Practical tips on making WiMAX field measurements, Part 1
By Technical Staff, Anritsu Company

Worldwide Interoperability for Microwave Access (WiMAX) digital communications technology is based on wireless transmission methods defined by the IEEE-802.16 air interface standard. Intended for wireless metropolitan area networks, it provides broadband wireless access up to 30 miles for fixed stations, and 3 to 10 miles for mobile stations. WiMAX has a range of potential uses, including providing high-speed mobile data and telecommunications services (such as 4G), as a wireless alternative to cable and DSL for last mile broadband access, and to connect Wi-Fi hotspots with each other and to other parts of the Internet.

The 802.16 air interface standard supports fixed, nomadic, portable and mobile access (Table 1). To meet the requirements of different types of access, two versions of WiMAX have been defined. The first, IEEE 802.16-2004 (fixed WiMAX), is optimized for fixed and nomadic broadband wireless access to homes and businesses. This point-to-multipoint version specifies WiMAX for systems in the 10 to 66 GHz and sub 11-GHz frequency ranges. The second version, IEEE 802.16e (mobile WiMAX), is designed to support portability and mobility and offers the full mobility of cellular networks at true broadband speeds. Both fixed and mobile applications of WiMAX are engineered to help deliver ubiquitous, high-throughput broadband wireless services at a low cost.

 


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Table 1. Types of Access to a WiMAX network.

This How To article focuses on the measurement procedures for fixed WiMAX networks. Its objective is to present practical field measurement tips and procedures which will help a field-based network technician or RF engineer conduct measurements on WiMAX networks.

Understanding WiMAX Technology
The 802.16 broadband wireless access (BWA) standard is designed to provide increased bandwidth and range, efficient bandwidth interference avoidance, and stronger encryption. Conformance and interoperability of the standard is overseen by the WiMAX Forum . In favorable circumstances, wireless connectivity is possible between network endpoints without the need for direct line of sight. Non-line-of-sight propagation (NLOS) performance is also possible and relies on a clever use of multi-path signals.

Some of the main advantages of WiMAX networks include:
 

Because 802.16 was designed to cover application to diverse markets it contains allowances for a number of physical layers to accommodate different frequency bands and region-by-region frequency regulatory rules. These options may leave implementers facing some tough decisions. To address this issue and help speed WiMAX adoption in real-world networks, the WiMAX Forum has created a limited number of system profiles. These profiles specify which features are mandatory or optional for the various MAC or PHY scenarios that are most likely to arise in the deployment of real WiMAX systems. As a result, vendors addressing the same market can build systems for that market which are interoperable but that do not require the implementation of absolutely every feature.

The choice of profiles is driven by market demand, spectrum availability, regulatory constraints, the services to be offered, and vendor interest. For example, the availability of spectrum for BWA services in several countries motivated the creation of profiles in the 3.5-GHz band. By the same token, the availability of license-exempt spectrum and demand for fixed services drove the creation of a profile in the 2.4-MHz and 5.8-GHz bands.

A list of fixed WiMAX system profiles is specified in Table 2. Here, the most globally harmonized band is the licensed 3.5 GHz band (3400 - 3600 MHz).

 


 

Table 2. Fixed WiMAX Initial Profiles (source: WiMAX Forum).

A list of Release-1 mobile WiMAX profiles is provided in Table 3. These profiles cover a range of channel bandwidths for licensed worldwide spectrum allocations in the 2.3, 2.5, 3.3 and 3.5 GHz frequency bands. Selection of frequency for mobile WiMAX deployment directly affects the quality and cost of the network. Lower frequency bands are generally preferred as they offer lower attenuation and longer reach which, in turn, leads to a smaller number of required cells to provide mobility coverage. The most preferred bands of many would-be mobile WiMAX operators today are between 1.9 GHz and 2.1 GHz. These bands, though, have already been assigned to 3G operators. There is also a growing interest in the 700 - 800 MHz bands traditionally used by analog TV broadcast, but it is unclear when these bands will be completely vacated.

 


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Table 3. Release-1 Mobile WiMAX System Profiles (source: WiMAX Forum). For 7 and 8.75 MHz channel bandwidths, sampling factor (instead of FFT size) is made variable.

 

The 802.16-2004 Fixed WiMAX standard covers both the Media Access Control (MAC) and the physical (PHY) layer protocols for extending broadband wireless access to the metropolitan area network. Fixed WiMAX is based on Orthogonal Frequency Division Multiplexing (OFDM). It can accommodate either time division duplexing (TDD) or frequency division duplexing (FDD) deployments, allowing for both full and half-duplex terminals in the FDD case. In licensed bands, the duplexing method can be either TDD or FDD. Unlicensed operation is limited to using the TDD format. As a cost-effective alternative to cable and DSL services, Fixed WiMAX supports scalable channel bandwidths from 1.25 to 20 MHz. The varying channel bandwidth is necessary to address international markets and regional spectrum regulations. It also supports both fixed and nomadic access in line-of-sight and NLOS environments. At higher frequencies, line of sight is a must as it eases the effect of multi-path and allows for wide channels (e.g. typically greater than 10 MHz in bandwidth). As a result, the standard provides very high capacity links on both the uplink and the downlink. For sub 11 GHz, the NLOS capability is a requirement.

802.16-2004 PHY
While multiple PHYs are defined for the 802.16-2004 standard, the one which dominates the industry is the 256-carrier OFDM format (Figure 1). With this modulation scheme, basic OFDM symbols are based on a 256 Fast Fourier Transform (FFT). Out of 256 subcarriers, 55 subcarriers are nulled and set aside for guard bands; the center frequency subcarrier is set to zero, since it is susceptible to carrier leakage; the remaining 200 subcarriers are used for data (192) and pilot (8).

 


 

1. OFDM Subcarriers.

Note that pilot subcarriers are always binary phase shift keying (BPSK) modulated, while the data carriers can be BPSK, quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16 QAM) or 64 quadrature amplitude modulation (64 QAM). These adaptive modulation and coding (AMC) rate configurations can be used to trade off data rate for system robustness under various wireless propagation and interference conditions.

In WiMAX systems with narrow bandwidth, the subcarriers are very closely spaced, thereby providing a relatively long symbol period. Note that a symbol period is defined as 1/subcarrier spacing rather than 1/bandwidth, thereby providing a relatively long symbol period. Closely spaced subcarriers and long symbols can be quite beneficial as they help overcome channel impairments (e.g. multipath). The long symbol period also offers significant advantages for long distances and non-line-of-sight applications.

802.16-2004 MAC
The 802.16-2004 MAC is designed specifically for the point-to-multipoint wireless access environment and supports higher layer or transport protocols such as ATM, Ethernet or Internet Protocol (IP), as well as future protocols. It features very high bit rates (up to 268 mbps each way) of the broadband physical layer, while delivering ATM compatible QoS including: unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS), and best effort service.

As compared to other popular wireless network standards (e.g. 802.11 WLAN), the 802.16-2004 MAC uses a scheduling algorithm that enables the SS to only compete once for initial entry into the network. Once it gains entry, the SS is allocated a time slot by the base station. While the time slot can increase or decrease, it remains assigned to the SS. The slot therefore, cannot be used other subscribers. The scheduling algorithm is stable under overload and over-subscription, and is bandwidth efficient. It also allows the base station to control quality of service (QoS) parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

As an example consider the TDD frame. It consists of one DL subframe followed by one or multiple UL subframes. An adaptive boundary lies between the DL and UL subframes of the TDD signal's flexible frame structure. A short transition gap, known as the transmit/receive transition gap (TTG), is placed between the DL and UL subframes. After completion of the UL subframes, another short gap, the receiver/transmit transition gap (RTG), is added between this subframe and the next DL subframe. The time durations of the transition gaps are a function of the channel bandwidth and the OFDM symbol time and are specified in the 802.16 standard.

In a TDD configuration, the base station and subscriber equipment each transmit on the same RF frequency but are separated in time. Transmissions on both the uplink and downlink always begin with a preamble which allows receivers to synchronize with the transmitter and is used for channel estimation. The DL subframe begins with a long preamble: two OFDM symbols used for synchronization and channel estimation at the SS. It is followed by the frame control header (FCH), which contains decode information for the SS and one or more downlink bursts of payload data. While from burst to burst, the modulation can change, within each burst, the modulation type is constant. Bursts using robust modulation types are required to be transmitted first, followed by less robust modulation types. For example, a downlink subframe containing all four types of modulation would need to be in this order: BPSK followed by QPSK, 16 QAM, and finally 64 QAM. The UL subframe begins with a short preamble: one OFDM symbol that is used at the base station for synchronization to the individual SS. The time duration of the long and short preambles is determined by the specified length of the OFDM symbol.

To simplify transmitter and receiver designs, all symbols in the FCH and DL data bursts are transmitted with equal power. Because the OFDM symbols use four different modulation types it is necessary to scale each, such that the average symbol power from each symbol is approximately equal. The 802.16-2004 MAC offers a number of key benefits. Its frame structure, for example, allows terminals to be dynamically assigned uplink and downlink burst profiles according to their link conditions. This allows a trade-off between capacity and robustness in real-time, and provides roughly a two times increase in capacity on average when compared to non-adaptive systems, while maintaining appropriate link availability. An efficiency increase comes from the use of, among other things, a variable length Protocol Data Unit (PDU). To save PHY overhead, multiple MAC PDUs may be concatenated into a single burst. Multiple Service Data Units (SDU) for the same service may also be concatenated into a single MAC PDU, to save on MAC header overhead. The overhead and delay of acknowledgements is eliminated due to the MAC's use of a self-correcting bandwidth request/grant scheme.

Mobile WiMAX
Mobile WiMAX specifies air interfaces for broadband wireless access systems and is based on Orthogonal Frequency Division Multiple Access (OFDMA) technology. The inherent advantages of OFDMA in terms of channel bandwidth scalability, throughput, latency, excellent QoS, spectral efficiency, tolerance to multipath and self-interference, and advanced antenna support, enable mobile WiMAX to provide higher performance than today's wide area wireless technologies. The specification was created to improve support for combined fixed wireless and mobile NLOS operation in frequencies below 6 GHz. It is considered the first 4G next-generation network solution.

 
Mobile WiMAX supports adaptive modulation and coding in both downlink and uplink with variable packet size. The uplink can support 16QAM modulation (64QAM optional) due to OFDMA orthogonal uplink subchannels. Although the scheduling overhead is higher to support variable packet size, the overhead for fragmentation and padding is reduced. Specific characteristics of the OFDMA PHY include:
 
  • Downlink modulation: 64QAM OFDM, 16QAM OFDM, and QPSK OFDM
  • Downlink code rate: Turbo and Convolutional Code (CC), Repetition: 1/12, 1/8, 1/4, 1/2, 2/3, 3/4, and 5/6
  • Uplink modulation: 16QAM OFDM, QPSK OFDM, and 64QAM OFDM (optional)
  • Uplink code rate " Turbo, CC, Repetition: 1/12, 1/8, 1/4, 1/2, 2/3, 3/4, and 5/6 (optional)
  • FFT size: 128, 512, 1024, or 2048
  • Cyclic Prefix (G): 1/32, 1/16, 1/8 and 1/4
  • Error correcting code: BTC (Block and Convolutional Turbo code), CC, and CTC (Convolutional Turbo Coding)

 


 

2. Downlink and uplink subframes.

The typical 802.16-2004 MAC frame structure consists of a downlink (DL) subframe and an uplink (UL) subframe (Figure 2). Currently there are seven supported frame durations in the IEEE 802.16-2004 standard ranging from 2.5 to 20 ms.

Some of the specific features of mobile WiMAX include:
 

  • Applies fast scheduling in both downlink and uplink. WiMAX performs scheduling on a per-frame basis and broadcasts the downlink/uplink scheduling in the MAP messages at the beginning of each frame. With Mobile WiMAX, the scheduling can change very quickly as can the amount of resources allocated ranging from the smallest unit to the entire frame. Therefore, it is well suited for bursty data traffic and rapidly changing channel conditions. Particularly since uplink subchannels are orthogonal. With uplink scheduling, the uplink resource is more efficiently allocated, performance is more predictable, and QoS is better enforced.
  • Flexible configuration of zones, bursts, and MAC PDUs (Protocol Data Units)
  • Variable bandwidths: mobile WiMAX supports channel bandwidths of 5 MHz, 7 MHz, 8.75 MHz, and 10 MHz and can optionally support channel bandwidths ranging from 1.25 MHz to 20 MHz. With the flexibility to support a wider bandwidth, Mobile WiMAX enjoys high aggregate sector throughput, which allows more efficient multiplexing of data traffic, lower latency and better QoS.
  • FCH (Frame Control Header), downlink MAP (which defines the access to the downlink information) and uplink MAP message (which allocates access to the uplink channel) automatically generated
  • Standards-based, raw, or fully-coded data
  • Partial Usage of Subchannels (PUSC), Full Usage of Subchannels (FUSC) and OFUSC Permutation Zone for downlink, PUSC and OPUSC Permutation Zone for uplink. Note that a permutation formula maps the subchannels to physical subcarriers in the OFDMA symbol. The formula varies for uplink and downlink.
  • Channel coding: CC and CTC
  • Space-Time-Coding (STC) with two antennas sources for both downlink and uplink. STC is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas.
  • SUI channel model, ITU channel model and 3GPP multiple input multiple output (MIMO)channel model
  • CC decoding with soft decision (with channel state information - CSI)

 


 

3. Possible OFDMs for 802.16e OFDMA.

Mobile WiMAX introduces support for an enhanced OFDMA modulation method known as Scalable OFDMA. SOFDMA allows for a variable number of carriers, in addition to the previously-defined OFDM and OFDMA modes (Figure 3). The carrier allocation in OFDMA modes is specifically designed to minimize the effect of the interference on user devices with omni-directional antennas. 802.16e also offers improved support for hard handoffs and features improved power-saving capabilities for mobile devices.

In particular, SOFDMA improves on OFDM 256 for NLOS applications in the following ways:
 

  • Improves coverage via advanced antenna diversity schemes and hybrid-Automatic Retransmission Request (hARQ). To improve packet reception, Chase Combining (CC) is implemented at the receiver to jointly process the packets in error and new retransmission.
  • Increases system gain and improves indoor penetration though use of denser sub-channelization.
  • Enhances security and NLOS performance by using high-performance coding techniques like Turbo Coding and Low-Density Parity Check (LDPC).
  • Allows administrators to trade coverage for capacity or vice versa by introducing downlink sub-channelization.
  • Improves coverage by using Adaptive Antenna Systems (AAS) and MIMO technology.
  • Eliminates channel bandwidth dependencies on subcarrier spacing, which allow for equal performance under any RF channel spacing (1.25-14 MHz)
  • Increases resistance to multipath interference by using an enhanced FFT algorithm which can tolerate larger delay spreads.