WiMAX MAC Protocol: An In-Depth Introduction

This page of the WiMAX tutorial covers the MAC protocol used between a base station and a subscriber station, compliant with the fixed WiMAX IEEE 802.16-2004 PMP profile. This article describes WiMAX MAC layer protocols as per the IEEE 802.16-2004 or 16d OFDM standard, with its functions mainly for schedule management and resource allocation.

This article also aids in the design of the WiMAX physical layer, as the WiMAX MAC layer plays a very important role in determining the modulation-code rate and other useful physical layer parameters.

It covers WiMAX MAC layers viz. convergence sub-layer, MAC common part sub-layer, and security sub-layer.

Description

Before we delve into the WiMAX MAC layer, let’s summarize the protocol layers at L1 and L2 of fixed WiMAX. As mentioned below, the MAC layer comprises three sub-layers: convergence sub-layer, MAC common part sub-layer, and security sub-layer.

The WiMAX MAC layer comprises three sub-layers:

1. Convergence Sub-layer: Interfaces with the layer above (i.e., the network layer) and the layer below (i.e., MAC CPS). This includes classifying external network service data units (SDUs) and associating them with the proper MAC service flow identifier (SFID) and connection identifier (CID). Hence, it takes care of the delivery of the CS PDU to the MAC SAP associated with the service flow for transport to the peer MAC SAP and receipt of the CS PDU from the peer MAC SAP. The optional feature supported by this layer is Payload header compression.

2. MAC Common Part Sub-layer: The core MAC functionality is performed by this sub-layer. It handles the access mechanism of the SS with BS, bandwidth allocation, connection establishment, and connection maintenance. This is explained in detail in this article.

3. Security Sub-layer: It takes care of security aspects viz. authentication, secure key exchange, and encryption.

To understand MAC CPS, we need to understand the WiMAX frame structure, which is mentioned below. For the discussion point of view, let us assume the following system parameters, and we will assume that the number of subscriber stations (SS) is one.

Let us assume that the WiMAX system BW is 3.5MHz; hence, we will have a sampling frequency of 4MHz. Let us assume a System Frame Duration of 10ms and a DL/UL Ratio of 56%, hence we will have total symbols in a WiMAX Frame equal to 125, No. of symbols in DL sub frame 70 and No. of Symbols in UL sub frame 55. As mentioned in the frame, the downlink sub frame is transmitted by the Base station (BS) to Subscriber stations (SSs) and the Uplink sub frame is transmitted by SSs to BS. TTG and RTG are the turnaround times to allow electronic circuitry in Base station and Subscriber stations to settle down in switching from Tx to Rx and vice versa to avoid loss of the data.

Let’s understand how the scheduling concept is mentioned in the standard. The standard defines DLFP which is 88 bits long and is always BPSK which is transmitted after L.P. (Long preamble) of 2 symbols. It carries the Length of the Downlink Bursts, and it is in units of OFDM symbols.

DLFP=(..., Rate_ID, Length_B1, DIUC, Length_B2, DIUC, Length_B3, DIUC, Length_B4....)

DLFP is decoded by SS based on a known modulation-code rate, which is always BPSK1/2. DLFP’s extracted parameters are required as a scrambler seed. Once SS decodes this, it will come to know about Downlink B1 which carries system critical messages namely DLMAP, ULMAP, UCD and DCD as described in the 16d standard.

Refer DLMAP vs ULMAP and UCD,UIUC,DCD,DIUC for more.

ULMAP structure with important fields mentioned below.

ULMAP= (..., Allocation start time,...start_time_B1,...duration_B1,...Start_time_B2,...duration_B2,...)

Allocation start time field, which defines the starting point of the uplink sub frame which is in units of Physical slots (PS), and mentions after how many PSs starting from DL Sub frame, the uplink allocation is going to start.

Physical slot is the basic unit of resource allocation is fixed WiMAX and it is 4/Fs.

For ranging standard defines fixed location after TTG but Bandwidth request can be sent anywhere in the uplink sub frame except region dedicated for ranging request. ULMAP which is transmitted by BS contains start_time_field by which SS will come to know when in the frame they have to initiate the transmission. SS will transmit the burst appending Short preamble (S.P.) as defined in the standard. As modulation-code rate for SS can vary in the system, request for bandwidth is designed in units of bytes. The Bandwidth Request PDU consists of header alone, which consist of field BR, which defines the number of bytes requested by SS.

Each MAC PDU starts with Generic MAC Header (GMH) which is 6 bytes long and consists of mainly following fields. As mentioned each MAC messages will have type field by which MAC layer will decide what action need to be taken with the received MAC PDU on both the sides(BS and SS)

GMH = (Header type,...., MAC msg type,..., CID, HCS)

There are two major fields UCD and DCD which is called uplink channel descriptor and downlink channel descriptor respectively. These fields in conjunction with UIUC and DIUC will decide modulation-code rates of the downlink and uplink bursts.

As in all the standards and wireless systems, before actual conversation (voice) or data transfer starts there will be exchange of messages between requesting system (Here SS) and serving system (Here BS). Similarly in WiMAX following messages are exchanged between BS and SS before internet connection is established.

WiMAX MAC Layer Messages

The following figure covers various MAC layer messages exchanged between BS and SS which include Broadcast message(DLMAP,ULMAP,UCD and DCD),Ranging request(RNG REQ),Bandwidth request(BW REQ),DSA REQ and DSA ACK.

The MAC layer is implementation specific, here MAC layer is implementated into two parts lower MAC layer and upper MAC layer. Lower MAC layer does CRC calculation and HCS calculation for GMH(Generic MAC Header). Upper MAC layer does scheculing and takes care of MAC layer messages. Following table describes different WiMAX MAC management messages and their type number, message description and connection type.

MAC message flow between BS and SS before data transfer takes place:

1. Broadcast message - DLMAP, ULMAP, UCD, and DCD (BS to SS)
2. RNG REQ (SS to BS), RNG RSP (BS to SS). Refer REG REQ and REG RSP page and WiMAX Ranging Procedure for more.
3. BW REQ (SS to BS), SBC REQ (SS to BS), SBC RSP(BS to SS).
4. BW REQ (SS to BS), REG REQ (SS to BS), REG RSP (BS to SS)

One can read REG REQ and REG RSP and SBC REQ and SBC RSP MAC management messages for details on fields carried by them.

At this stage network entry for SS is completed and now it will establish service flow. For Service flow there are two methods, BS initiated and SS initiated. We will talk about BS initiated Service flow for both downlink and uplink connection.

5. DSA REQ for downlink (BS to SS), DSA RSP for downlink (SS to BS)
6. DSA ACK for downlink (BS to SS), DSA REQ for uplink (BS to SS)
7. DSA RSP for uplink (SS to BS), DSA ACK for uplink (BS to SS)

At this stage, the connection is established, and both sides can transfer data to each other, i.e., the Network layer can start communicating through MAC-PHY.