WiMAX: Next-generation Mobile Broadband Wireless Technology

By: Dr. Ramesh Chandran, SeaSolve Software Inc.
( 1 May 2008 )

The spread of mobile broadband wireless technology has revolutionized the way to communicate and share data. The fourth generation (4G) wireless systems are expected to provide data rates with mobility in the order of hundreds of Mbps.

The IEEE 802.16 working group has been tasked with defining broadband wireless access technology that comes under the category of Wireless Metropolitan Area Network (WMAN). This standard defines the access methodology for Point-to-Point (PTP) and Point- to-Multipoint (PTM) broadband wireless links. The IEEE 802.16 standards do not define end-to-end architectures and nor do they address interoperability issues across products.

Worldwide Interoperability for Microwave Access, commonly called WiMAX, is a wireless networking standard, which does define an end-to-end system architecture and addresses interoperability issues for IEEE 802.16-based products. The WiMAX Forum acts as a certification and verification body for all vendor equipment and is similar to the Wi-Fi Alliance for Wireless Local Area Network (WLAN) products.

WiMAX certification aims at ensuring that a WiMAX system conforms to the IEEE 802.16/ETSI HiperMAN specifications selected by the WiMAX Forum and that WiMAX equipment from different vendors is interoperable. A Wave 1 certification profile includes basic functionalities for WiMAX operation. Wave 2 certification is backward compatible with Wave 1 certification. In addition to Wave 1's physical layer features, AMC (Adaptive Modulation and Coding), Effective CINR using pilots, Multiple-Input Multiple-Output (MIMO) and Beamforming are added for Wave 2 certification to improve system performance. IEEE 802.16/WiMAX promises to be the core enabling technology for next generation broadband mobile wireless application including last mile solutions, hotspots, cellular backhauls and high speed broadband access that supports mobility.

WiMAX is considered as a potential candidate for 4G wireless communication which can provide data rates up to 100Mbps with full mobility and a greater coverage area (typically up to a 5km range). Finally, WiMAX will be a serious competitor to third generation (3G) cellular broadband wireless systems and other candidates for 4G (such as the Long Term Evolution or LTE).

The IEEE 802.16 standard supports both timedivision duplex (TDD) and frequency division duplex (FDD) modes. Although both modes are supported, broadband operators appear to prefer the TDD mode, because most broadband networks support Internet access which typically requires asymmetric bandwidth.

The Physical Layer (PHY) specifications allow WiMAX devices to operate in various frequency bands: 700MHz, 2.3GHz to 2.4GHz, 3.4GHz to 3.6GHz and 5.7GHz to 5.8GHz. IEEE 802.16d/e devices can operate in scalable bandwidths varying from 1.25MHz to 20MHz. Most of the countries prefer to operate in the 3.5, 5, 7, 10 and 20MHz bandwidths to take care of interoperability issues across all WiMAX vendor equipments.

IEEE 802.16e transceivers use a modulation technique known as Scalable Orthogonal Frequency-Division Multiple Access (S-OFDMA) for data transmission. OFDMA combined with higher order data modulation techniques can support high data rate applications. OFDM/A is also considered as a key physical layer technology for all future wireless communication like UMTS LTE and UMB (Ultra Mobile Broadband). Similar to the frequency division multiplexing (FDM) technique, OFDM employs a multicarrier technique where data is sent over a large number of channels called subcarriers. The large numbers of low bit rate carriers are transmitted in parallel in an OFDM symbol duration. An OFDM symbol consists of all modulation symbols (in parallel) that occur at the same time on all subcarriers.

All these carriers are synchronized in time and frequency to form a single block of spectrum. The longer the modulation symbol time (the slower the modulation), the closer together the subcarriers, and thus more subcarriers can be in the allotted bandwidth.

Figure 1 describes the concept of OFDM in a three-dimensional view. Subcarrier spacing is the difference between the two adjacent center frequencies of the subcarriers. The subcarriers are equally and closely spaced at exactly the inverse of the modulation rate. The OFDM symbol in the figure includes six modulation symbols. One can observe only pure, unmodulated sine waves on each of the subcarriers making up an OFDM symbol.

The baseband frequency of each subcarrier is chosen to be an integer multiple of the inverse of the symbol time. OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. The cyclic prefix is added with the OFDM symbol in order to prevent the effect of the channelinduced ISI from the previous symbols. The cyclic prefix is a repetition of the last data symbols in an OFDM block.

OFDMA BASESTATION IQ GENERATION AND ANALYSIS
SeaSolve Software Inc.'s SeaMAX suite is divided into a host of signal generation and analysis modules that enable the user (in most cases, a test engineer) to check the receiver and transmitter characteristics of WiMAX devices during the design and manufacturing stages, in the production line and even in the field (post-production).

Through signal generation, users are capable of testing the receiver characteristics of a DUT at the PHY layer with the added ability of setting crucial MAC level parameters for the downlink and uplink sub-frames. The SeaMAX Generator suite features the creation of multiple MPDUs in each DL/UL burst with user-defined payload lengths. The Mobile-WiMAX generators additionally support signal characteristics specific to OFDMA such as variable FFT sizes, bandwidth, cyclic prefix length, boosting and sub-channelization, thus making their signals completely standard-compliant. PUSC (Partial Usage of Subchannels), FUSC (Full Usage of Subchannels) and AMC are the different zones used in the WiMAX downlink subframe. The usage of subcarriers is heavier in FUSC as compared to PUSC to improve the overall system¡¯s data transmission. The objective of the AMC scheme is to increase the per user throughput by changing the modulation-order coding-rate based on channel conditions. The channel conditions can be estimated based on the feedback from the MS through a given feedback channel. The WiMAX Forum's Wave 2 Certification profile includes AMC testing, which the SeaMAX 16e BS Generator provides with different modulation and coding rates for AMC.

In order to characterize the receiver performance over a broad range of signal qualities, the SeaMAX Generator Suite also allows the emulation of various RF impairments and channel models. These include AWGN, frequency offset, IQ imbalance, phase noise and SUI/mobility channel models. SeaMAX also allows a signal to be previewed through plots of IQ, CCDF, constellation diagrams and power spectrum, and also has a dedicated frame view that shows the layout of the DL subframe including the configured FCH, MAP messages and bursts.

The SeaMAX Analyzer Suite comprises of solutions that act as virtual WiMAX receivers compliant to both IEEE 802.16d¨C2004 and 802.16e-2005. The SeaMAX Analyzers are capable of demodulating the complete PHY of a received signal and displaying its baseband characteristics for the received WiMAX frame. The analyzer module is capable of decoding the basic MAC payload data received during an acquisition and saving it to file in hex/binary format. The demodulation is shown with a number of plots and the measurements including Power spectrum, EVM, Signal constellation and channel frequency response (CFR). The Error Vector Magnitude (EVM) is a measure that is used to quantify the performance of digital transmitter and receiver. CFR gives the behavior of the wireless channel for all the subcarriers across the system bandwidth.

A channel estimation process is carried out in the SeaMAX Analyzer in two steps. Initial channel estimation, coarse frequency offset and timing offset correction are done using the preamble which is present in the beginning of the frame. The channel tracking and fine offset estimation are carried out using the pilot subcarriers transmitted along with data subcarriers. The population of pilot subcarriers is more in PUSC as compared to FUSC.

Figure 3 shows SeaSolve¡¯s Mobile subscriber station (MSS) Generator and Analyzer solutions. The Generator includes extra options like the selection of ranging over data bursts. The ranging option includes initial ranging, periodic ranging, bandwidth request and handover ranging. Multiple-Input and Multiple-Output (MIMO) and Adaptive Antenna System (AAS) options are also being included in the SeaMAX suite. MIMO options include emerging features like closed or open-loop MIMO. Users can configure the number of transmit and receive antennas as per the certification profile and select the features as per the test¡¯s requirement.

REFERENCES
- IEEE Standard 802.16-2004 Air Interface for Fixed Broadband Wireless Access Systems

- IEEE Standard 802.16-2005 Air Interface for Fixed and Mobile Broadband Wireless Access Systems

- Mobile WiMAX-Part 2: A Technical Overview and Performance Evolution ¨C WiMAX Forum

- WiMAX Forum Stage 2&3-NWG Document: WiMAX End-to-end Architecture, 2005


About the Author
Dr. Ramesh Chandran is Member Research Engineer at SeaSolve Software Inc., www.seasolve.com.


CAPTIONS
Figure 1: A three-dimensional view of the OFDM signal.

Figure 2: SeaMAX OFDMA Base Station IQ Generator and Analyzer.

Figure 3: SeaMAX OFDMA MSS IQ Generator and Analyzer.

Click here for Illustrations:

Figure 1, Figure 2, Figure 3