Understand connectivity options, optimal usage models for your next mobile handset design
Find out what your options are and which ones are best for your mobile design
Wireless headsets, affordable wireless internet, financial transactions using the handheld device, convenience and ease of use when transferring small amounts of data from mobile to mobile or large amounts of data between the mobile device and PC/laptop, list but a few of the attractive applications associated with the handset today.
Enabling all of this functionality requires different connectivity technologies, both wired and wireless, to support these required features and their associated peripherals. These include, among others, Bluetooth, Wi-Fi, IrDA, and USB. There are various tradeoffs with these technologies with respect to performance, power, and ease of use. Different usage models tend to suit certain connectivity options over others. This article will compare these connection methods and describe the benefit of each in its particular application space. It may also act as a guide for mobile handset architects to make better choices in their designs.
As the mobile handset moves closer and closer to becoming the all-encompassing communication and entertainment tool for the generations of today and tomorrow, handset manufacturers are on the constant prowl for functionality that adds the ease of use and convenience we've come to expect in today's information filled hectic lifestyles. Because of this, we, as consumers of these devices, are becoming increasingly demanding.
We want (and may be required) to be available 24/7, and as we spend more and more time on calls, whether in our cars, on public transportation, at the gym, or simply while multi-tasking in today's fast paced lifestyles, convenience becomes a key requirement, and something we are willing to pay more and more for. It is convenient, and can be safer, if we do not have to touch or be preoccupied with our mobile handset in order to make/receive calls, hence the demand for hands-free wireless headsets. As we become more and more dependent on the internet we want to be connected all the time in order to access email and have all the information we have when using our traditional PCs at our fingertips, and at the lowest cost possible. In Europe and Asia the idea of paying for goods and services at point-of-sale (POS) with our handsets is catching on, and service providers are only too happy to oblige particularly if it helps to increase the Average Revenue per User (ARPU). Also being able to exchange business cards and other contact information by simply bringing our handsets in proximity with the desired recipient provides a certain level of convenience over entering the data manually. With the integration of cameras and music players into mobile handsets, we expect the same experience and performance that we are traditionally used to with stand-alone counterparts. These examples account for a lot of what we desire today from our handsets and we can only begin to imagine what the future holds!
Each of the examples mentioned above requires its own unique connectivity option that has been optimized for that particular application: Bluetooth provides the most optimal connection for wireless headsets, Wi-Fi for the faster and lower-cost internet access that we are used to from our PCs, IrDA for close-proximity secure transactions, and USB for transferring large amounts of picture and music files to/from our PCs. Handset designers are therefore on the constant look out for ways to make their handsets better, at the expense of having to be familiar with all of the underlying technologies that enable these features.
Table 1. Connection options comparison
Bluetooth has become the standard for wireless headset connection. You would be hard pressed to find a high-end phone these days without Bluetooth capabilities, and as time goes on Bluetooth will find itself in more and more low-end phones as well. With Bluetooth integration into common mobile handset processors, virtually all handsets will eventually have Bluetooth capabilities. Buoyed by its popularity and growth in the handset market, Bluetooth is expanding into other devices such as laptops, for wireless modem and data connection to mobile handsets, and automobiles for hands-free calling. The goal of Bluetooth is to provide a cheap solution for short-range wireless voice and data connectivity to all gadgets in a simple and easy-to-use manner, while consuming as little power as possible. This is a tall order, and the technology has not been without its share of problems.
While Bluetooth is dominating in the short-range voice application space, it has yet to make a real impact in regards to data transfer. There are several reasons for this, the most important one being ease of use. When someone buys a Bluetooth headset for his or her phone, the most ideal scenario would be to be able to open the package, turn it on, and it works. This is, unfortunately, not the case. In reality the user has to go through a syncing process which requires menu navigation and mode selection on the phone (which is different from phone to phone) as well as mode selection on the headset. Not the easiest scenario but manageable by anyone who can read directions (and wants to take the time to figure it out). However, if the user is trying to connect to their PC using Bluetooth for data syncing purposes, the complexity increases dramatically. The user must create a Bluetooth Personal Area Network (PAN) on the PC, set the PC to search for Bluetooth enabled devices, set the phone to be in "visible" mode, and if they don't see each other (which they probably will not at first) there is a slew of troubleshooting steps that must be taken. The user almost has to be an IT professional to get it to work, and it certainly is not a task you would give to your grandma.
Another shortcoming of Bluetooth when it comes to data transfer is the data rate. The maximum data rate is 1 Mbps for version 1.2 and 3 Mbps for version 2.0. This data rate is suitable for data syncing to a PC data such as user contacts and calendar information. It also will work for phone-to-phone communications, to transfer a business card for example. Depending on the bandwidth of the service provider's network, it may also be sufficient for modem connectivity to the laptop. However if the phone has an MP3 player and the user would like to transfer gigabytes of music to the phone, such a transaction could take hours. The frustration of waiting this long for the transfer to complete will far outweigh the convenience of the connection being wireless.
Bluetooth power consumption varies heavily depending on the mode of operation, the transfer type, and the silicon being used. One thing that all users will agree on is that there is a noticeable difference in the battery life of the handset when using a Bluetooth headset. On average Bluetooth can consume 45mA while active, but the real problem is that it still consumes around 15mA when not active, i.e. in standby mode, to remain linked to a device. The result is that those users who leave their Bluetooth headset on constantly linked in anticipation of a phone call will see a very noticeable reduction in the battery life of the handset. Another artifact is that the Bluetooth headset may not last the entire day between charges, depending on how heavy your voice usage is. This is a real problem and Bluetooth silicon manufacturer are working feverishly to bring these power numbers down. Some processors are even absorbing the Bluetooth processing to help out. Turning off the Bluetooth headset between calls is not an option because of, as mentioned earlier, the lengthy handshaking process required when the Bluetooth headset is initially powered on. This sequence is far too lengthy to have to go through while your phone is ringing from an important caller.
Wi-Fi, which is based on IEEE 802.11 specifications, was originally developed with the goal of creating wireless internet connectivity to the LAN via a laptop PC. With Wi-Fi chipsets shipping in every new laptop PC today, it has become ubiquitous in business office environments, enabling users to now roam around the office with their laptop and connect to the Internet and email wirelessly from whichever conference room or office they choose. With wireless routers being affordable for quite some time now, wireless home networking is quickly reaching saturation as users move away from being tethered to a desk in order to access the internet. Security has been improved over time and users now feel comfortable that no one is snooping their data while they are connecting wirelessly. Wi-Fi data rates are also comparable to their wired counterpart so there is no reason for consumers and businesses to not go wireless.
With ubiquity in its initial target market, Wi-Fi is now finding its way into new applications such as handsets in various roles. For example, users could use cellular data networks, such as HSPA, while they are out of the home/office and outside the range of an access point or 'hotspot' to access the Internet, switching over to their higher bandwidth Wi-Fi network as soon as they enter their home or office or come back into range. This provides always-on connectivity, but with lower cost implications than using cellular data networks alone. With VoIP capable handsets, users can switch over to the Wi-Fi network when in their home/office networks for voice calls also, thereby cutting down the cost of minutes used when within range of an access point. This is especially attractive for businesses whose 'always available' employees spend a lot of money on long-distance calls for their global interactions.
With regard to usability, however, Wi-Fi suffers from similar symptoms to Bluetooth in that there is a somewhat lengthy connection process involved, especially when first connecting to a new and secure network. Generally this pain is eased for each subsequent connection, but in the ideal model for mobile handsets, users should be able to connect seamlessly whenever they come into range of an access point. This means that wherever they go, if a handset can connect, it should switch over and connect seamlessly, without any user intervention. The benefit of this solution to the consumer is immense as it provide access to higher bandwidth more of the time and could cut down significantly on the cost of long distance international calls.
Wi-Fi data rates can be described by referring to the letter appended to the standard to indicate the different specifications (it is worth mentioning that these specifications vary by channel usage, radio technology, and other factors also). 802.11b provides data rates between 5.5 and 11Mbps, 802.11a 6-54Mbps, and 802.11g up to 54Mbps. 802.11n is currently in development and has not yet become a mainstream technology, but it is promising data rates of 300-600Mbps. Comparing this to the bandwidth that the cellular data networks based on technologies such as HSPA, which is promising peak data rates of 28Mbps downlink and 11.5Mbps uplink in the 2008 timeframe, and LTE, which is promising 100Mbps in the downlink and 50Mbps uplink in the 2009 timeframe, it can be seen that Wi-Fi will keep its competitive position particularly if it can provide similar or greater bandwidth at a fraction of the cost.
Power consumption, especially for a battery-powered application is an area where Wi-Fi, although improving, is found wanting. Looking at a datasheet from a major Wi-Fi chip player reveals power consumption in the order of 90mA receive and 125-190mA transmit. Using higher data rates will of course consume more power, and therefore Wi-FI in today's handsets tends to be somewhat crippled in order to achieve the optimum bandwidth versus battery life. Advances in this area will need to be made to keep Wi-Fi competitive in handsets, but these problems are being overcome as we can now see a whole slew of handset products in the market boasting Wi-Fi support, a number that will continue to grow this year.
Infrared Data Association (IrDA) is a connection technology gaining popularity in European and Asian markets, largely due to point-of-sale (POS) applications. It is extremely convenient for passengers to be able to pay for their morning commute subway train tickets simply by bringing their phone close to a sensor as they walk through a turnstile, rather than queuing for their tickets beforehand. While infrared communication technology has been around for some time now, popularized by remote controls, it has only recently been seen penetrating the mobile handset market.
IrDA is a point-to-point, low bandwidth, short range, line-of-site, communications technology. It allows two devices to transfer data at distances up to 1 meter with data transfer rates of up to 16 Mbps, under ideal conditions. The two devices must have an unobstructed view of each other, which has advantages and disadvantages. Obviously, holding the device in place for a long period of time could be annoying to the user if there was no way of setting the device down, and still keeping an unobstructed line-of-site. For this reason it is not an ideal technology for passing large amounts of data. With other wireless and wired connection methods this is not an issue because line-of-site is not an issue. IrDA does, however, have a distinct advantage over other wireless technologies when it comes to security. Data is not being transmitted in all directions for anyone standing in the vicinity to pick up, as it is with Bluetooth and Wi-Fi. With IrDA, users knows who is receiving the data because they must provide line of sight. This is especially important when executing a POS transaction as the user's personal information is being transmitted. People would think twice about using this method to make payments if someone standing around the corner could virtually pick their pocket by stealing their information wirelessly!
Power consumption of the IrDA transceiver can be classified into three categories: shutdown, standby, and transmitting. During shutdown, power consumption is negligible; however, the user must turn it on (often by navigating multiple windows) in order to use it. Most users would prefer to leave it on so they can receive connections from other devices at any time and initiate connections quickly. During standby the user may receive and initiate connections, and the power consumption is approximately 100μA, having minimal effect on battery life. When transmitting (a connection has been made) the power consumption jumps up to at least 100mA, and even higher as the data rate increases. For applications where connection times are short, such as a POS transaction or a contact exchange with another handset, this is acceptable. But for applications where a long connection time is needed, such as modem connectivity or music/picture file transfers, the battery would be drained in no time.
USB began its life as a connectivity method to replace the serial (RS-232), PS-2 (mice and keyboards), and parallel ports (Centronix and IEEE-1284 for printers) on the PC with a single universal connector and underlying protocol. Due to its ubiquity today in the PC world, and the continuous efforts of the USB-IF to add features and stay abreast of the industry and market needs (such as the definition of mini-, and now micro-USB connectors for handheld applications), it has become the interface of choice in order to connect the handset to the PC, especially for Mass Storage transfers. The demand for being able to connect one's handset to the PC comes from the need to be able to transfer files and synchronize contacts quickly and easily, which has in turn arisen from the handset integrating features such as digital cameras and portable music players, with the expectation from consumers that their experiences will be the same with the handset as it is with discrete counterparts such as digital still cameras and PCs.
In terms of usability, USB tops the list among the connection options described here. You just plug it in and it works. That was not always the case and most of us will remember having to manually install drivers to recognize our USB peripherals. But those days are long gone now and USB is ubiquitous due to its ease of use and plug-and-play capabilities. Everyone, including grandma, can use it.
In terms of data rates, FS-USB at 12Mbps has been shipping in handsets for quite some time now, providing a low cost way to add USB functionality to the handset. However the move to HS-USB at 480Mbps is well and truly upon us as users expect to move large amounts of picture and music files between handsets and PCs with the same experience in terms of performance they are used to with iPods and digital cameras. Innovative implementations improving the performance of Mass Storage transfers such as Cypress Semiconductor's West Bridge Antioch device shown in Figure 1 are also materializing. This implementation provides an optimized path from PC to Mass Storage (Flash, SD card, HDD, etc) without processor intervention, enabling users to download large files in the blink of an eye and faster than has been possible to date. And because this implementation is targeted at mobile handset applications, small form factor and low-power features come standard.
Figure 1. West Bridge Antioch application implementation in a cellular phone
Wired USB
With wired USB, power consumption is generally not of concern as power is provided via the host through Vbus/GND in the USB cable along with the signal pair. USB ports on hosts such as laptop PCs are required to provide up to 500mA to a connected and active downstream peripheral. Therefore, when the handset is connected to the PC and data is being transferred, the current requirements for the USB silicon can be supplied by the upstream host. When the handset is not connected to the PC, the USB silicon is put in standby mode with the power consumption dropping to the μA range. Also, there is never a need for USB silicon to be active when not tethered so parts of the USB data path such as the PHY can be powered from Vbus only. That is, when Vbus is not connected, power is removed completely from the PHY and the current consumption could potentially go to zero. Another advantage of being tethered and having access to 500mA is of course that the excess current can be used to charge the handset battery, which is really convenient as users can charge the battery in their handsets from any PC, with one caveat. The current USB specification does not provide a method for efficiently charging dead batteries, i.e. USB devices must be powered up and able to enumerate with the host in order to request the full 500mA. If they cannot do this, they can only draw 2.5mA until such time as they can enumerate and request the full 500mA, which would make the charging process take longer than necessary (consider that typical wall chargers provide charge currents in the 1-1.5A range). The USB-IF is currently working on an addendum to the USB 2.0 and OTG specifications which will define, among other things, a dead battery waiver as a way to overcome this limitation. This specification should be completed this year. China has also created its own similar specification to deal with charging the handheld device battery using USB, and its specification is already complete. You might think that handset OEMs implement charging via USB today, and handsets can be charged via a host USB connection or with the supplied wall chargers with USB connectors, and you would be right. But these handsets use proprietary mechanisms, and will not charge efficiently if the battery is fully discharged, i.e. if the battery is dead (defined as the USB silicon cannot power up to enumerate with the host) the phone will seem to take forever to charge (at least up until the point where it has enough charge to power up and enumerate). And because they use proprietary mechanisms to detect a wall charger, a wall-charger from one OEM cannot be used to charge another's phone, and vice versa. Moving forward handset vendors will move to support the amended USB specification in order to get USB certification. The USB-IF is also defining a standard way for the handset to identify what it is connected to via USB, for example high current wall chargers. This will enable OEMs to use standard low-cost USB wall chargers that will undoubtedly materialize as a result of the open specification, as wall charger manufacturers can offer the same product to many OEMs.
Wireless USB
Certified Wireless USB is worth a quick mention as it will make USB connections wireless. This approach needs to achieve the same level of penetration in PC and PC chipsets before it will really become a serious option for handsets so it is quite some time away, considering its wired counterpart was first introduced in 1995! Of course, users will not the have the advantage of being able to charge their phones with Certified Wireless USB. One thing it does bring, which may lead to faster adoption in handsets, is the fact that both Certified Wireless USB and Bluetooth 2.0 use ultra-wideband (UWB) radio technology. This means that the same radio could support Bluetooth and Certified Wireless USB in different usage modes at different times. This kind of convergence could be key to allowing the handset continue its march to becoming the all-in-one communications entertainment device.
Summary
With the multitude of diverse features being integrated in today's handsets comes the many different connectivity options that must be supported, each optimized for a particular feature and/or application. Handset designers must familiarize themselves with the pros and cons of each of these technologies in order to understand and account for the trade-offs of performance, power, and ease of use associated with each in order to design the most feature-rich, cost-sensitive handsets for their target markets.
Some of the connectivity options in use today were designed with another primary application in mind and are usually adopted by handsets as an after thought. As the handset strives to become the all-encompassing portable device to support our communication and entertainment needs, handset customers and service providers will not wait for handset optimized new specifications. Rather than re-inventing the wheel, handset designers must use the connection options already defined for that feature or function and adapt as necessary. This may change as we move forward and is already evident in the fact that both Bluetooth 2.0 and Certified Wireless USB both use an UWB radio. This will potentially allow for a single radio to enable two connectivity options in different usage scenarios. This is a good example of how convergence may reduce the design complexity of handsets and allow for more efficient and cost-effective solutions.
About the authors
Ray Casey is a Strategic Product Marketing Manager in Cypress Semiconductor's Consumer and Computation Division. His focus is on strategic product definition. Ray received his B.Eng. in Electronic Engineering from the University of Limerick, Ireland, and is today based in San Jose, CA. Ray can be contacted at rev@cypress.com.
Triton Hurd is a Senior Applications Engineer in Cypress Semiconductor's Consumer and Computation Division. His focus is on HS USB peripherals with an emphasis on its application to mobile handsets. Triton received his BS in Electrical Engineering from Cal Poly, in San Luis Obispo, CA. He is currently based in classy San Diego, CA and hopes it stays that way. Triton can be contacted at teh@cypress.com.
