The smartphone represents one of the most significant shifts in our world. In less than thirteen years, we went from some people owning a dumb phone to the majority of the planet having a smartphone (~83.7% as of 2022, according to Statista). There are very few things that a larger percentage of people on this planet have. Not clean water, not housing, not even food.
How does a smartphone work? Most people have no idea; they are insanely complicated devices. However, you can break them down into eight submodules, each of which is merely complex. What makes them work is that each of these components can be made small, at massive economies of scale, and are tightly integrated, allowing easy assembly.
So without further ado, the fundamental eight building blocks of the modern cellphone are: the application processor, the baseband processor, a SIM card, the RF processor, sensors, a display, cameras & lenses, and power management. Let’s have a look at them all, and how they fit together.
When you think computer or, by extension, smartphone, you think of a single cohesive unit powering what your smartphone does. However, even putting aside the universal multicore processors, the processing inside your phone is not a single voice but a cacophony of voices chattering away at each other. The application processor (AP) is what we think of as this traditional brain. It features a large multi-core CPU, almost always ARM-based.
These APs might come as part of an extensive system on a chip (SoC). The SoC can include memory, the GPU, digital signal processors, image processing units, the modem (which we will get to later), and other minor things like video decoders and AI engines.
But back to the CPU. It runs your phone’s operating system and is primarily the ring leader in your phone’s decisions. These days, it often has big powerful cores that consume much more power and small cores that consume much less energy. This is usually done by making the smaller cores single dispatch, in-order, while the larger cores are massive multiscalar out-of-order cores. We could spend quite a long time breaking down the various parts of a modern processor, but we think you might learn better by making your own or fixing a broken simulation.
The baseband processor (BP) is an entirely separate processor that the AP can talk to, often referred to as a modem. Even the iPhone, which has its own custom AP, uses a Qualcomm or Intel BP. Generally, Wi-Fi, Bluetooth, and other radio-type communications are handled by the AP and the BP focuses entirely on cellular communications. Even the specs for the ancient 3G protocol are long and dense, with each revision adding intricacy on top. Complex power amplifiers, multiband-multimode transceivers, and other techniques add to a complex circuit.
Baseband processors have their own firmware and RAM, and the codebases are opaque manufacturer-specific binaries. There’s a good talk on hacking an old 3G USB modem. In fact, some of the Intel modems in some iPhone models had x86 processors in them. [Comsecuris] detailed a buffer overflow they found in many iPhone modems that gives some helpful information on the internal workings.
We think of them as just being storage, but the small piece of carrier-branded plastic with the handfull of gold contacts has a small chip inside it. It’s all according to spec, but the interactions between AP, BP, and SIM cards has grown quite complex. SIM cards come into the security limelight, as they are updated over the air with little control from the user. They can talk to the BP without the AP knowing. Can you imagine your phone calling or texting someone, and the only way you would know would be by auditing your phone records? Your phone already does so. We’re slowly moving away from SIM and towards eSIM, but that’s a ways off for most of us.
Sensors make your phone much more aware of its surroundings. Cameras, GPS, microphones, distance sensors, proximity sensors, gyroscopes, accelerometers, altimeters, magnetometers, LIDAR, fingerprint sensors, radar sensors, ambient light level sensors, and pressure sensors are all common additions to adorn our smartphones.
The amount of data streaming in can be truly astounding. Many of these sensors have their own microprocessor to handle the load and present a more cohesive view to the AP. In fact, some phones have coprocessors to handle these different sensors in more power-efficient ways. So we have the AP talking to a coprocessor, talking to microprocessors, and reading sensors. All these sensors go a long way towards making a smartphone much more useful in day-to-day life. You can call, get directions, rotate for portrait mode, or use your phone for physics experiments.
Smartphone screens are a marvel. They have a resolution that matches or beats many household TVs while being high quality and affordable. Used phone screens are often used in projects here on Hackaday as they’re easy to find and then drive. We see advancements here with OLED displays and microLED displays. As the screen is often the front of your phone, having it heal itself would be pretty handy. There’s nothing exceptional about the displays in smartphones as they’re just miniaturized versions of larger panels.
Modern cameras on smartphones are crazy.
As an example, modern smartphone lenses are crazy. Here's the design used on the iPhone 7; every single lens element is highly aspherical with a very large number of correction coefficients; this lens completely breaks the existing poly optics implementation from the 2016 paper. pic.twitter.com/cYr9qsa5Bj
— Yining Karl Li (@yiningkarlli) February 27, 2022
Lenses and sensors are unique in many ways. Unlike displays, where you can shrink the process, light and lenses get weird when they’re small. So, how do you capture more light in a smaller area? An incredible (though long) resource on lens design is here that details how we do that. The lens design is a complex art with a long history of iteration and improvement since the early 1800s. There are several notable inventions listed in the link above. We could do a whole article just about camera lenses. In particular, the long exposures of old cameras had just as much to do with lens design as it did film.
The lens and sensor are going crazy, but the Image Signal Processor (ISP) that processes the photos is going crazy as well, in some cases blatantly lying. Like a traditional Signal Processor, the ISP takes an image and processes it, applying some transformation. However, while some filters are done on the AP — think the Snapchat filters — the actual reading of the sensor and forming a picture are done by the ISP.
Phones are mobile and expected to last a long time on battery. Like other battery-powered products, there’s a battery management system (BMS) that supports the battery. However, there are some pretty high demands placed on it. It has to charge wirelessly and at incredibly high rates and often. On the flip side, efficiency is considered in all the other parts that we’ve discussed. There are power modes on every module. Every component is scrutinized against the budget, both monetarily and against the power budget.
Put it All Together and Throw it Away!
In our modern society, we replace our smartphones frequently, and our world seems to have no end to our hunger for them. So with more and more discarded smartphones on hand each year, packed with features and processors, why aren’t we hacking on them more? They make a perfect companion for your 3D printer.
There have been some attempts to make smartphones modular, allowing you to swap out the baseband processor, the display, or the camera. However, the concept seems to have stalled. Particularly the application processor, baseband processor, and sensor unit are so tightly connected in many modern phones, that swapping out any of the component parts would be like redesigning the whole thing from scratch. Smartphones are valuable because of their scale and tight integration, and it’s just hard to beat that.
Still, it’s incredible how far we’ve come in smartphones in general. We at Hackaday can’t wait to see what the next decade holds.