Q21Principle of Electronic Communication

(a) GSM vs CDMA Cellular Systems

GSM (Global System for Mobile Communications) uses a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA): the available frequency spectrum is divided into channels (FDMA), and each channel is further divided into time slots (TDMA), with each active user assigned a specific frequency channel and time slot combination. CDMA (Code Division Multiple Access) instead allows all users to transmit simultaneously across the entire available bandwidth, distinguishing each user's signal using a unique spreading code (as described in relation to an earlier question in this examination) rather than separating users by time or frequency.

In terms of security, GSM uses a SIM card containing a secret authentication key and the A3/A5/A8 algorithm suite for authentication and encryption, though older GSM encryption (A5/1) has known cryptographic weaknesses; CDMA systems typically incorporate the spreading code itself as an inherent layer of security and interference resistance (since a receiver without the correct code sees the CDMA signal as noise-like), generally considered to provide somewhat stronger inherent security and resistance to eavesdropping than early GSM implementations, though both technologies have been superseded in overall security sophistication by later 3G/4G/5G security architectures. In terms of typical applications, GSM historically dominated in Europe and much of Asia/Africa as the dominant 2G standard, while CDMA (IS-95, and later CDMA2000) was more prevalent in North America and parts of Asia, though both technologies' relevance has diminished considerably with the global shift to LTE/4G and 5G, which use OFDMA rather than either pure TDMA/FDMA or CDMA as their multiple access scheme.

(b) RFID - Working Principle and Applications

RFID (Radio Frequency Identification) uses a small tag containing a microchip and antenna attached to an object, and a reader device that emits a radio frequency signal to communicate with the tag. Passive RFID tags (the most common, low-cost type) have no internal battery; instead, they harvest the small amount of energy from the reader's incoming radio signal itself to power the chip briefly enough to transmit back the tag's stored identification data via backscatter modulation. Active RFID tags include their own battery, allowing longer read range and more frequent transmission, at higher cost. Applications of RFID include retail inventory management and anti-theft tagging, automated toll collection (highway toll tags), livestock and asset tracking, access control and ID badges, and supply chain/logistics tracking, valued for enabling fast, contactless, line-of-sight-free identification of tagged items compared to barcode scanning.

It is also worth noting the broader significance of this GSM/CDMA comparison for understanding cellular technology evolution: the industry's eventual convergence on OFDMA-based LTE/5G standards for essentially all mobile carriers worldwide (regardless of whether they originally used GSM or CDMA for their 2G/3G networks) reflects OFDMA's superior ability to combine high spectral efficiency with robustness against multipath fading in mobile radio channels, effectively ending the historical GSM-versus-CDMA technology divide as both technology families migrated to a common underlying access scheme for modern high-speed mobile data services.

It is also worth distinguishing RFID from the related but distinct NFC (Near Field Communication) technology, which operates over a much shorter range (a few centimeters) and is widely used for contactless payment and simple pairing applications - while both technologies share underlying radio-frequency and inductive-coupling principles, RFID is generally optimized for longer-range, one-way identification/tracking applications (reading many tags quickly from a distance), whereas NFC is optimized for very short-range, often two-way, deliberate 'tap-to-connect' interactions between two active devices.