desirable use case for infotainment systems where some of the devices use 5GHz and some use 2.4GHz. However, in use cases where only one band is needed for communication, 2x2 MIMO can be used so that the system can transmit and receive data using multiple antenna and provided two-times throughput for the given band compared to a SISO operation.
The automotive industry is going through an in-car connectivity revolution. Multiple devices need to be connected simultaneously and wirelessly. In-car connectivity has evolved from basic hands-free calling to needing to support sophisticated infotainment systems. Throughput requirements have increased as well, require enough throughput to support multiple displays, internet access, screen sharing, and Bluetooth connectivity.
The 11ac RSDB WiFi standard is the only protocol able to address the continuously growing demand of throughput. To provide reliable and efficient connectivity to all technologies available in a car – Bluetooth and WiFi in most cases – a WiFi + Bluetooth radio implementation is the most efficient and economical solution. As coexistence is handled by the device itself, OEMs do not need to invest a lot of time and money addressing coexistence issues that have already been solved. Selecting an industry-supported approach provides the backbone for commercially successful in-car connectivity as it already works with every WiFi or Bluetooth device that could ever be intended to be connected. In addition, this approach reduces design investment and time, enabling these resources to be spent on interoperability testing.