2.2.1 IEEE 802.16 OFDM/TDD Frame Structure
The PMP mode is considered the well-adopted network configuration in IEEE 802.16 net-works wherein the BS is responsible for controlling all the communication among SSs. Two duplexing techniques are supported for the SSs to share common channels, i.e., time divi-sion duplexing (TDD) and frequency dividivi-sion duplexing. The MAC protocol is structured to support multiple physical (PHY) layer specifications in the IEEE 802.16 standard. In this
DL PHY PDU
DLFP DL-MAP UL-MAP DCD UCD MAC PDUs MAC PDUs MAC PDUs
Frame n-1 Frame n Frame n+1 Frame n+2
time
Figure 2.1: Schematic diagram of IEEE 802.16 PMP OFDM frame structure with TDD mode.
work, the WirelessMAN-OFDM PHY, utilizing the orthogonal frequency division multiplex-ing (OFDM), with TDD mode is exploited for describmultiplex-ing the design of the proposed APC approach.
Fig. 2.1 illustrates the schematic diagram of the IEEE 802.16 PMP OFDM frame structure with TDD mode. It can be observed that each frame consists of a DL subframe and a UL subframe. The DL subframe contains only one DL PHY protocol data unit (PDU), which starts with a long preamble for PHY synchronization. The preamble is followed by a frame control header (FCH) burst and several DL bursts. A DL frame prefix (DLFP), which is contained in the FCH, specifies the burst profile and length for the first DL burst (at most four) via the information element (IE). It is noted that each DL burst may contain an optional preamble and more than one MAC PDUs that are destined for the same or different SSs. The first MAC PDU followed by the FCH is the DL-MAP message, which employs DL-MAP IEs to describe the remaining DL bursts. The DL-MAP message can be excluded in the case that the DL subframe consists of less than five bursts; nevertheless, it must still be sent out periodically to maintain synchronization. A UL-MAP message immediately following
the DL-MAP message denotes the usage of UL bursts via UL-MAP IEs. An interval usage code, corresponding to a burst profile, describes a set of transmission parameters, e.g., the modulation and coding type, and the forward error correction type. The DL interval usage code (DIUC) and UL interval usage code (UIUC) are specified in the DL channel descriptor (DCD) and UL channel descriptor (UCD) messages respectively. The BS broadcasts both the DCD and UCD messages periodically to define the characteristics of the DL and UL physical channels respectively.
On the other hand, as can be seen from Fig. 2.1, the UL subframe starts with the con-tention intervals that are specified for both initial ranging and bandwidth request. It is noted that more than one UL PHY PDU can be transmitted after the contention intervals. Each UL PHY PDU consists of a short preamble and a UL burst, where the UL burst transports the MAC PDUs for each specific SS. Moreover, the transmit/receive transition gap (TTG) and the receive/transmit transition gap (RTG) are inserted in between the DL and the UL subframes and at the end of each frame respectively. These two gaps provide the required time for the BS to switch from the transmit to receive mode and vice versa.
2.2.2 Packet Transmission Mechanism
A connection in IEEE 802.16 PMP networks is defined as a unidirectional mapping between the BS and an MS, which is identified by a 16-bit connection identifier (CID). Two kinds of connections, including management connections and transport connections, are defined in the IEEE 802.16 standard. The management connections are utilized for delivering MAC management messages; while the transport connections are employed to transmit user data.
During the initial ranging of a SS, a pair of UL/DL basic connections are established, which belong to a type of the management connections. It is noted that a single Basic CID is assigned to a pair of UL/DL basic connections, which is served as the identification number for the corresponding SS. Thus the SS uses the individual transport CID to request bandwidth for each transport connection while the BS arranges the accumulated transmission opportunity by addressing the Basic CID of the SS.
SS1
SS2
BS
Internet (or other networks) Intra-cell traffic
Inter-cell traffic
Wired connection Wirelses connection
(a)
DL subframe UL subframe Frame n
DL subframe UL subframe Frame n+1
The jth packet:
from SS1 to BS
The jth packet:
from BS to SS2
Packet-rerouting delay
Other data packets Target intra-cell data packet
(b)
Figure 2.2: Example of packet transmission in IEEE 802.16 PMP networks: (a) network topology and (b) conventional transmission scheme in time sequence.
Fig. 2.2 depicts the conventional packet transmission mechanism of IEEE 802.16 PMP networks. An exemplified network topology that consists of one BS and two neighboring SSs is shown in Fig. 2.2(a). Two types of traffic exist in the network: inter-cell traffic and intra-cell traffic. For the inter-cell traffic, the source and the destination for each traffic flow are located in different cells, e.g., the traffic flow of 𝑆𝑆2 for accessing the Internet. On the other hand, the intra-cell traffic is defined while the source and destination are situated within the same cell network, such as the traffic flow between 𝑆𝑆1and 𝑆𝑆2in Fig. 2.2(a). Considering the scenario that 𝑆𝑆1 intends to communicate with its neighboring station 𝑆𝑆2, two transport connections are required to be established via the service flow management mechanism for the intra-cell traffic, i.e., the UL transport connection from 𝑆𝑆1 to the BS and the DL transport connection from the BS to 𝑆𝑆2. Fig. 2.2(b) illustrates the conventional transmission mechanism of IEEE 802.16 PMP networks in time sequence. In the most ideal case, the 𝑗th intra-cell packet, transmitted from 𝑆𝑆1 to the BS in the 𝑛th frame, will be forwarded to 𝑆𝑆2 in the (𝑛+1)th frame by the BS. The rerouting process apparently requires twice of communication bandwidth
for achieving the intra-cell packet transmission, which consequently increases communication overhead resulted from the duplication of data packets. Moreover, the delay time for packet-rerouting can be more than one half of a frame duration while the packet transmission from the BS to 𝑆𝑆2 is postponed to a latter DL subframe.
2.2.3 Problem Statement
Based on the aforementioned drawbacks of the conventional transmission mechanism in IEEE 802.16 PMP networks, the object problem of this work is described as follows:
Problem 1 (Direct Communication Problem). Given a pair of SSs that are actively in-volved in packet transmission, how to conduct efficient communication for the pair in order to increase the user throughput as well as to reduce the communication bandwidth and control overheads?