The IEEE 802.16 specifies the data and control plane of the MAC and PHY layers, as illustrated in Figure 1. More specifically, the MAC layer consists of three sublayers: the service-specific convergence sublayer (SSCS), the MAC common part sublayer (MAC CPS), and the security sublayer. The SSCS receives data from the upper layer entities that lie on top of the MAC layer, for example, bridges, routers, hosts. A different SSCS is specified for each entity type, including support for asynchronous transfer mode (ATM), IEEE 802.3, and Internet Protocol version 4 (IPv4) services. The MAC CPS is the core logical module of the MAC architecture, and is responsible for bandwidth management and QoS enforcement. Finally, the security sublayer provides SSs with privacy across the wireless network, by encrypting data between the BS and SSs.
Figure 1: Scope of the IEEE 802.16 standard—data/control plane.
This section reports the basic IEEE 802.16 MAC CPS and PHY layer functions so as to introduce the notation that will be used in the rest of this work. The interested reader can find all the details of the IEEE 802.16 specifications in the standard document.
The IEEE 802.16 standard specifies two modes for sharing the wireless medium: point-to-multipoint (PMP) and mesh. With PMP, the BS serves a set of SSs within the same antenna sector in a broadcast manner, with all SSs receiving the same transmission from the BS. Transmissions from SSs are directed to and centrally coordinated by the BS. On the other hand, in mesh mode, traffic can be routed through other SSs and can occur directly among SSs. As access coordination is distributed among the SSs, the mesh mode does not include support to parameterized QoS, which is needed by multimedia applications with stringent requirements. In this study we focus on the PMP mode alone.
In PMP mode uplink (UL) (from SS to BS) and downlink (DL) (from BS to SS) data transmissions occur in separate time frames. In the DL subframe the BS transmits a burst of MAC payload data units (PDUs). As the transmission is broadcast all SSs listen to the data transmitted by the BS. However, an SS is only required to process PDUs that are addressed to it or that are explicitly intended for all the SSs. In the UL subframe, on the other hand, any SS transmits a burst of MAC PDUs to the BS in a time division multiple access (TDMA) manner. DL and UL subframes are duplexed using one of the following techniques, as shown in Figure 2: frequency division duplex (FDD) is where DL and UL subframes occur simultaneously on separate frequencies, and time division duplex (TDD) is where DL and UL subframes occur at different times and usually share the same frequency. SSs can be either full-duplex, that is, they can transmit and receive simultaneously, or half-duplex, that is, they can transmit and receive at nonoverlapping time intervals.
Figure 2: Frame structure with frequency and time division duplexes.
The MAC protocol is connection-oriented: all data communications, for both transport and control, are in the context of a unidirectional connection. At the start of each frame the BS schedules the UL and DL grants to meet the negotiated QoS requirements. Each SS learns the boundaries of its allocation within the current UL subframe by decoding the UL-medium access protocol (MAP) message. On the other hand, the DL-MAP message contains the timetable of the DL grants in the forthcoming DL subframe. Both maps are transmitted by the BS at the beginning of each DL subframe, as shown in Figure 2.
As the BS controls the access to the medium in the UL direction, bandwidth is granted to SSs on demand. For this purpose, a number of different bandwidth request mechanisms have been specified. With unsolicited granting a fixed amount of bandwidth on a periodic basis is requested during the setup phase of an UL connection. After that phase, bandwidth is never explicitly requested. A unicast poll consists of allocating to a polled UL connection the bandwidth needed to transmit a bandwidth request. If the polled connection has no data awaiting transmission (backlog, for short), or if it has already requested bandwidth for its entire backlog, it will not reply to the unicast poll, which is thus wasted. Instead, broadcast (multicast) polls are issued by the BS to all (multiple) UL connections. The main drawback of this mechanism is that a collision occurs whenever two or more UL connections send a bandwidth request by responding to the same poll. In this case all collided connections [1] need to resend the bandwidth requests, but a truncated binary exponential backoff algorithm is employed to reduce the chance of colliding again.
Bandwidth requests can also be piggybacked on PDUs. For instance, assume that a bandwidth request is sent by an SS for its connection x. Then, before the entire backlog of connection x is served, more data is received from upper layers. In this case, the SS can notify the BS of the increased bandwidth demands by simply adding a Grant Management subheader to any outgoing PDU of connection x. Finally, while a connection is being served by the BS, the SS can use part of the bandwidth scheduled by the BS for data transmission to send a standalone PDU with no data that updates the amount of backlog notified to the BS. This mechanism is called bandwidth stealing.
It is worth noting that an SS notifies the BS of the amount of bytes awaiting transmission at its connections' buffers, but the BS grants UL bandwidth to the SS as a whole. Due to this hybrid nature of the request/grant mechanism (i.e., requests per connection, grants per SS), an SS also has to implement locally a scheduling algorithm to redistribute the granted capacity to all its connections.
Finally, the BS and SSs can fragment a MAC service data unit (SDU) into multiple PDUs, or they can pack multiple SDUs into a single PDU, so as to reduce the MAC overhead or improve the transmission efficiency. A hybrid analytical simulation study of the impact on the performance of this feature of the MAC layer has been carried out by Hoymann. Results showed that, if the use of fragmentation is enabled, the frame can be filled almost completely, which can significantly increase the frame utilization, depending on the size of SDUs. These optional features have also been exploited in a cross-layer approach between the MAC and application layers, so as to optimize the performance of multimedia streaming.
2 PHY Layer
The IEEE 802.16 standard includes several noninteroperable PHY layer specifications. However, all the profiles envisaged by the WiMAX forum for fixed BWA specify the use of orthogonal frequency division multiplexing (OFDM) with a Fast Fourier Transform (FFT) size of 256, which is thus the primary focus of this study. This PHY layer has been designed to support non-line-of-sight (NLOS) and operates in the 2–11-GHz bands, both licensed and unlicensed. Transmitted data is conveyed through OFDM symbols, which are made up from 200 subcarriers. Part of the OFDM symbol duration, named the cyclic prefix duration, is used to collect multi-path. The interested reader can find a technical introduction to the OFDM system of the IEEE 802.16 in recent survey papers.
To exploit the location-dependent wireless channel characteristics, the IEEE 802.16 allows multiple burst profiles to coexist within the same network. In fact, SSs that are located near the BS can employ a less robust modulation than those located far from the BS. The combination of parameters that describe the transmission properties, in DL or UL direction, is called a burst profile. Each burst profile is associated with an interval usage code (IUC), which is used as an identifier within the local scope of an IEEE 802.16 network. The set of burst profiles that can be used is periodically advertised by the BS using specific management messages, that is, downlink channel descriptor (DCD) and uplink channel descriptor (UCD). To maintain the quality of the radio frequency communication link between the BS and SSs, the wireless channel is continuously monitored so as to determine the optimal burst profile. The burst profile is thus dynamically adjusted so as to employ the less robust profile such that the link quality does not drop below a given threshold, in terms of the carrier-to-interference-and-noise ratio (CINR). However, as a side effect of the dynamic tuning of the transmission rate, it is not possible for the stations to compute the transmission time of MAC PDUs a priori. Therefore, SSs always issue bandwidth requests in terms of bytes instead of time, without including any overhead due to the MAC and PHY layers.
Although the link quality lies above a given threshold, it is still possible that some data get corrupted. To reduce the amount of data that the receiver is not able to successfully decode, several forward error correction (FEC) techniques are specified, which are employed in conjunction with data randomization, puncturing, and interleaving. Finally, each burst of data is prepended by a short physical preamble (or preamble), which is a well-known sequence of pilot subcarriers that synchronize the receiver. The duration of a preamble is one OFDM symbol, which can be accounted as PHY layer overhead. In the DL subframe a preamble is prepended to each burst, which can be directed to multiple SSs employing the same burst profile (Figure 2). On the other hand, in the UL subframe, each SS always incurs the overhead of one preamble for each frame where it is served.
Nice post.
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