1. WiMAX and Broadband Wireless Access
WiMAX, short for Worldwide Interoperability for Microwave Access, refers to a family of systems and profiles based on IEEE 802.16. They were designed for broadband wireless access over a large area, including the "last mile" problem.
The refers to the final segment of a network that connects the main internet backbone, which runs between cities and countries, to individual homes and businesses. Historically, this last mile was dominated by physical cables: the copper telephone lines used for DSL and the coaxial cables used for cable internet. Laying these cables is an expensive and time-consuming process, especially in rural, remote, or geographically challenging areas.
WiMAX was intended as a wireless alternative to DSL and cable access without laying a cable to every building. Fixed profiles targeted homes and businesses, while later mobile profiles targeted devices moving across a metropolitan area. Actual throughput and range depended on the band, profile, and radio conditions.
A quoted peak PHY rate is an upper bound for a selected profile, channel width, modulation and coding scheme, allocation, and favorable radio conditions. User throughput is lower after MAC and PHY overhead, guard intervals, scheduling, retransmissions, interference, and link-margin requirements. A nominal cell radius or line-of-sight distance is likewise not a guaranteed service area: practical range and rate depend on frequency, antennas, transmit power, terrain, and non-line-of-sight propagation.
2. Comparing WiMAX, Wi-Fi, and LTE
WiMAX can be compared with two other wireless technologies: Wi-Fi and cellular systems such as LTE.
Wi-Fi (IEEE 802.11)
Purpose: Designed for Wireless Local Area Networks (WLANs). It provides high-speed internet access in a relatively small, localized area such as a home, office, or coffee shop.
- Range: Very short, typically tens of meters.
- Spectrum: Operates in unlicensed bands such as 2.4 GHz and 5 GHz. This makes deployment accessible but increases exposure to interference from other networks and devices.
WiMAX (IEEE 802.16)
Purpose: Designed for Wireless Metropolitan Area Networks (WMANs). WiMAX profiles targeted broadband access from one base station over an area larger than a typical WLAN.
- Range: Usually longer than Wi-Fi and often measured in kilometers. Fixed line-of-sight links could reach farther, depending on the profile and transmit power.
- Spectrum: Many operator profiles were designed for licensed bands. A license does not guarantee quality, but it limits which users may transmit in the band.
Cellular LTE (4G)
Purpose: Designed for mobile communication and handovers between cells for users in motion, for example in cars and trains.
- Range: Similar to WiMAX, covering several kilometers from a base station.
- Spectrum: Also operates in licensed spectrum.
- Key Differentiator: LTE was developed as a cellular system with mobility procedures and an evolutionary path from GSM and UMTS. Mobility was added to WiMAX in later amendments to the 802.16 family.
3. The IEEE 802.16 Standard Family
The WiMAX air interface is defined in the 802.16 family of standards developed by the . Later amendments extended the family from fixed wireless access to profiles that support mobility.
IEEE 802.16 (2001) - Fixed Wireless Access
The first version of the standard was designed for Fixed Wireless Access (FWA). It was intended as a wireless alternative to cable and DSL for stationary users, such as homes and businesses. This initial version had one significant limitation: it required a path between the subscriber's antenna and the provider's base station. This meant an installation technician had to carefully mount an outdoor antenna on a rooftop and aim it precisely at the tower, which limited its practical deployment. It operated in a very high frequency band from 10 to 66 GHz.
IEEE 802.16-2004 (often called 802.16d) - operation below 11 GHz
This revised standard incorporated earlier amendments such as 802.16a and expanded the system's uses. It introduced two important elements:
- Lower Frequency Bands: It specified operation below 11 GHz, including ranges from 2 to 11 GHz. Lower frequencies generally reduce attenuation and help with paths that do not have direct line of sight, but they do not remove propagation limits.
- Non-Line-of-Sight (NLOS) Capability: Variants below 11 GHz included OFDM modes and support for NLOS channels. A link could therefore operate when obstacles blocked the direct path between the base station and user. This simplified some installations, although the result depended on the profile, antennas, and channel. The standard provided a basis for early "Fixed WiMAX" deployments.
IEEE 802.16e-2005 - Mobile WiMAX
This amendment added mobility support to the WiMAX family. Earlier versions targeted stationary or nomadic users, while IEEE 802.16e introduced procedures between base stations. The amendment targeted licensed bands below 6 GHz. A user could maintain a session while moving, for example in a vehicle, and the profile called "Mobile WiMAX" competed with 4G cellular systems such as LTE. Whether a handover avoided interruption depended on the procedure, terminal, and radio conditions.
The WiMAX Forum
While IEEE developed the technical standard, the industry alliance called the WiMAX Forum defined profiles and ran interoperability certification. The tests checked whether equipment from different vendors met the selected profile and could work together. This role is similar to the one the Wi-Fi Alliance plays for Wi-Fi products.
4. WiMAX Radio Technologies
WiMAX parameters come from the radio technologies defined by each profile. The Mobile WiMAX IEEE 802.16e profile uses OFDMA, as did contemporary LTE.
OFDMA (Orthogonal Frequency-Division Multiple Access)
Mobile WiMAX was one of the early broadband profiles based on OFDMA. Instead of one high-speed stream, data is divided into parallel streams, each transmitted on its own closely spaced . The base station assigns subsets of subcarriers to multiple users.
- Multipath Handling: Longer symbols and the cyclic prefix limit the effect of delayed signal copies. OFDMA therefore reduces sensitivity to Inter-Symbol Interference (ISI), which occurs among other places in urban channels.
- Flexible Allocation: OFDMA lets the base station assign groups of subcarriers to different users according to traffic needs and channel quality.
Scalable OFDMA (SOFDMA)
Mobile WiMAX (IEEE 802.16e) uses a variant called SOFDMA. The "Scalable" part refers to selecting parameters for the channel bandwidth. Profiles could use 5 MHz, 7 MHz, or 10 MHz channels. SOFDMA scales the size, which changes the number of subcarriers. For example, a profile could use a 512-point FFT for a 5 MHz channel or a 1024-point FFT for a 10 MHz channel. This helped adapt the system to different spectrum allocations.
In the PHY layer, subcarriers are organized into OFDMA symbols, while the MAC layer assigns them in time and frequency. Profiles also specify modulation and coding according to channel quality, so throughput is not a fixed property of the channel bandwidth alone.
A mobile profile uses frames with a downlink portion and an uplink portion. The base station sends allocation information to terminals, and terminals perform ranging and synchronization procedures before transmitting.
The selected profile also defined channel bandwidth, duplexing mode, and antenna options, so devices from different profiles did not necessarily offer the same parameters.
Certification profiles specified mandatory options, so IEEE 802.16 compliance alone did not imply interoperability between arbitrary equipment. Operators selected a profile according to spectrum and deployment model.
MIMO (Multiple-Input Multiple-Output)
WiMAX profiles also supported MIMO. Multiple antennas at the base station and subscriber station allowed spatial multiplexing, sending several data streams, or transmit diversity, sending the same data over different paths to improve reliability.
5. The WiMAX Network Architecture
The overall WiMAX network has a layered structure comprising user devices, the access network, and the core network.
Subscriber Station (SS/MS)
This is the user's device. For fixed WiMAX, this was typically an outdoor antenna or an indoor desktop modem. For Mobile WiMAX, this became a USB dongle, a PC card, or eventually, a chip integrated into a laptop or a mobile phone.
Access Service Network (ASN)
This is the WiMAX equivalent of the Radio Access Network. It consists of two main parts: the Base Stations (BS) that provide the radio coverage, and the ASN Gateway (ASN-GW), which acts as a control point, managing handovers and connecting the radio network to the core.
Connectivity Service Network (CSN)
This is the WiMAX All-IP core network. It provides IP connectivity, authenticates users through an AAA server, connects the WiMAX network to the internet or other networks, and maintains session state and IP address assignment.
The ASN handles network entry, radio resource allocation, and handovers between base stations. The CSN provides authentication, IP addressing, and connections to other networks. Separating these roles lets an operator keep access functions distinct from IP services.
6. WiMAX and LTE: Deployment Factors
In the mid-2000s, WiMAX and LTE were competing approaches to broadband wireless access. WiMAX had fixed and mobile deployments, including networks built by Clearwire in the United States and Netia in Poland. Over time, LTE became the dominant cellular technology.
Deployment and ecosystem factors
The outcome was not determined by the radio layer alone. Mobile WiMAX and LTE both used OFDMA and MIMO, but they differed in standards support, deployment base, and device availability. The operator and vendor ecosystem increased LTE's advantage.
- LTE's Backing:LTE was developed by 3GPP, a partnership of standards organizations including ETSI, ATIS, ARIB, CCSA, TTA, and TTC. Operators and vendors such as Ericsson and Nokia could develop LTE as an evolutionary path from GSM and UMTS networks. GSMA represented the ecosystem's interests but did not create 3GPP specifications.
- WiMAX's Backing: WiMAX was also promoted by the computing and data industries, with Intel among its known supporters. It had operators and certified equipment, but it did not build the same migration path through existing cellular networks.
- Evolution and Roaming: LTE operators could use existing 2G and 3G networks to supplement coverage while 4G deployment was incomplete. This eased device and service migration. WiMAX did not have an equivalent, widely deployed cellular layer to combine with new deployments.
- Economies of Scale: The larger number of LTE deployments increased production volumes for modems and infrastructure. This lowered costs and improved device availability, making it harder for WiMAX to compete in the cellular market.
The Legacy of WiMAX
WiMAX did not become the dominant technology for mobile broadband, but its profiles remained useful in selected Fixed Wireless Access (FWA) deployments. OFDMA, MIMO, and broadband wireless access are part of the shared development of later radio systems. New FWA networks more often use other technologies, so WiMAX use depends on existing infrastructure and the operator.