There have been a number of ill-fated efforts to make data services come to life over fixed broadband wireless links. From LMDS to MMDS to proprietary approaches, designers have struggled to make fixed broadband wireless a true competitor to DSL and cable modem connections.
Until now. New work being conducted at the 802.16 committee has breathed new life into developing systems that delivery data services over broadband wireless links. And, with the development of the 802.16 specification (also known as WiMAX), the IEEE is providing a technology platform for developing low-cost radios that can make fixed broadband wireless soar.
In this two-part tutorial, we'll take a detailed look at the key technical elements defined under the 802.16a specification. In Part 1, we'll provide a basic overview of the specification, examine the physical layer requirements, and look at the frame and media access control structures. Then in Part 2, we'll further the discussion by detailing the key technologies required for MAC operation while also looking at some future enhancements on the horizon.
Brief Overview
The core components of a WiMAX system are the subscriber station (SS) otherwise known as the CPE and the base station (BS). A BS and one or more SSs can form a cell with a point-to-multipoint (P2MP) structure. On air, the BS controls activity within the cell, including access to the medium by SSes, allocations to achieve quality of service (QoS) and admission to the network based on network security mechanisms.
An 802.16-based system often uses fixed antenna at the subscriber station site. The antenna is mounted to the roof or an eave. Provisions such as adaptive-antenna systems (AAS) and sub-channelization are also supported optionally by the standard for enhanced link budget required for in-door installation. IEEE 802.16e sub-committee is currently working on extension to the standard required for mobility and support for the power limited SS terminals.
A BS typically uses either sectored/directional or omni-directional antennas. A fixed SS typically uses directional antenna while mobile or portable SS usually uses an omni-directional antenna.
Multiple BSes can be configured to form a cellular wireless network. When orthogonal frequency division multiplexing (OFDM) is used, the cell radius can ideally reach up to 30 miles, however this requires a favorable channel environment and only the lowest data rate can be achieved. Practical cell sizes usually have a small radius of around 5 miles or less. The 802.16 standard also can be used in a point-to-point (P2P) or mesh topology, using pairs of directional antennas. This can be used to increase the effective range of the system relative to what can be achieved in P2MP mode.
802.16: What Features it Supports
WiMAX supports both time division duplex (TDD) and frequency division duplex (FDD) modes of operation on air, along with a range of channel bandwidths. The OFDM PHY mode, which is also known WirelessMAN-OFDM, is specified for use between 2 and 11 GHz.
The 802.16 MAC controls access of the BS and SSes to the air through a rich set of features. The on-air timing is based on consecutive frames that are divided into slots. The size of frames and the size of individual slots within the frames can be varied on a frame-by-frame basis, under the control of a scheduler in the BS. This allows effective allocation of on air resources to meet the demands of the active connections with their granted QoS properties.
The 802.16 MAC provides a connection-oriented service to upper layers of the protocol stack. Connections have QoS characteristics that are granted and maintained by the MAC. The QoS parameters for a connection can be varied by the SS making requests to the BS to change them while a connection is maintained.
QoS service in the 802.16 MAC service takes one of four forms: constant bit rate grant, real time polling, non-real-time polling, and best effort. Media access control packet data units (MPDUs) are transmitted in on-air PHY slots. Within these MPDUs, MAC service data units (MSDUs) are transmitted. MSDUs are the packets transferred between the top of the MAC and the layer above. MPDUs are the packets transferred between the bottom of the MAC and the PHY layer below.
Across MPDUs, MSDUs can be fragmented. Within MPDUs, MSDUs can be packed (aggregated). Fragments of MSDUs can be packed within a single packed MPDU. Automatic retransmission request (ARQ) can be used to request the retransmission of unfragmented MSDUs and fragments of MSDUs.
The MAC has a privacy sublayer than performs authentication, key exchange and encryption of MPDUs. Layering of the 802.16 protocol is shown in Figure 1.
Figure 1: Diagram illustrating the layers of the 802.16 protocol.
Through the use of flexible PHY modulation and coding options, flexible frame and slot allocations, flexible QoS mechanisms, packing, fragmentation and ARQ, the 802.16 standard can be used to deliver broadband voice and data into cells that may have a wide range of properties. This includes a wide range of population densities, a wide range of cell radii and a wide range of propagation environments.
Convergence sublayers at the top of the MAC enable Ethernet, ATM, TDM voice and IP (Internet Protocol) services to be offered over 802.16.
Basic Profiles
WiMAX defines interoperable system profiles targeted for common licensed and unlicensed bands used around the world. This enables 802.16-based equipment to be used in diverse spectrum allocations around the world. Table 1 lists WiMAX MAC and PHY basic profiles for 2-11GHz WirelessMAN and WirelessHUMAN OFDM.
Table 1: WiMAX Basic Profiles for WirelessMAN OFDM
WiMAX system profiles consist of one of the two basic MAC profiles—P2MP or Mesh—and one of the six PHY profiles listed in Tab le 6. Three bandwidth sizes 1.75, 3.5 and 7 MHz are primarily considered for 3.5GHz ETSI band, 3 and 5.5 MHz channelizations are considered for MMDS band, while the 10 MHz channelization can be used for unlicensed bands.
The OFDM PHY
Figure 2 shows the physical layer transmit pipeline for WirelessMAN OFDM as specified in IEEE 802.16. The OFDM signaling format was selected in preference to competing formats such as single-carrier (SC) and CDMA due to its superior non line-of-sight (NLOS) performance, which permits significant equalizer design simplification to support operation in multipath propagation environments.
Figure 2: WirelessMAN OFDM transmit signal processing pipeline.
Reel-Solomon and convolutional coding are mandatory forward-error correction techniques that must be used when implementing the WirelessMAN OFDM PHY. Table 2 lists the corresponding rate set along with the modulation types.
Table 2: Mandatory Channel Coding Per Modulation.
Note: The code rates mentioned in this table are the overall code rates achieved by Reed-Solomon and convolutional coding.
In WirelessMAN OFDM, the Fast Fourier transform (FFT) size is 256. In this 256 block, 55 sub-carriers (28 low and 27 high) are set aside for guard band and 8 sub-carriers are used for pilot signal.
Fixing the number of used sub-carriers to 192, the system uses different over-sampling rates of higher than 1, 8/7 and 7/6, to maximize the achieved throughput while meeting spectral masks of different regulatory requirements. The WirelessMAN OFDM supports a wide range of guard time sizes relative to the OFDM symbol period (Table 3). At the maximum, 25% guard time can be considered while the minimum value of 3% can be used in relatively benign channel condition.
Table 3: OFDM Symbol Parameters
Table 4 provides typical root-mean-square (rms) delay spread values corresponding to an omni antenna for the IEEE 802.16a channel models. The link distance (cell radius) for these delay-spread values is 7 km. It is important to highlight the range of rms delay spread, depending upon terrain conditions, shown as Terrain Type A, B, or C in the table:
- Terrain Type A: The maximum path loss category; hilly terrain with moderate-to-heavy tree densities.
- Terrain Type B: The intermediate path loss category.
- Terrain Type C: The minimum path loss category; mostly flat terrain with light tree densities.
Table 4: IEEE 802.16 Channel Models
The worst-case delay spread of 5.24 μsec can be supported with 1/4 guard time option for a wide channel bandwidth of 10MHz. In a mobile environment with omni antenna in subscriber stations, a typical 10 μs delay spread can be supported with 1/4 guard time for 5 MHz channel bandwidth.
Table 5 shows the data rates achieved for various bandwidths and combination of modulation types and coding rates. A guard time value of 1/32 is used. The rates here consider the effect of PHY overhead but MAC overhead and preamble overhead are not included in calculation.
Table 5: Data Rates Achieved at Various 802.16 Bandwidths
IEEE 802.16 also considers optional sub-channelization in uplink. This feature is particularly useful when a power-limited platform such as a laptop is considered in the subscriber station in an indoor portable or mobile environment. With a sub-channelization factor of 1/16, a 12-dB link budget enhancement can be achieved. Sixteen sets of 12 sub-carriers each are defined, where one, two, four, eight or all sets can be assigned to a subscriber station in uplink. The eight pilot carriers are used when more than one set of sub-channels are allocated.
To support and handle time variation in the channel, the 802.16 standard provisions optional, more frequent repetition of preambles. In the uplink path, short preambles, called mid-ambles for this purpose, can be repeated with a programmable repetition period. The options are:
- Preamble only
- Mid-amble after every L = 8 data symbols
- Mid-amble after every L = 16 data symbols
- Mid-amble after every L = 32 data symbols
In the downlink direction, a short preamble can be optionally inserted at the beginning of all downlink bursts in addition to the long preamble that is presented by default at the beginning of the frame. A proper implementation of base station scheduler guarantees minimum required repetition interval for channel estimation.
Analysis shows that with implementation of appropriate channel estimation interpolation scheme, mobility speed of up to 150 km/hr can be supported with L better than 10.
Framing and Media Access
In 802.16, the PHY part of the specification is responsible for the on air framing, media access, and slot allocation. The MAC itself is concerned with performing the mapping from MSDUs to the MPDUs carried in the on air transmissions. This places the division of labor between the PHY and MAC slightly higher in the stack than is common.
On air transmission time is divided into frames. In the case of an FDD system, there are uplink (SS to BS) and downlink (BS to SS) subframes that are time aligned on separate uplink and downlink channels. In the case of a TDD system, each frame is divided up into a downlink subframe and an uplink subframe.
In both TDD and FDD modes, the length of the frame can vary (under the control of the BS scheduler) per frame. In TDD mode, the division point between uplink and downlink can also vary per frame, allowing asymmetric allocation of on air time between uplink and downlink if required.
The downlink frame format includes a preamble, a DL_MAP, a UL_MAP, and downlink slots. The DL_MAP is a directory of the slot locations within the downlink subframe. The UL_MAP is a directory of slot locations within the uplink subframe. It is through the DL_MAP and UL_MAP frame descriptors that the BS allocates access to the channel for both uplink and downlink.
The SS uses the DL_MAP to identify the location of MPDUs within the frame and listens to each of the MPDUs in turn, receiving those that match a connection ID targeted at that SS.
Uplink framing is more complex, since for best effort delivery and network entry, a contention-based multiple access scheme is required in order to mediate between SSes that are simultaneously seeking access to the medium. Based on the QoS service used for a connection, a connection may have either a guaranteed slot, may get access to a guaranteed slot on a per frame basis through polling from the BS or it may have to contend for uplink access on a contention basis in a multiple access (TDMA) slot.
Contention access takes place in slots set aside for the purpose in the uplink, the contention slot for initial ranging slots and the contention slot for bandwidth requests. Each of these slots is divided into minislots. SSes contending for access use a truncated binary exponential backoff algorithm to elect which mini slot to begin its transmission in.
The initial ranging contention slot is used as part of the network entry algorithm. An SS transmits a ranging request (RNG-REQ) packet in the initial ranging contention slot. The RNG-REQ packet has a long preamble, enabling the BS to better identify the timing of the received RNG-REQ packet. If the RNG-REQ is received, the BS responds a RNG-RSP (ranging response) giving timing and power adjustment information to the SS. The SS can then adjust the timing to account for transit delays and path loss of its transmissions such that the timing and power of the signal as received at the base station aligns with transmissions from other SSes.
The bandwidth request contention slot is used by SSes to content for access to the channel. Bandwidth requests are transmitted into this slot. Once a bandwidth request has been received and granted, the SS may use non-contention slots allocated by the BS.
The BS dictates the length of the contention slots. The optimal length for either of the contention slots might change based on any of a variety of parameters such as the number of SSes, the number and type of QoS connections allocated and current activity levels.
On to Part 2
That wraps up our looking at the 802.16 PHY layer and framing structures. In Part 2, which will appear online tomorrow, we'll examine the 802.16 MAC layer while looking at some enhancements to the spec that are under development.
About the Authors
David Johnston is senior communication engineer with Intel Corporation. He is actively involved in the development of the 802.16 standard and has a background in the development of wireless communication systems. David is also presently the interim chair of the 802.21 committee. David can be reached at dj.johnston@intel.com.
Hassan Yaghoobi is a technical marketing engineer for Intel's Broadband Wireless Division. He also serves as secretary of the 2-11 GHz Technical Working Group for the WiMAX Forum, an industry group focused on interoperability of systems that conform to the IEEE 802.16a standard. Hassan can be reached at hassan.yaghoobi@intel.com.
