Wireless spectrum is a controlled asset in almost every country and allocated under license to selected operators – typically at very high cost to the operator. To stimulate growth of broadband wireless, regulators in many countries permit operation in bands not occupied by licensed operators. In some cases, operators are required to register for operation – receiving a “license” to operate. In other cases no registration is required. However in all cases, operation is on a non-exclusive basis. There are four sub bands in 5 GHz range typically used for license-exempt operation globally. In the US, operators may register for operation in the 3.65 GHz band. Operation on a non-exclusive basis raises the probability of interference and difficulties finding clear spectrum. License-Exempt WiMAX refers to the use of WiMAX technology to deliver broadband services in these non-exclusive bands. Some WiMAX systems employ leading edge interference mitigation and spectrum management techniques overcoming the challenges of unlicensed operation. WiMAX employs OFDM waveform for higher resistance to multi-path fading, farther reach and higher bandwidth and capacity in license-exempt bands.
WiMAX can operate in the almost 300 MHz of spectrum in the four sub-bands at 5 GHz. The ISM band is 125 MHz wide from 5.725 to 5.850 GHz. There are three Unlicensed National Information Infrastructure (U-NII) bands, each 100 MHz wide: 5.150 to 5.250 GHz, 5.250 to 5.350 GHz, and 5.725 to 5.825 GHz. Many countries allow operation in one or more of these bands. The United States FCC permits operation of WiMAX in the 50 MHz existing from 3.65 GHz to 3.70 GHz.
3. I wish to offer services near the airport or other government or military installation. Will License-Exempt WiMAX cause interference?
While the 5GHz spectrum has been deregulated in many countries, commercial installations and government radar often continue to co-exist in this spectrum. It is absolutely necessary to ensure that growth of wireless broadband networks do not interfere with the proper operation of these sensitive radar systems. Regulators have suggested the use of a feature called Dynamic Frequency Selection (DFS) to eliminate the possibility of wireless broadband systems occupying the same spectrum as government radar installations. DFS periodically scans the airwaves to detect and avoid channels being used by radar signals. Transmission stops immediately when interference occurs, the system searches for a clear channel, and the new frequency is broadcast from the base unit to all subscriber units within the sector. If no clear channel can be found, the wireless broadband system enters a “sleep mode” where no transmission occurs until the radar has vacated the channel. Aperto Networks’ 802.16d compliant WiMAX systems meet both US FCC and ETSI standard Dynamic Frequency Selection (DFS) regulations.
Consumer and enterprise grade products cannot withstand the extreme temperature conditions in places like Siberia, Egypt and others. Carrier-class systems are recommended for mission-critical reliability and redundancy. Aperto’s WiMAX products have been deployed and tested in temperatures ranging from -40 degree Celsius in Russia to +50 degrees in Egypt.
- 802.16 Standard– The use of OFDM eliminates inter-symbol interference (ISI) problems and the complexities of adaptive equalization, ensuring high quality voice services on par with that of wired networks. OFDM results in higher resistance to multi-path fading, farther reach and higher bandwidth and capacity in the license-exempt bands.
- Suitable network design – A good site-survey must provide multiple access points, such as towers and base stations, close to the clients such that a client can select the best RF connection at a particular time. Global Positioning System (GPS) synchronization prevents inter-sector and inter-base station interference by identifying the base station location and location of possible interference as well as synchronizing RF transmit times.
- Dynamic Frequency Selection –DFS can scan the airwaves periodically for “blacklisted” signals (radar or external system). If the base station encounters any interference, it searches for a clean frequency and informs all connected subscriber units to start transmitting on the new frequency.
- Smaller Channel Bandwidth –Large channel sizes are useful for increasing the data throughput of the wireless channel. However, they also cause more interference and make network planning more difficult. WiMAX allows the use of a flexible range of channel sizes to make optimum use of available spectrum and have granular control on the radio resource.
Yes. WiMAX has the ability to prioritize delivery of different types of traffic. Classes of service such as Unsolicited Grant Service (UGS) allows scheduling of fixed-size grants at a recurring interval with as little latency as possible, providing excellent performance for voice and other real-time applications. WiMAX uses four different service classes.
WiMAX uses clearly defined service classes for prioritizing traffic over the wireless link. In 802.16d systems, Unsolicited Grant Service (also commonly referred to as constant bit rate), real-time polling service, non real-time polling service and best effort delivery prioritization can be assigned to each traffic type within the sector. Service classification and prioritization needs to be an end-to-end function. Therefore, base stations should support traditional IP-styled quality of service (DSCP) and the mapping of WiMAX service classes into DSCP at the edge of the wired network. This is in contrast to tweaks made to 802.11 systems for prioritization based QoS, that does not scale well with increase in voice traffic.
Dynamic or manual channel selection is used to eliminate interference. Additionally, breaking up long backhaul paths into shorter ones will help to minimize fading and interference. Finally, successfully polarizing antennas (cross-polarization can provide approximately 20-25 db of discriminations of unwanted signals). Shielded cabling is extremely important when it comes to interference.
The ability to operate a standardized solution in both a licensed and a license-exempt band is one of the benefits of WiMAX solutions for deployments around the world. The adoption of license-exempt and licensed WiMAX solutions over proprietary and WiFi systems is being driven by the following additional benefits:
Scalability. The 802.16d standard supports Transmit Power Control (TPC) and channel quality measurements as additional tools to support efficient spectrum use. The standard also supports flexible radio frequency (RF) channel bandwidths and reuse of these frequency channels as a way to increase network capacity. The standard has been designed to scale up to hundreds or even thousands of users within one RF channel. Operators can re-allocate spectrum through sectoring as the number of subscribers grows.
Cost-effectiveness. Standardization encourages interoperability between equipment of different vendors and drives down costs for the end user. The wireless medium used by WiMAX enables service providers to circumvent costs associated with deploying wires, such as time and labor. License-exempt WiMAX gives operators the double benefit of deploying carrier-grade wireless systems at lowered cost-barriers.
The availability of 5GHz band for license-exempt use is directed by the regulatory authorities of each country. Many countries are changing regulations to include 5GHz spectrum for license-exempt use along with the 2.4GHz band. It is best to check with your country's regulatory authority before deciding whether to deploy 5GHz. As of August 2007, the following countries did not allow license-exempt use of 5GHz:
"UNITED ARAB EMIRATES"
WiFi is based on the IEEE 802.11 standard. It operates in both the 2.4GHz band and the various 5GHz bands. The 802.11 standard has been designed for local area networks extending up to a few meters and supporting a few hundred users. On the other hand, 802.16 standards for WiMAX have been designed for metro area networks (MAN) that have radius of at least 2-3 kilometers (more in LOS conditions) and are capable of supporting thousands of simultaneous users. WiMAX has scheduled MAC and Quality of Service built-in that allows for more robust and reliable link quality under high interference conditions such as license-exempt 5GHz band. The 256-point FFT OFDM TDD PHY is inherently resilient to multi-path fading, unlike WiFi. The advantages of WiMAX over WiFi in license-exempt band are summarized in the table below:
License-free operation drive down the cost-barrier to enjoying high quality broadband wireless access for small ISPs and enterprises that can now enjoy attractive business case without locking up their investment in license-fee to enter the market. With the low cost and high performance of standards based WiMAX products, it is possible for them to exploit the 5GHz band like never before. 5GHz WiMAX allows the ISP to offer SLA backed enterprise services, tiered service plans, differentiated premium voice and data services, WiMAX backhaul for WiFi mesh or hotspots and other such offerings. This not only generates higher revenues for the ISP but also opens up 5GHz band for use where QoS and long-distance link reliability is an issue. Enterprises and campuses looking for an affordable, high-speed and robust solution for non-commercial use are also deploying 5 GHz WiMAX. Capital expenditure is no longer a deterrent to rich IP connectivity for anyone requiring a public or private network, thanks to license-free WiMAX.
Mobility is an application and is not a characteristic of the frequency band. Mobility does require low latency for faster hand-off between sectors and larger wavelengths to accommodate high number of subscribers. For this reason, lower frequencies such as 2.5GHz are better suited for mobile applications than high frequencies. At the same time, it is possible to implement efficient algorithms to provide some level of mobility even within the 5GHz band.
WiMAX MAC layer implements a 256 point OFDM sub-channelized waveform that allows hundreds of fixed sites to share a single base station within tens of kilometers or miles The 802.16 MAC layer dynamically assigns bandwidth to subscriber devices using time-division multiple access (TDMA). In contrast, the 802.11 MAC layer uses carrier sensing and contention mechanisms to provide bandwidth control. This results in drastically reduced performance in higher noise conditions as compared to 802.16.