Wireless Access Techniques
FDMA
FDMA (Frequency Division Multiple Access) assigns separate frequency bands to each user to avoid interference.
- Each user gets a unique frequency
- Simple and easy to implement
- Limited capacity for many users
FDMA is a basic method for separating users in frequency, but it doesn’t scale well for large networks.
FDMA was the earliest multiple access technology put into practice.
TDMA
TDMA (Time Division Multiple Access) splits access into time slots so users take turns using the channel.
- Users share the same frequency over different time slots
- More efficient than FDMA
- Reduces interference between users
TDMA allows multiple users to share a channel efficiently by organizing access over time.
Global System for Mobile Communication (GSM) uses TDMA as its frequency access technology. GSM is a second-generation technology and, although generally replaced by newer technologies such as 4G LTE and 5G, is still used in some parts of the world.
DSSS
DSSS (Direct Sequence Spread Spectrum) spreads a signal over a wider frequency using a code for better reliability.
- Signal is spread using a unique code
- Resistant to interference and noise
- Provides some security benefits
DSSS improves communication reliability by spreading the signal and reducing the chance of interference.
CDMA
CDMA (Code Division Multiple Access) allows multiple users to share the same frequency using unique codes.
- Each user has a unique code to separate signals
- Supports many users at the same time
- Resistant to interference
CDMA makes efficient use of spectrum by allowing many users to coexist without collisions.
OFDM
OFDM (Orthogonal Frequency Division Multiplexing) is a modulation technique used to transmit data efficiently over multiple subcarriers.
- Splits a channel into many narrow, orthogonal subcarriers
- Each subcarrier carries a part of the data
- Reduces interference and multipath fading
OFDM is used in Wi-Fi, LTE, and digital TV to send high-speed data reliably. It does not assign subcarriers to different users by itself.
OFDMA
OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access technique based on OFDM, allowing multiple users to share the same channel.
- Assigns different subcarriers or resource blocks to different users
- Each user can transmit simultaneously without interference
- Used in LTE and 5G for uplink and downlink
OFDMA builds on OFDM but adds multi-user allocation, making it suitable for cellular networks.
Key difference:
- OFDM = just a way to send data on multiple orthogonal subcarriers
- OFDMA = uses OFDM but also splits subcarriers among multiple users
Think of OFDM as a single highway with many lanes (subcarriers), and OFDMA as letting multiple cars (users) drive on specific lanes at the same time.
SC-FDMA
SC-FDMA (Single Carrier Frequency Division Multiple Access) is a single-carrier version of OFDMA, used mainly in uplink channels.
- Uses single-carrier modulation
- Reduces peak-to-average power ratio
- Commonly used for LTE uplink
SC-FDMA is energy-efficient for mobile devices and ensures reliable uplink transmission.
FHSS
FHSS (Frequency Hopping Spread Spectrum) takes the total amount of bandwidth (spectrum) and splits it into smaller subchannels.
- Changes the transmission frequency in a known sequence
- Signal hops between multiple frequencies
- Reduces interference and jamming
- Used in Bluetooth and older Wi-Fi (802.11b)
FHSS improves reliability and security by making the signal harder to intercept or disrupt.
DFS
DFS (Dynamic Frequency Selection) allows devices to switch frequencies automatically to avoid interference.
- Detects radar or other high-priority signals
- Moves to a different channel when needed
- Required in some Wi-Fi bands (5 GHz) for compliance
DFS ensures Wi-Fi devices do not interfere with radar systems and maintains smooth operation across shared spectrum.