Base Stations, Packet Switches, Operational Support System

A base station is a radio access transceiver (combined transmitter and receiver) that is used to connect subscriber stations to a WiMAX system. Base stations convert and control the sending of data packets and can connect one or many wireless devices to a backbone network.

Base stations can perform one or many types of data transfer functions including bridging (linking networks), retransmitting (repeating), distributing (hubs), directing packets (switching or routing) or to adapt formats for other types of networks (gateways).

Indoor Base Stations

Indoor base stations are devices or assemblies that enable systems or system connections to receive and convert radio or communication signals that are enclosed and/or have connections that allow them to be used in indoor environments.

Outdoor Base Stations

Outdoor base stations are devices or assemblies that enable systems or system connections to receive and convert radio or communication signals that are enclosed and/or have connections that allow them to be used in outdoor environments.

Packet Switches

A packet switch is a device in a data transmission network that receives and forwards packets of data. The packet switch receives the packet of data, reads its address, searches in its database for its forwarding address, and sends the packet toward its next destination.

Packet switching is different than circuit switching because circuit switching makes continuous path connections based on a signal’s time of arrival (TDM) port of arrival (cross-connect) or frequency of arrival. In a packet switch, each transmission is packetized and individually addressed, much like a letter in the mail. At each post office along the way to the destination, the address is inspected and the letter forwarded to the next closest post office facility. A packet switch works much the same way.

Operational Support System (OSS)

Operations support systems are combinations of equipment and software that are used to allow a network operator to perform the administrative portions of the business. These functions include customer care, inventory management and billing. Originally, OSS referred to the systems that only supported the operation of the network. Recent definition includes all systems required to support the communications company including network systems, billing, and customer care.

Subscriber Stations (SS)

Subscriber stations (SS) in a WiMAX system are transceivers (transmitter and receivers) that convert radio signals into digital signals that can be routed to and from communication devices. The types of WiMAX subscriber stations range from portable PCMCIA cards to fixed stations that provide service to multiple users.

Subscriber stations may be capable of full frequency division duplex (FDD), half frequency duplex (H-FDD), time division duplex (TDD) or any combination of these access types.

Figure 1 shows the different types of WiMAX access devices. WiMAX access devices include external boxes that connect to an Ethernet or USB port, PCMCIA card with external antennas or portable devices with built in radio modems.

Figure 1: WiMax Devices

Indoor Subscriber Stations

Indoor subscriber stations are devices or assemblies that enable users to receive and convert radio or communication signals that are enclosed and/or have connections that allow them to be used in indoor environments.

Outdoor Subscriber Stations

Outdoor subscriber stations are devices or assemblies that enable users or devices to receive and convert radio or communication signals that are enclosed and/or have connections that allow them to be used in outdoor environments.

Wireless Broadband System Parts

Wide-area wireless broadband systems are typically composed of end user subscriber stations (access devices), base stations (access nodes), packet switches and gateways.

Network Topology

Network topology is the physical and logical relationship between nodes in a network as well as the layout and structure of a network. The WiMAX system can be setup as a point to point (PTP), point to multipoint (PMP) or a mesh network.

Point to Point (PTP)

Point to point communication is the process of transferring information from one device (or point) to one other device (single receiving point). The WiMAX system can use PTP communication for high-speed communication links for backhaul (system interconnection) applications.

Point to Multipoint (PMP)

Point to multipoint communication is the process of transferring information from one device (or point) to multiple points or devices (multiple receiving points). The WiMAX system can use PMP to provide broadband access to multiple users per base station.

Mesh Network

A mesh network is a communication system where each communication device (typically a computer) is interconnected to multiple nodes (connection points) in the network allowing data packets to travel through alternate paths to reach their destination.

Some or all of the resources of a WiMAX system can be configured to provide mesh network services so that the need to interconnect base stations to access points (such as Internet gateways) can be reduced or eliminated. When a WiMAX system is setup as a mesh network, packets can hop across neighboring base stations to reach other points in the network.

WiMAX transceivers that are part of a mesh network are called nodes. Each mesh node is assigned a unique NodeID and each link between nodes is assigned a LinkID. Packets that enter into the mesh network contain their mesh network destination address (NodeID) and their current mesh link address (LinkID). A mesh node receives and forwards packets towards their destination (NodeID). As nodes in the network transfer packets towards their destination, they change the LinkID within the packet to reflect the next link that will be used in the mesh network.

Mesh nodes that can communicate with each other are part of a neighborhood. Neighborhoods can be small with neighbors that are directly adjacent to each other (immediate neighborhood) or they can be part of a larger neighborhood where nodes must communicate through other nodes so packets can reach their destinations (extended neighborhood).

Mesh node operation may be independent (distributed) or controlled ( scheduled) by another network device. When a mesh network is centrally coordinated, one of the nodes is designed as a master synchronization node. The master synchronization node receives requests from mesh nodes, analyzes the bandwidth and transmission requirements and distributes coordinating (scheduling) to other mesh nodes within the network. Mesh nodes that are part of a coordinated mesh system are called a mesh cell.

Mesh nodes regularly broadcast neighbor lists which contain information about available link connections that can be used as part of the mesh network and their associated scheduling times. These lists contain link quality, burst profile, RF power levels and control slot information that will be used to gain access and transmit on the link.

Mesh networks can be setup as either logical or physical connections. A logical mesh network is a system that uses existing WiMAX transceivers to receive and forward packets or data towards its destination. For logical mesh networks, links can be dynamically setup and removed as desired by the mesh network. Physical mesh nodes use transceivers that have their links preestablished so they are not addressable (cannot have their addresses dynamically assigned).

When a WiMAX device wants to attach to a mesh network (gain network entry), it must initially search to see if radio channels are available for a mesh network. If it finds an available radio channel, it needs to communicate with a mesh node that is willing to help it attach to the mesh network (called a sponsor node). The sponsor node will allow it to make a radio connection and negotiate basic communication parameters. It will then relay the request to join the mesh network to the node or part of the network that will authorize and assign resources to the new mesh node (e.g. the master synchronization node).

Figure 1 shows a WiMAX system configured as a mesh network. The base station (BS) devices in a WiMAX system are part of a neighborhood domain and the devices within this neighborhood can be configured as relay point nodes so they can help transfer packets from their neighbors towards their destination. A distant subscriber station that cannot directly communicate with the mesh base station can send packets to a neighbor that can relay the packets to the mesh base station that connects to Internet gateway. This example shows that a mesh network contains links between mesh nodes that are uniquely identified by link identifiers (LinkID) and that the destination point for packets traveling through the mesh network are identified by a node identifier (NodeID). Packets that travel through the mesh network contain both the destination address (NodeID) and the current link address (LinkID). As packets travel through mesh nodes, the link address changes.

Figure 1: WiMax Mesh Network

A directed mesh network is communication system where each communication device (typically computers) is interconnected to multiple nodes ( connection points) through the use of highly directional connections (e.g. directional antennas). In directed mesh networks, data packets travel through the directed paths to reach their destination. WiMAX systems may use a directed mesh network to interconnect areas to a central facility or gateway (such as a gateway to the Internet).

Figure 2 depicts a WiMAX system that is setup as a directed mesh network. This diagram shows how a WiMAX operator can use directional antennas that interconnect to base stations to allow packets to be relayed from distant locations to a central location (such as an Internet gateway) using WiMAX frequencies. This example shows that the use of directional antennas results in highly focused transmission beams which have little impact on the overall radio coverage of the WiMAX system.

Figure 2: WiMax Directed Mesh Network

WiMAX Broadband Applications

Broadband communication service is the transfer of digital audio (voice), data, and/or video communications at rates greater than wideband communications rates (above 1 Mbps). Broadband connections allow for the providing of multiple services such as telephone (voice), data, and video on one network.

WiMAX VoIP

Digital telephony is a communication system that uses digital data to represent and transfer analog signals. These analog signals can be audio signals (acoustic sounds) or complex modem signals that represent other forms of information.

WiMAX systems can provide telephone services through the use of IP Telephony (voice over Internet protocol – VoIP). These IP networks initiate, process, and receive voice or digital telephone communications using IP protocol. WiMAX systems can provide digital telephone service through the use of analog telephone adapters (ATAs) or IP telephones. ATAs convert IP signals into standard telephone (dial tone) formats.

WiMAX Standards

WiMAX standards are the operational descriptions, procedures or tests that allow manufacturers to produce devices that reliably operate and can work with devices produced by other manufacturers. The development of WiMAX standards is overseen by the Institute of Electrical and Electronics Engineers (IEEE).

The WiMAX standard is given the standard identifier of 802.16 (there are several variations of 802.16). 802.16 specifications primarily cover the lower layers including the physical layer and media access control (MAC) layer and define the different levels of quality of service (QoS) that can be provided.

Even within the WiMAX specification, there are multiple radio interface types. The need for these variations is typically the result of different industry requirements such as fixed point to point communications or mobile broadband communications, which require tradeoffs in radio access types in exchange for key requirements such as mobility, higher speed data communication rates or longer transmission distance.

To enable the 802.16 system to provide mobile operation (not fixed location), the 802.16e specification was created and released in 2006. 802.16e defines mobile broadband operation in the 2-6 GHz frequency range. 802.16e added mobility management, extensible authentication protocol (EAP), handoff (call transfer), and power saving sleep modes to the WiMAX system.

In addition to the basic 802.16 radio interface specifications, several other 802.16 specifications have been created to the standards of the network and operation of WiMAX devices and systems.

802.16f is an IEEE standard that defines the network management information base (MIB) parts that are used for the 802.16 WiMAX system. 802.16g is an IEEE standard that defines the management processes (management plane) that are used for the 802.16 WiMAX system. 802.16.2-2004 is an IEEE standard that defines how to plan and setup broadband radio transmitters in 802.16 systems so they can co-exist.

Figure 1 offers a comparison between the original fixed WiMAX standard and the WiMAX standard that can be used for fixed, mobile and portable. This table shows that the original 802.16 standard was released in 2004 and it was only capable of providing fixed wireless data services. It used OFDM modulation and could be deployed in both TDD or FDD formats. The 802.16e standard was released in 2005 (now merged into the original 802.16 standard) was designed for fixed, mobile and portable operation. It used OFDMA modulation with TDD and optionally FDD duplexing capability.

Figure 1: WiMax Standards

Figure 2 depicts the evolution of the 802.16 wireless broadband specification over time. The original 802.16 specification offered fixed wireless broadband service at 10-66 GHz. To provide fixed wireless broadband in the 2-11 GHz range, the 802.16a specification was created. Additional variations of the original 802.16 specification were created until in 2004, these specifications were merged into a single 802.16-2004 main specification. Since the 802.16-2004 specification was released, an 802.16e addendum was approved that adds portability to the 802.16 WiMAX system.

Figure 2: Wireless Broadband 802.16 Evolution

Mobile WiMAX

Mobile wireless is the use of wireless technology to provide voice, data or video service to locations that may change over time. Mobile wireless services include mobile telephone, mobile data and broadcast video services. Mobile wireless systems may replace or augment wireless telephone service, highspeed data communication links, and broadcast television systems.

Figure 1 depicts how a WiMAX system provides communication services to mobile devices. The WiMAX system has various options that can improve the signal quality to mobile users, which operate in a variety of urban, pedestrian and rural environments. The WiMAX system can use OFDM modulation which enhances mobile performance in multipath signal conditions and it can use MIMO and beam forming capabilities to increase signal reliability and system capacity.

Figure 1: WiMax Mobile Communication Services

Fixed WiMAX

Fixed wireless is the use of wireless technology to provide voice, data, or video service to fixed locations. Fixed wireless services include wireless local loop (WLL), point-to-point microwave, wireless broadband, and free-space optical communication. Fixed wireless systems may replace or bypass wired telephone service, high-speed telephone communication links, and cable television systems.

Figure 1: WiMax Spectral Efficiency

The initial WiMAX 802.16 standard was developed to provide high-speed data communication for licensed fixed applications at microwave frequencies (10-66 GHz). Shortly after the development of the initial 802.16 standard started, several versions were created to provide different service types and to operate in lower frequencies (2-11 GHz).

The 802.16a specification was created to allow WiMAX to operate in the 2-11 GHz range. This was followed by the 802.16c specification, which contained profiles for 10-66 GHz systems. Development of an 802.16d specification was started to define profiles for 2-11 GHz range. Eventually, all of the variations (802.16a, 802.16c and 802.16d) were merged into a single 802.16 specification (802.16-2004).

Figure 2 illustrates that point to point communication can reach up to approximately 50 km (30 miles) with a data rate of over 72 Mbps. For multipoint communication that can operate in partially blocked (near line of sight) terrain, the WiMAX system can reach up to 10 km (6 miles) with a data rate of over 40 Mbps.

Figure 2: WiMax Fixed Communication Services

Channel Loading & Spectral Efficiency | WiMAX

Channel Loading

Channel loading is a ratio of the number of users authorized to operate on a particular channel or systems compared to the number of users that actively transmit on a system. An example of channel loading on a WiMAX system is the number of broadband subscribers that can be effectively served by a single WiMAX radio channel.

The amount of channel loading depends on a variety of factors including the type of use (e.g. bursty web browsing or watching continuous digital telephony). For many types of applications, the subscriber station does not usually continuously transmit data while it is connected to the WiMAX system. For Internet browsing, the typical data transmission activity is less than 10%. This could allow channel loading of 10:1 or more. For example, in a WiMAX cell or single radio coverage area that has a channel capacity of 70 Mbps, 700 broadband Internet customers could be provided with data rates of 1 Mbps each.

The service provider can affect the channel loading through their price plans. Price plans can range from usage based service (charge per megabyte transferred) to unlimited rate plans. In 2006, the services and rate plans offered by WiMAX service providers were similar to digital subscriber line (DSL) and cable modem services rate plans.

Spectral Efficiency

Spectral efficiency is a measurement characterizing a particular modulation and coding method that describes how much information can be transferred in a given bandwidth. This is often given as bits per second per Hertz.

Modulation and coding methods that have high spectral efficiency are typically very sensitive to small amounts of noise and interference and often have low geographic spectral efficiency. WiMAX was designed to use multiple types of modulation, which allows the system to offer very high spectrum efficiency when the signal quality permits. Because of the robust modulation type used for the GSM system, its spectral efficiency is approximately 1.0 to 1.5 bits per Hertz. More efficient modulation types are used in the WCDMA system providing 1.5 to 2.5 bits per Hertz. 802.11 Wi-Fi systems can use efficient modulation types when channel quality is acceptable (e.g. limited interference by other unlicensed devices), which can provide spectral efficiency of 2 to 3 bits per Hertz. The 802.16 WiMAX system can use very efficient modulation providing an efficiency of 3 to 5 bits per Hertz.

The approximate spectral efficiency for several different types of systems. This diagram shows that the spectral efficiency of early mobile telephone systems (e.g. GSM) is approximately 1.0 to 1.5 bits per Hertz and that newer cellular systems (such as WCDMA) have spectral efficiencies of 1.5 to 2.5 bits per Hertz. The spectral efficiency of 802.11 WLAN system can be 2 to 3 bits per Hertz and the spectral efficiency of the 802.16 WiMAX system is approximately 3 to 4 bits per Hertz.

Radio Coverage Area

Radio Coverage Area

Radio coverage occurs when a geographic area receives a radio signal above a specified minimum level. WiMAX can operate up to 50 km under line of sight (LOS) and up to 8 km under non-line of sight (NLOS) conditions. Practical cell sizes are limited to approximately 5 miles.

WiMAX radio coverage varies based on the options installed and used (such as diversity transmission) in the equipment and the modulation (such as QAM –vs- QPSK), frequency and the parameters that are set.

For the most part, there is a tradeoff between data transmission rate and distance. As the modulation type becomes more efficient (more bits per Hertz), the higher the channel quality has to be at the receiver which means the maximum distance that can be used from the transmitter is reduced.

Radio signal attenuation varies from approximately 20 dB per decade in free space to between 40 to 60 dB per decade when signals travel through objects (resulting in building penetration loss). As the distance increases by a factor of ten in freespace, the signal level drops by a factor of 1000, whereas when radio signals travel through objects (walls and floors), the signal may decrease by a factor of 100,000 or more.

Figure 1 illustrates the maximum distance and data transmission rates for fixed and mobile WiMAX communication in a geographic setting. A 20 MHz wide WiMAX radio channel can provide approximately 75 Mbps of data transfer (when it is close to the base station) while the data transmission rate decreases as the distance from the base station increases.

Figure 1: WiMax Carrier Serving Area

Frequency Bands

Frequency bands are the range of frequencies that are used or allocated for radio services. There are two primary frequency bands defined for WiMAX systems; 10 to 66 GHz (the original frequency band) and 2 to 11 GHz. The WiMAX system is designed to allow operation on licensed or unlicensed radio channels.

A licensed frequency band is a range of frequencies that requires authorization for use (a license) from a regulatory agency or owner of the frequency band in a geographic area for permission to transmit radio signals in that area. Unlicensed frequency bands are a range of frequencies that can be used by any product or person provided the transmission conforms to transmission characteristics defined by the appropriate regulatory agency.

Data Transmission Rates & WiMAX Service Rates

Data transmission rate refers to the amount of digital information that is transferred over a transmission medium over a specific period of time and is commonly measured in the amount of bits that are transferred per second (e.g. bps, Mbps).

The data transmission rate for WiMAX systems varies based on factors including radio channel bandwidth (1.25 MHz to 28 MHz), modulation type (BPSK, QPSK, QAM) and channel coding type (percentage of bits dedicated to control and error protection). The raw data transmission rate of a WiMAX radio channel can be in excess of 155 Mbps using QAM modulation. The allocated data transmission rates to each user are typically 1 to 3 Mbps allowing WiMAX operators to have several hundred subscribers for each RF channel.

WiMAX Service Rates

A rate plan is the structure of service fees that a user will pay for services. Rate plans are usually divided into monthly and usage fees.

In the early 2000s, wide area wireless systems (e.g. mobile telephone systems) were limited to relatively low data transmission rates (regularly below 20 kbps). Users were often required to pay for bandwidth on a time or usage basis. Consequently, the cost of this data transmission was approximately 10 cents per kilobyte ($100 per megabyte).

The ability of WiMAX systems to provide high data transmission rates with a limited amount of equipment allows for a substantial reduction in operation cost which thereby leads to significant lowering of service rates. The service rate cost for WiMAX systems in 2006 was approximately 2 to 3 cents per megabyte (99.97% lower than mobile data rates in the mid-1990s).

Figure 1 shows a sample wireless broadband service rate plan for a specific WiMAX system in 2006. This example shows that users may select from 3 rate plan options; basic, standard or premium. Each rate plan has a maximum downlink and uplink data transfer rate associated with it along with a monthly service fee. The amount of allocated data that can be transferred as part of the rate plan (at no cost) may be limited (e.g. 100 MByte maximum) or it may be unlimited (e.g. premium plan). If the user exceeds the allocated data transfer amount for their selected rate plan, they are charged a usage fee (e.g. 2 to 3 cents per MByte). In addition to the service fees, WiMAX service providers may charge a connection fee (setup fee) and there may be a leasing charge for equipment that is provided by the WiMAX service provider (e.g. WiMAX modem).

Figure 1: Sample Wireless Broadband Service Rate Plan 2006

WiMAX Compared to Mobile Telephone Data Systems

Mobile telephone systems are fully automatic wide-area high-capacity RF networks made up of a group of coverage sites called cells. As a subscriber passes from cell to cell, a series of handoffs ensures smooth call continuity.

Mobile telephone systems have evolved to offer a mix of voice and packet data services. These systems are composed of interlinked cells that have the capability to transfer connections from tower to tower. The radio channel bandwidth is relatively narrow compared to WiMAX systems and the modulation types are less efficient (i.e. more robust). Therefore, the maximum data rates of mobile telephone data systems are lower than that of WiMAX.

Figure 1 shows how WiMAX is positioned to fit with cellular data and Wi-Fi systems. WiMAX systems are designed to provide centrally managed highspeed data services over wide areas, whereas Wi-Fi systems are designed to provide self-managed wireless data services over relatively small geographic areas.

Figure 1: Comparison between WiMAX and 3G

Finally, mobile telephone data services are designed to provide a mix of voice and medium speed data services to customers as they move throughout a mobile system.

WiMAX Compared to 802.11 Wi-Fi

802.11 Wi-Fi is an industry standard developed by the IEEE for wireless network communication to provide wireless local area network (WLAN) services. It usually operates in the 2.4 GHz or 5.8 GHz spectrum and permits data transmission speeds from 1 Mbps to 54 Mbps.

WiMAX differs from Wi-Fi in various ways including service range, data transmission throughput, quality of service capability and security processes.

Outlined in Figure 1, 802.16 WiMAX and 802.11 Wi-Fi systems are designed to provide different capabilities. WiMAX systems are well suited for wide area networks that are managed by a service provider and Wi-Fi systems are a good choice for local area networks.

Figure 1: Differences between WiMAX and Wi-Fi