Definition
Multiple Access Technology (Multiple Access) allows multiple users to share the same wireless channel resources, achieving spectrum reuse. Different generations of cellular systems use different multiple access methods: 2G GSM uses FDMA+TDMA, 3G WCDMA uses CDMA, 4G LTE and 5G NR use OFDMA based on OFDM (Orthogonal Frequency Division Multiple Access). OFDMA divides the system bandwidth into several subcarriers, assigns different subcarriers or time slots to different users, and achieves flexible resource allocation and multi-user reuse through traffic scheduling.
Types and Evolution of Multiple Access
- FDMA (Frequency Division Multiple Access): Different users occupy different frequency bands, simple but low spectrum efficiency, used in early analog systems
- TDMA (Time Division Multiple Access): Different users occupy different time slots, GSM uses 200kHz carriers + 8 time slots, achieving 8-user reuse
- CDMA (Code Division Multiple Access): Different users use different spreading codes to share the same frequency band; used in WCDMA, strong anti-interference capability.
- OFDMA (Orthogonal Frequency Division Multiple Access): The bandwidth is divided into orthogonal subcarriers, and users are allocated different RBs (Resource Blocks); supports frequency-domain scheduling and MIMO; the core multiple access method for LTE/5G.
Multiple Access in LTE and 5G NR
LTE downlink uses OFDMA, uplink uses SC-FDMA (Single Carrier Frequency Division Multiple Access) to reduce the terminal's PAPR and save power amplifier consumption. 5G NR uses OFDMA for both downlink and uplink; the subcarrier spacing is configurable (15/30/60/120 kHz) and supports more flexible parameter sets (Numerology) to adapt to different services and frequency bands. OFDMA combined with modulation and channel coding enables multi-user scheduling and capacity maximization through MCS and RB allocation.
Typical Use Cases
Multiple access technology determines the capacity and multi-user access capability of cellular networks. IoT devices, as part of massive connections, share spectrum resources with services such as eMBB and URLLC via OFDMA; NB-IoT and eMTC use narrowband and dedicated resource blocks, and under QoS scheduling, they work cooperatively with regular terminals.
When SimtoLink's IoT SIM cards access carrier networks, devices share wireless resources with other users through multiple access technologies. In densely deployed scenarios, multiple access efficiency directly affects the number of IoT devices that can connect to a single cell and the available bandwidth per device. When evaluating large connection scenarios for customers, SimtoLink pays attention to carriers' resource allocation strategies for standards like NB-IoT and Cat.1, as well as the impact of multiple access scheduling on device access latency and rate, helping customers optimize deployment density and business models.
Orthogonal Partition and Code-Division Sharing Are Not the Same Kind of Occupancy
Multiple access has to answer how several transmitters on the same stretch of spectrum can be separated at the receiver. Frequency division puts users on different carriers. Time division puts them in different slots. Code division lets them transmit at the same time and on the same frequency with different spreading codes, and the receiver separates them by code correlation. OFDMA cuts the bandwidth into subcarriers that are orthogonal to each other, then assigns resource blocks to different users. With ideal synchronization, users do not overlap. When orthogonality fails, leftover frequency error and timing error become interference between users, and the “full reuse” in a capacity calculation no longer holds.
LTE downlink uses OFDMA. The uplink uses SC-FDMA so the device waveform stays closer to a single carrier, the peak-to-average ratio falls, and the power amplifier can run in a more efficient region. 5G NR returns to OFDMA in both directions and relies on device capability, power control, and optional waveform processing to live with the peak-to-average ratio. “4G and 5G are both orthogonal frequency division” is roughly true only on the downlink. The uplink mechanisms cannot be mixed. NB-IoT and eMTC carve a narrow band or repeated resources out of this grid. They are not a new multiple-access method that ignores scheduling.
Whether a Resource Block Can Be Given Out Depends on Synchronization and Scheduling
OFDMA can place different users on adjacent subcarriers only when their arrival time and frequency offset both fall inside the orthogonality condition. If timing advance does not keep up, or the crystal offset is too large, an edge user leaks into a neighbor’s resource block. The scheduler also has to account for symbols taken by the control channel. Resource blocks left for data are fewer than the nominal bandwidth. If an IoT device wakes in only a few subframes, the multiple-access resource it can use is the free blocks at those wake instants, not the average bandwidth across the day.
With a mass of small packets, the limit often comes from random access and scheduling requests, not from the number of bits on the traffic channel. Each device has to contend for a preamble and then ask for resources. When preambles collide, orthogonal resources behind them cannot be handed out no matter how many there are. Lower the share of devices that wake at the same moment, or stagger the report times. Do not only upgrade the carrier bandwidth. The line with traffic scheduling is this: multiple access decides how the resource is cut, and scheduling decides who receives which piece on this occasion.
Dividing the Resource-Block Count Straight into a User Count
A common error is “20 MHz has a hundred resource blocks, so it can take a hundred users.” One transmission from one user can occupy several blocks. Control channels, broadcast, and retransmissions take blocks too. A block at the edge carries far fewer bits than a block near the site. Another error is to ignore the single-carrier character of the uplink and estimate device power on LTE uplink with the downlink’s way of stacking subcarriers. The real peak-to-average ratio and amplifier current will not match.
The CDMA habit that “everyone is on the same frequency” also does not transfer to OFDMA. OFDMA users are supposed to be orthogonal. Interference comes mainly from neighboring cells, not from other codes in this cell. When the rate in this cell will not rise, first ask whether resources are short, access is colliding, or a neighbor is leaking on the same channel. Calling every problem “multiple-access capacity is used up” misses synchronization and scheduling, which are the more common causes.
The number of random-access preambles, and how simultaneous the reports are, often runs out before the traffic resource blocks do. A meter reading on the hour makes thousands of devices pick a preamble in the same second. After collisions everyone backs off and retries, while the orthogonal data resources are still empty. Spreading the report second is more effective than growing the carrier from ten megahertz to twenty. An uplink single-carrier waveform also cannot be spliced like downlink subcarriers when estimating current. The peak-to-average ratio is different, the amplifier operating point is different, and the battery calculation comes out biased.