#mac layer

Till Now we have an introduction on what is WiMAX. WiMAX as a whole encompasses a wide range of communication protocols. Since it’s difficult to cover everything, in this blog I will narrow down my approach to mobile WiMAX technology protocol i.e. 802.16e. Let’s dive deep into the world of Mobile WiMAX to know more about it.

As soon as we see mobile WiMAX, an obvious question pops to our head - Is there a fixed WiMAX also? If yes how both are different? I will try to give a give a justification to it. After that we can will go more into details of mobile WiMAX and its mode of operations.

To give a headstart, there is just one common point between 802.16d fixed WiMAX and 802.16e mobile WiMAX – they share the same last name. Beyond that there is virtually no relevant relationship between the two technologies. 802.16d was developed for the sole purpose of linking the “last mile” to the home or enterprise that has long been the gap between broadband Internet “haves” and “have nots”. There is no viable competitive technology to fixed WiMAX for providing last mile connectivity other than some proprietary solutions like Motorola’s Canopy. 802.16e was developed to compete with existing cellular solutions and to expand mobile voice services by adding high-speed mobile data services[1].  I am restricting the protocols just to definition. Moving forth I will discuss only about “802.16e mobile WiMAX”.

How Mobile WiMAX came into existence

In December, 2005 the IEEE ratified the 802.16e amendment to the 802.16 standard. This amendment adds the features and attributes to the standard necessary to support subscriber stations moving at vehicular speeds. The WiMAX Forum is now defining system performance and certification profiles based on the IEEE 802.16e Mobile Amendment and, going beyond the air interface, the WiMAX Forum is defining the network architecture necessary for implementing an end-to-end Mobile WiMAX.

Mobile WiMAX is a broadband wireless solution that enables convergence of mobile and fixed broadband networks through a common wide area broadband radio access technology and flexible network architecture. The Mobile WiMAX Air Interface adopts Orthogonal Frequency Division Multiple Access (OFDMA) for improved multi-path performance in non-line-of-sight environments. Scalable OFDMA (SOFDMA) is introduced in the IEEE 802.16e Amendment to support scalable channel bandwidths from 1.25 to 20 MHz. The Mobile Technical Group (MTG) in the WiMAX Forum is developing the Mobile WiMAX system profiles that will define the mandatory and optional features of the IEEE standard that are necessary to build a Mobile WiMAX-compliant air interface that can be certified by the WiMAX Forum. The Mobile WiMAX System Profile enables mobile systems to be configured based on a common base feature set thus ensuring baseline functionality for terminals and base stations that are fully interoperable.

FEATURES OF STATE OF ART MOBILE WiMAX TECHNOLOGY

Mobile WiMAX products and certification follow the IEEE 802.16e air interface specifications. The figure below gives a more detailed view of the construction of the mobile WiMAX system profile from the air interface perspective. The system profile is composed of five subprofiles, namely, physical layer (PHY), media access control (MAC), radio, duplexing mode and power classes. Even though there are many different combinations of center frequencies and channel bandwidths accommodating different regional spectrum regulations, all Release 1 mobile WiMAX products share the same PHY and MAC features (profiles) and the same duplexing mode which is Time Division Duplex (TDD). In the following, some detailed descriptions of key PHY and MAC features in the mobile WiMAX system profile are offered.

PHYSICAL LAYER

In the following we give the key PHY features of mobile WiMAX technology and provide short descriptions.

OFDMA Basics

Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexing technique that subdivides the bandwidth into multiple frequency sub-carriers. In an OFDM system, the input data stream is divided into several parallel sub-streams of reduced data rate (thus increased symbol duration) and each sub-stream is modulated and transmitted on a separate orthogonal sub-carrier. The increased symbol duration improves the robustness of OFDM to delay spread. OFDM exploits the frequency diversity of the multipath channel by coding and interleaving the information across the sub-carriers prior to transmissions. OFDM modulation can be realized with efficient Inverse Fast Fourier Transform(IFFT). Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple-access/multiplexing scheme that provides multiplexing operation of data streams from multiple users onto the downlink sub-channels and uplink multiple access by means of uplink sub-channels.

The OFDMA symbol structure consists of three types of sub-carriers:

Data sub-carriers for data transmission

Pilot sub-carriers for estimation and synchronization purposes

Null sub-carriers for no transmission used for guard bands and DC carriers

The WiMAX OFDMA PHY supports sub-channelization in both Downlink and Uplink. The minimum frequency-time resource unit of sub-channelization is one slot, which is equal to 48 sub-carriers.

Scalable OFDMA

The IEEE 802.16e-2005 Wireless MAN OFDMA mode is based on the concept of scalable OFDMA (S-OFDMA).S-OFDMA supports a wide range of bandwidths to flexibly address the need for various spectrum allocation and usage model requirements. The scalability is supported by adjusting the FFT size while fixing the sub-carrier frequency spacing at 10.94 kHz. Since the resource units sub-carrier bandwidth and symbol duration is fixed, the impact to higher layers is minimal when scaling the bandwidth. It supports adaptive modulation and coding, so that in conditions of good signal, a highly efficient 64 QAM coding scheme is used, whereas when the signal is poorer, a more robust BPSK coding mechanism is used. In intermediate conditions, 16 QAM and QPSK can also be employed.

Other Advanced features of PHY layer

Adaptive modulation and coding (AMC), Hybrid Automatic Repeat Request (HARQ) and Fast Channel Feedback (CQICH) were introduced with Mobile WiMAX to enhance coverage and capacity for WiMAX in mobile applications. The combinations of various modulations and code rates provide a fine resolution of data rates. WiMAX provides signalling to allow fully asynchronous operation. The asynchronous operation allows variable delay between retransmissions which gives more flexibility to the scheduler at the cost of additional overhead for each retransmission allocation. HARQ combined together with CQICH and AMC provides robust link adaptation in mobile environments at vehicular speeds in excess of 120 km/hr.

MAC LAYER

The 802.16 standard was developed from the outset for the delivery of broadband services including voice, data, and video. The MAC layer is based on the time-proven DOCSIS standard and can support bursty data traffic with high peak rate demand while simultaneously supporting streaming video and latency-sensitive voice traffic over the same channel. The resource allocated to one terminal by the MAC scheduler can vary from a single time slot to the entire frame, thus providing a very large dynamic range of throughput to a specific user terminal at any given time.

Quality of Service (QoS) Support

With fast air link, asymmetric downlink/uplink capability, fine resource granularity and a flexible resource allocation mechanism, Mobile WiMAX can meet QoS requirements for a wide range of data services and applications. The service flow based QoS mechanism applies to both DL and UL to provide improved QoS in both directions. Mobile WiMAX supports a wide range of data services and applications with varied QoS requirements.

MAC Scheduling Service

The Mobile WiMAX MAC scheduling service is designed to efficiently deliver broadband data services including voice, data, and video over time varying broadband wireless channel.

Mobility Management

Battery life and handoff are two critical issues for mobile applications. Mobile WiMAX supports Sleep Mode and Idle Mode to enable power-efficient MS operation. Mobile WiMAX also supports seamless handoff to enable the MS to switch from one base station to another at vehicular speeds without interrupting the connection.

Security

Mobile WiMAX supports best in class security features by adopting the best technologies available today. Support exists for mutual device/user authentication, flexible key management protocol, strong traffic encryption, control and management plane message protection and security protocol optimizations for fast handovers.

References: