# A MAP Overhead Aware Two-Dimensional OFDMA Burst Construction Algorithm
## 2 Background and Related Works  
### OFDMA Network  
The conventional OFDMA network architecture consists of two fixed stations, namely the base station (BS in Figure 1) and subscriber station (SS in Figure 1), as shown in Figure 1. The BS is typically a service provider, which connects to a public network and provides each SS with the last-mile access to public networks. The SS may be a portable device which is called a mobile station (MS). Each frame consists of downlink (DL) and uplink (UL) subframes. The DL carries information from a BS to SSs, while the UL carries information from the opposite direction (i.e., subscriber to base station).  
Downlink and uplink subframes are duplexed by either using frequency division duplex (FDD) or time division duplex (TDD). Frequency division duplex is suitable for bi-directional communication since it uses different frequency bands for transmitting DL sub-frame and UL sub-frame at the same time domain. In TDD mode, the BS divides one channel for transmitting DL and UL sub-frames at two distinct domains, as shown in Figure 2. Time division duplex allows the service provider to decide the ratio of uplink and downlink transmission times, and thus the flexibility of TDD mode allocated to serve the un-balanced traffic load is selected in this paper.  
One of the major functions and services performed by the physical layer (PHY) is modulation coding scheme (MCS) selecting rule in the data transmission and the data reception. Several MCSs in this PHY are supported, such as quadrature phase shift keying (QPSK) and several quadrature amplitude modulation (QAM) schemes. Subscriber stations at different locations may encounter different channel qualities and then adopt different MCSs, resulting in different bandwidth resource allocations. If the SSs randomly acquire channels for data transmission, they are likely to suffer poor channel quality and low transmission rate.  
The OFDMA frame for TDD, as shown in Figure 2, begins with a downlink preamble (Preamble in Figure 2) which synchronizes each SS, and performs channel estimation. The frame control header (FCH in Figure 2) is used to specify DL sub-frame prefix (DLFP) and the length of the MAP message. The burst (Burst in Figure 2) means the bandwidth region in which data is placed in and formed by slots. Each user can be described by a burst profile, which constitutes a set of transmission parameters (MCSs). The download link/ upload link map (DL/UL-MAP) messages describe the information for each DL/UL burst. On uplink, each SS uses the assigned burst allocated by BS to transmit its data  
Figure 1: Orthogonal frequency division multiple access (OFDMA) network.  
Figure 2: OFDMA frame structure.  
through UL-MAP. On downlink, each SS determines when and where the data designated for it is via MAP. In addition, TTG described in Figure 2 means Transmit Transition Gap, and RTG in Figure 2 means Receive Transition Gap (RTG).  
The MAP message starts with the header followed by the burst information list, as shown in Figure 3[1]. After computing the total number of bits in the MAP, the size of the MAP header can be made. The burst information (called Burst Info. in Figure 3) contains information element (IE) of each burst, and each burst is formed by the header and the payload. The total number of bits in the MAP IE [1] is \(44+16\times n\) (bits) (\(4+8+16\times n+8+2+22\)).  
The MAP overhead is mainly dominated by the size of IEs. Let \(B\) denote the number of bursts in this map and \(n_{j}\) denote the number of some connection identifiers (CIDs) in the burst \(j\). In the downlink, one connection is allowed to be assigned into more than one burst with the increased MAP IE size, i.e., 16 bits. The size of the MAP formula is shown in Equation (1), accordingly. It is even possible to pack multiple connections into one burst for reducing MAP overhead. In this scenario, the unique CID is used to specify the connections.  
\[104+\sum_{j=1}^{B}\left(44+16\times n_{j}\right) \tag{1}\]