Mobile WiMAX PHY (RF) operation and measurement, Part 1

Part 1 of this article introduces the WiMAX standard and provides in-depth explanation of the IEEE 802.16 profiles.
Part 2 profiles control mechanisms and the mobile station.
Part 3 covers the RF Test Suite and transmitter measurement set up.
Part 4 covers transmitter power, including spectral flatness, spurious emissions, and adjacent channel power. .
Part 5 covers modulation and frequency tests.

Adding user mobility features to the conventional orthogonal frequency division multiplexing (OFDM) signal used in 802.16d for fixed WiMAX introduces substantially more flexibility in the way that the radio signals are constructed. Mobility features give the network operator the freedom to adapt the operation of the base station (BS) to specific requirements of the physical location. It also introduces many variables that need to be understood and tested at several levels—from basic parametric tests, to end-to-end performance evaluation.

WiMAX and the IEEE 802.16 standard
WiMAX is a broadband wireless access (BWA) technology based on the IEEE 802.16 standard. The WiMAX Forum uses the 802.16-2004 specification and subsequent corrigenda (corr 2) as a base for it to build the mobile WiMAX standard. Where the original fixed WiMAX profiles defined a set of parameters to meet the needs for fixed and nomadic access, the mobile WiMAX profiles support high mobility broadband services at speeds greater than 120 km/h. Among the important features in mobile WiMAX are OFDMA and sub-channelization. With sub-channelization, users can be allocated spectrum components across the bandwidth available to everyone. This brings the benefit of frequency diversity without having to frequency hop.

The IEEE 802.16 Working Group originally developed the broadband point-to-multipoint (PMP) standard as a wireless extension from a wired network infrastructure. The first approved air interface was based on a time division multiple access (TDMA) protocol. The interface supported time division duplex (TDD) and frequency division duplex (FDD). The architecture was originally configured for fixed antenna terminals with line of sight (LOS) propagation across the 10 to 66 GHz frequency range. The standard was later expanded to include operation in the 2 to 11 GHz range with non line of sight (NLOS) capability using a robust OFDM technique.

Mobile WiMAX is based on 802.16-2004 [1] and 802.16e-2005 [2], which is now being combined into a single document. The updated standard combines fixed and mobile services into a network architecture similar to a cellular system where a single BS can support fixed, portable, and mobile terminals. Unlike existing cellular systems, mobile WiMAX uses an all internet protocol (IP) backbone. The standard includes an OFDMA PHY with sub-channelization that allows the time and frequency resources to be dynamically allocated among multiple users across the downlink (DL) and uplink (UL) sub-frames.

The dynamic allocation of the time and frequency resources can be examined using a spectrogram. A spectrogram shows how the use of the spectrum changes with time. Figure 1 shows a spectrogram of an OFDMA frame including DL and UL sub-frames. In this simplified example, two users are assigned groups of frequency subcarriers during the first three symbols in the UL sub-frame. As shown in the figure, the subcarrier or sub-channel assignments may change dynamically over the time duration of the sub-frame in response to changes in the radio channel due to fading, interference, and performance quality requirements. The physical subcarrier to sub-channel assignment or mapping is specified in the 802.16 documents. Also of note, and clearly visible in the DL signal, are the pilot subcarriers. These provide the receivers with the information needed to remove the frequency and time response of the radio channel.

WiMAX Forum waves and profiles
The WiMAX Forum is comprised of industry experts whose charter is to bring the IEEE 802.16 standard to the marketplace and to create the process for certification and inter-operability between equipment vendors. The WiMAX Forum tests operational performance based on the standard through the use of radio and protocol conformance test documentation. Subsets of system features are known as profiles, which specify the mandatory and optional features from the 802.16 standard required for baseline functionality and interoperability. The choice of profiles has been driven by spectrum availability, regulatory constraints, and market demand. To reduce the complexity involved when releasing a new radio system, the certification process has been broken down into waves where basic system functionality is introduced during Wave 1 and more advanced features, such as MIMO, are added during Wave 2. Table 1 shows some of the basic requirements for Wave 1. Over time, it is expected still further enhancements will be introduced, for example operation in a hybrid FDD mode.

The WiMAX Forum specifies a series of protocol and radio conformance tests (RCT) for compliance and interoperability between various equipment manufacturers. Certification test houses, such as AT4 Wireless in Spain and Telecommunications Technology Association (TTA) in Korea, were among the first to be approved by the WiMAX Forum to provide conformance testing to the WiMAX profile specifications.

Mobile WiMAX and WiBRO
WiBRO, short for wireless broadband, is a portable internet service based on the 802.16e standard that is currently being rolled out in South Korea. WiBRO operates in the 2.3 GHz spectrum and uses the same PHY and media access control (MAC) as defined in one of the mobile WiMAX profiles. Mobile WiMAX PHY
The mobile WiMAX PHY uses a combination of TDD and OFDMA for downlink and uplink signaling and multiple user access. The unique features within the TDD/OFDMA frame provide frequency diversity, frequency reuse, and cell segmentation which improve the performance against fading and inter-cell interference.

TDD
The WiMAX OFDMA frame is configured to support a point-to-multipoint network. The 802.16e PHY supports TDD, FDD, and half-duplex FDD operation. The initial release of the Mobile WiMAX profile will only include TDD as shown in Table 1. Future releases may include FDD variants to match spectrum regulatory requirements in specific countries. For interference mitigation, system-wide synchronization is required when using TDD. Synchronization is typically achieved using a global positioning system (GPS) reference at the BS. In the event that network synchronization is lost, the BS will continue to operate until synchronization is recovered, using a local frequency reference. TDD, as specified in the WiMAX profile, enables asymmetric DL and UL traffic. Asymmetric traffic using TDD may improve the spectrum utilization and system efficiency as compared to FDD operation which typically requires equal UL and DL bandwidths. TDD uses a common channel for both UL and DL transmission allowing for a lower cost and less complex transceiver design. TDD also assures channel reciprocity which may benefit applications such as MIMO and other advanced antenna technologies.

OFDM and OFDMA
An OFDM system is implemented by multiplexing a single high data rate input stream into a parallel combination of low data rate streams. The parallel streams are modulated onto separate subcarriers in the frequency domain through the use of an inverse fast Fourier transform (IFFT) [4]. In a typical OFDM system, one user data occupies all the subcarriers in the channel with the exception of pilot, guard, and null subcarriers. An example of a DL sub-frame where three users are multiplexed into the same OFDM channel is shown in Figure 2. At any one instant, only one user's data is modulated onto all the available subcarriers or sub-channels across the frequency domain. In the time domain, a single user will occupy one or more OFDM symbols within the sub-frame. Multiple access is achieved by time slotting the sub-frame using TDMA. As shown in Figure 2, multiple users can sequentially share a portion of the sub-frame in time.

With the addition of subscriber mobility to the IEE802.16 standard, a frequency domain multiple access scheme, referred to as OFDMA, was added to the WiMAX PHY. For the OFDMA system, several users may now be assigned different sets of frequency subcarriers, which effectively allow them to transmit simultaneously in time. Users may be dynamically added or dropped over the duration of the sub-frame. Figure 3 shows an example where three users share the available frequency sub-channels and can simultaneously transmit data in time. As shown in Figure 3, sub-channels may be re-allocated to other users over the duration of the sub-frame. This dynamic allocation in both time and frequency greatly improves the efficiency of the available resources but at the cost of considerable complexity to the air interface and the BS scheduler.

Time and frequency parameters
The IEEE 802.16e air interface as adopted by the WiMAX Forum specifies channel bandwidths ranging from 1.25 to 20MHz. The first release of the mobile WiMAX system profile incorporated 5, 7, 8.75, and 10 MHz bandwidths as shown in Table 1. The bandwidth scalability in Mobile WiMAX OFDMA is achieved by adjusting the FFT size and the subcarrier spacing. For a given channel bandwidth, the subcarrier spacing is inversely proportional to the number of subcarriers and, therefore, the FFT size. The time duration of the OFDMA symbol is set by the inverse of the subcarrier spacing. Therefore by fixing the subcarrier spacing, the symbol time is automatically specified. The inverse relationship between subcarrier spacing and symbol duration is a necessary and sufficient condition to ensure that the subcarriers are orthogonal. Table 2 shows the subcarrier spacing and symbol time for the Mobile WiMAX 10 and 8.75 MHz (WiBRO) profiles using nominal bandwidths of 10 and 8.75 MHz respectively.

The Mobile WiMAX frame contains 48 symbols. The symbol time contains the actual user data and a small extension called the guard time. The guard time is a small copy from the end of the symbol that is inserted before the start of the symbol. This guard time is also called the cyclic prefix (CP) and its length is chosen based on certain assumptions about the wireless channel. As long as the CP interval is longer than the channel delay spread, inter-symbol interference (ISI) introduced by the multi-path components can be eliminated. The 802.16 standard specifies a set of CP values but the initial profile specifies a CP value of 1/8, meaning that the guard time is 1/8 the length of the symbol time. Table 2 shows the guard time and symbol duration for the Mobile WiMAX and WiBRO using the nominal bandwidth of 10 and 8.75 MHz respectively.

Frame structure
The OFDMA frame consists of a DL sub-frame and an UL sub-frame. The flexible frame structure of the TDD signal consists of a movable boundary between the DL and UL sub-frames. A short transition gap is placed between the DL and UL sub-frames and is called the transmit-receive transition gap (TTG). After the completion of the UL sub-frame, another short gap is added between this sub-frame and the next DL sub-frame. This gap is called the receiver-transmit transition gap (RTG). The minimum time durations for these transition gaps are called out in the 802.16 standard and are a function of the channel bandwidth and the OFDM symbol time. It is typical to define these transition gaps in terms of physical slot (PS) units. A PS is a unit of time defined as 4/(sampling frequency). The sampling frequency is equal to the FFT size multiplied by the channel spacing. Table 2 shows the PS for the 10 and 8.75 MHz cases using a 1024-point FFT.

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