Mobile Sensing: WiFi Indoor Localization

Introduction

In recent years, the issue of location has become increasingly important. Services such as Google Maps have made it easy to navigate and locate specific points anywhere in the world using satellites. As these systems already work accurately outdoors, the next step is to apply similar technology inside buildings, such as museums or commercial centers. Analog systems can guide people to places of interest, such as a work of art in a museum, or offer content based on location, such as advertising when passing by a store in a shopping center. However, in closed environments, satellite precision is low, making it necessary to resort to other technologies for effective positioning.

Indoor Location Technology

Indoor location-based services have generated great interest due to their social and monetary significance, with a projected market value of 10 billion dollars by 2020. Due to the inefficiency of GPS in indoor environments, alternative technologies such as WiFi or Bluetooth are necessary. Beacons always send out a signal that can be picked up by other BLE-compatible devices. They are good for positioning systems because they are cheap, use less energy, and are easy to set up. However, existing applications of different indoor tracking algorithms using wireless technologies are often imprecise. Algorithms based on RSSI measurements in WiFi networks are usually imprecise due to the large variation of the signal measured at each instant. Although many aspects of WiFi are improving, precision still has a long way to go. 

Wireless Communications

In wireless communications, the medium for sending information is always shared. This is a big difference from wired technologies. To avoid frequency band overlap and interference, there must be strict rules about how space is used. The communication protocols used by each participating device are responsible for coordinating access to the medium. The effectiveness of a wireless network depends on several factors, including the number of computers sharing the network, environmental conditions, electromagnetic interference, obstacles, and latency.

Maximum data transmission rates never represent the maximum “useful” data transmission rate since part of the frame is occupied by information on access control to the medium, flow control, encryption, etc. The real data transfer rate is always below what is defined in the standards. Since wireless networks are broadcast over the air, security becomes a critical factor, requiring authentication and data encryption to prevent unauthorized access.

WiFi Indoor Localization

WiFi, which stands for Wireless Fidelity, was born from the need to establish a wireless connection mechanism that was compatible between different devices. In other words, Wi-Fi is a wireless technology that allows users to connect wirelessly to each other through devices known as Access Points (APs) or hotspots. It generally operates on the 2.4 GHz and 5 GHz frequencies and was standardized by the Institute of Electrical and Electronics Engineers (IEEE). Over the years, numerous WiFi protocol standards have been developed, improving several aspects of its predecessors.

The Evolution of Wi-Fi Technology

The first official standard was 802.11b, followed by 802.11a, 802.11g, 802.11n, and 802.11ac. The b and g versions work in the 2.4 GHz band and are the most widely used. In 2019, 802.11ax (called Wi-Fi 6) was launched, which is estimated to have four times the performance of 802.11ac. While 5GHz networks achieve faster speeds than 2.4 GHz, they are less effective at traversing obstacles such as walls and furniture, resulting in a lower coverage range. An average Wi-Fi network has a range of 300 feet outdoors or 150 feet indoors.

The main benefit of using Wi-Fi networks for positioning is that the infrastructure is already there in buildings.Due to the rapid growth of wireless networks for home and business use, it is common to find more than one Wi-Fi access point in a building. These access points can be installed on their own or come from other places nearby.On the other hand, the biggest problem is that the 802.11 protocol was not made to be used in this way. Every time you want to find out where you are, you have to send messages to all Wi-Fi access points asking for the right information, which slows down the network.In turn, it may happen that some access points belonging to a third party block such requests.

The IEEE 802.11 wireless standards use a medium access protocol called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). Its name is similar to that used in wired Ethernet networks (CSMA/CD: Carrier Sense Multiple Access with Collision Detection), but its operation is different. In the wireless case, CA refers to collision avoidance, while in Ethernet it is about collision detection. Wi-Fi networks are half-duplex, meaning devices cannot transmit and receive at the same time on the same radio channel. A device cannot “listen” at the same time it is transmitting, so it cannot detect collisions. Because of this, IEEE experts used a collision avoidance mechanism that they called DCF (Distributed Control Function).

WiFi Network Security

Since the transmission medium is the air, which is naturally accessible by any device, it is essential to ensure that access to the network is restricted to authorized devices. To achieve this, the 802.11i standard defines different systems, such as WEP, WPA, and WPA2, in which devices use keys for authentication. APs periodically broadcast an advertisement containing the SSID (Service Set Identifier), allowing users to identify the correct AP and connect to it. The connection process begins with an authentication procedure, for which a key is generated. WiFi networks have three types of authentication.

1. Wired-Equivalent Privacy Key (WEP):

The goal of this security system is to make wireless networks as secure as wired ones. Unfortunately, it was quickly compromised, and its use is not currently recommended. At the start of the authentication process, the client device sends an unencrypted text message, which the AP encrypts using a shared key and returns to the client. The keys are usually 128 or 256 bits. The main problem with WEP is key management. Generally, keys are distributed manually or through another secure route. WEP uses shared keys, meaning it uses the same key for all clients, so if the key is discovered, all users are at risk. To obtain the key, it is only necessary to listen until obtaining the return of the authentication frames. Using WEP is better than nothing; when there is nothing better, it is advisable to use it. A good recommendation is to use security in the upper layers, such as SSL, TSL encryption, etc.

2. Wi-Fi Protected Access (WPA)

To overcome the security flaws of WEP, WPA was developed. This system was designed under the auspices of the WiFi Alliance and used a part of the 802.11i standard, which was later updated to replace WEP. One of the key elements of WPA is TKIP (Temporal Key Integrity Protocol), which is part of the 802.11i standard and operates by generating dynamic keys. WPA can optionally use AES-CCMP (Advanced Encryption Standard – Counter Mode with Cipher Block Chaining Message Authentication Code Protocol) as a replacement for TKIP.

3. WPA2/WPAv2:

It is currently the best technique available to secure a WiFi network. It uses mandatory AES-CCMP and is used in all devices manufactured today.

Standards

We currently have access to various wireless networks that offer us connectivity to the multiple devices we use daily. Different communication standards commonly used are under the name “WiFi,” which include:

• 802.11a: a wireless network with a carrier in the 5 GHz ISM band and a data transfer speed of up to 54 Mbps.

• 802.11b: a wireless network with a carrier in the 2.4 GHz ISM band and a data transfer rate of up to 11 Mbps.

• 802.11g: a wireless network with a carrier in the 2.4 GHz ISM band and a data transfer rate of up to 54 Mbps.

• 802.11i: authentication and encryption.

• 802.11n: a wireless network with a carrier in the 2.4 GHz and 5 GHz ISM band, with data transfer rates of up to 600 Mbps.

• 802.11ac: a wireless network with a carrier below 6 GHz, with data transfer rates of at least 1 Gbps in multi-station operation and 500 Mbps in a single link.

The Bottom Line

keeping your WiFi network secure is like keeping your front door locked—you wouldn’t want just anyone walking in, right? While Wired-Equivalent Privacy (WEP) was once an option for securing your network, it’s now outdated and has known flaws. Wi-Fi Protected Access (WPA) and its successor, WPA2/WPAv2, are much more effective security measures. Different WiFi communication standards offer various data transfer rates and frequencies, so it’s important to choose the right one for your needs. By taking these measures, you can keep your WiFi network safe without sacrificing the convenience of wireless connectivity.