Editor's Note: This is a multi-part excerpt from Agilent Technologies' Application Note: Mobile WiMAX PHY Layer (RF) Operation and Measurement.
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.
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Part 5 covers modulation and frequency tests.
This Part profiles control mechanisms and the mobile station.
Control mechanisms
Ranging
Ranging is used for several functions such as bandwidth requests, timing and power control, periodic maintenance, and handover. Initial ranging, using the special CDMA ranging burst, is used by any MS that wishes to synchronize to the system for the first time. Default time and power parameters are used to initiate communication and then adjusted until they meet acceptance criteria. Periodically these parameters are updated during a maintenance interval. A ranging channel is defined in the UL and composed of one to eight adjacent sub-channels. A PN code sequence is chosen by the MS and this sequence is modulated onto the subcarriers. The PN sequence provides a spreading gain that allows multiple ranging transmissions to be simultaneously received by the BS thus defining this technique as CDMA ranging [5].
Fast feedback
Fast feedback slots are assigned to individual MSs when the BS requires information from the MSs in a very timely manner. Information concerning CINR, MIMO, antenna, and spatial multiplexing are all examples of time critical parameters that can make use of the fast feedback channel. A portion of the beginning of the UL sub-frame is allocated for this function. Orthogonal encoding is used to allow for multiple MSs to transmit simultaneously within the same slot.
Hybrid automatic repeat request (HARQ)
A MAC level automatic repeat request (ARQ) is used by the receiver to provide feedback on successfully received and missing blocks of data. An ARQ mechanism greatly reduces the error rate at the expense of adding a time delay. The major differences between ARQ and HARQ are that ARQ discards previously transmitted data while HARQ combines the previous and retransmitted data to gain time diversity. HARQ also makes use of faster responding physical layer ACK and NACK packets, transmitted from the receiving device. In operation, HARQ operation is typically planned to operate with approximately a ten percent packet error rate, minimizing delays in repeat packet transmission, which has a dramatic effect on data throughput. HARQ is one element in the Mobile WiMAX profile that can deliver high performance with mobility at speeds in excess of 120 km/h.
Media access control (MAC) layer
Mobile WiMAX is based on IP or packet-switched services, which provides for a common network core. Data transfer involves the creation of a service flow, a unidirectional transport mechanism from the MAC through the PHY.
The MAC layer of the 802.16 standard was developed to support voice, data, and video under bursty conditions and high peak demands. The MAC is connection-oriented with each connection assigned a service class based on the type of quality of service (QoS) that is required by the MS. The MAC layer manages the radio resources to efficiently support the QoS for each connection established by the BS. There are five types of data delivery services available in the Mobile WiMAX profile. Table 4 shows a summary of the available service classes based on the typical application.
Table 4: Mobile WiMAX data delivery and services application
The network entry process
Each air interface in the MS will have a 48-bit universal MAC address that is used during the initial ranging and authentication process. The MS begins network entry and initialization by searching over a predetermined list of channel frequencies for the preamble of an OFDMA frame. When a preamble is found, the MS begins to determine the DL and UL parameters using the FCH and management data bursts, the DL channel descriptor (DCD), and the UL channel descriptor (UCD.) At this point, the MS has the necessary information to transmit on an UL ranging slot. The MS randomly selects an available slot from the list contained in the UL-MAP and sends a ranging request (RNG-REQ) message. The BS replies with a message with the power, timing, and frequency adjustments required by the requesting MS.
Once the MS's timing and transmit power have been properly adjusted, the BS and MS negotiate which PHY and MAC capabilities will be used for subsequent communication. Signals from the MS are distinguished at the PHY using different codes called the permutation base (like the separation of the BS signals using the preamble ID.) Unlike WLAN, which uses a MAC address, each data burst is logically marked for a specific receiver using a connection identifier (CID.) Next, the MS is authenticated and receives an IP address using DHCP. At this point the MS is managed by the BS and additional timing and power adjustments are preformed during a maintenance cycle called periodic ranging.
MAC scheduler
The MAC scheduler is located at each BS and is responsible for efficiently allocating the time and frequency resources in response to bursty traffic and time-varying channel conditions. The resource allocations are delivered to the MSs using the DL-MAP and UL-MAP located at the beginning of each frame. The scheduler determines the ordering of the data packets and the type of zone(s) on a frame by frame basis.
Handover
The handover (HO) process allows a MS to switch to another BS in order to improve its QoS, which may have degraded due to movement across cell boundaries or changing channels conditions. The Mobile WiMAX profile specifies three techniques for hard HO and soft HO. Hard HOs use a break-before-make approach and are typically sufficient for data services. Soft HOs, while complex to implement and administer, are beneficial for applications that require low-latency such as VoIP.
The Mobile Station
This section describes the architecture of the MS. In many ways it is very similar to that of a WLAN module. Like a WLAN device, or other network card, the WiMAX card is an adapter. It takes data packets designed for wired transmission and converts them to a different packet structure suitable for radio use.
The MS, sometimes referred to as the SS or customer premises equipment (CPE), consists of four major parts:
- Driver software
- Digital interface
- Baseband processor
- Radio transceiver
The driver software is loaded onto the host processor, where a suitable application can then call the resources of the card. For a Microsoft application, the driver will typically take the form of a dynamic link library (DLL).
The host application plays an important role in testing. The type of application can be split into two mains groups: those for calibration and testing, and those for use by the subscriber to the network.
The driver makes use of a specific digital interface, for example a PC card (PCMCIA), mini PC, or USB. The WiMAX device has to contain this hardware interface adapter.
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