IEEE 802.16

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.

1 MAC Layer

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.

Frequency Allocation in Mobile WiMAX

In orthogonal frequency division multiple access (OFDMA), the frequency band is divided into subcarriers. These subcarriers are grouped into sets, called subchannels in WiMAX, which are allocated to users. We will consider in our numerical applications the case of an Fast Fourier Transform (FFT) size of 1024 subcarriers, with an intercarrier spacing of 7.8 kHz.

Two allocation modes are possible: distributed and adjacent.

Distributed Allocation

In the distributed subcarriers allocation, full channel diversity is obtained by distributing the allocated subcarriers to subchannels using a permutation mechanism. This mechanism is designed to introduce frequency diversity, thus minimizing the performance degradation due to fast fading which is characteristic of mobile environments. In addition to that, WiMAX standards specify two different distributed allocation modes: the fully used subchannelization (FUSC) mode where all subcarriers are used to form subchannels in each cell, and the partially used subchannelization (PUSC) mode where the frequency band is divided into three segments.

For illustration, with an FFT size of 1024 and after reserving the pilot and guard subcarriers, a FUSC allocation will correspond to 16 subchannels of 48 data subcarriers each, whereas a PUSC allocation will correspond to 30 subchannels, each containing 24 data subcarriers. Note that assigning subcarriers to subchannels in PUSC is a bit complicated, as it employs two permutations:

§  An outer permutation divides the subcarriers into six major groups of clusters using a specific renumbering sequence.

§  An inner permutation operates separately on each major group, distributing subcarriers to subchannels within the group and is based on the FUSC permutation with distinct parameters for the odd and even major groups.

This is illustrated in Figure 1, where two groups are assigned to one segment corresponding to a sector of the cell. Note that a segment can also be allocated to a cell in an omni-directional setting.

Figure 1: Construction of groups and segments in the partially used subchannelization allocation mode.

Adjacent Allocation

This method uses adjacent subcarriers to form subchannels. It corresponds to the WiMAX AAS (Advanced Array Systems) mode, designed to support Multiple-Input Multiple-Outpt (MIMO) techniques and adaptive modulation. Note that, to achieve a frequency diversity, mobiles using adjacent allocation may hop rapidly between different subchannels during their communication times.

Energy Conservation via Sleep Mode

Need for "Sleep" and "Wakeup" Modes
The simplest way to save energy and thus prolong the battery life of a SS is to put the SS to "sleep" (i.e., kill all processes running on the SS except for the minimum required to sustain the connection) when it is not involved in any communications. During the sleep period, the SS will not transmit, but "listen" to the channel occasionally to maintain connectivity. This feature is not specified in the 802.16-2004 standard, as that standard was proposed for stationary SSs and it was assumed that power supply for the SSs would not be a critical problem.

Clearly, an SS in the sleep mode requires a complementary mechanism for "waking up" so that it can resume transmitting or receiving. The signaling message that accomplishes this is usually called the "paging" signal. Again, the 802.16-2004 standard does not include a paging signal either.

Message Exchanges to Enter Sleep Mode
We propose the following sequence of steps before an SS enters the sleep mode.

Based on a lack of traffic on the DL and UL, the SS decides that it should enter the sleep mode. Thus, the decision to enter the sleep mode is SS-initiated. The algorithm that the SS applies to arrive at this decision is arbitrary, and can be specific to that SS alone while being unknown to the BS.

The SS uses a CID belonging to one of its current sessions to send a bandwidth request (BR) message to the BS. This is the mechanism for requesting additional bandwidth specified in the current 802.16-2004 standard. As it is very unlikely for a SS to specify 0 bytes in the BR message (i.e., requesting zero additional bandwidth), we propose for the BS to interpret such a message from the SS as a request for permission to enter the sleep mode. A key advantage of such use of BR message with 0-byte request is that if the BS is not enabled to support this sleep/wakeup function, then a request for an additional bandwidth of zero bytes will simply be ignored or discarded, thereby causing no changes to the current session.

If the BS is capable of supporting sleep/wakeup in the SS, then it includes a pre-specified Uplink Interval Usage Code (UIUC) that serves as an acknowledgment to the SS that it is allowed to enter into the sleep mode. The specific UIUC is known and hard-wired in the BS and SS.

On receipt of this acknowledgment message from the BS, the SS enters the sleep mode after a fixed time interval (measured in the number of frames), which is set by the network operator and assumed known to both the BS and the SS. This obviates the need to transmit this interval defining the sleep start time from BS to SS or vice versa, thereby eliminating the need to change the standard to accommodate a message that does so.

During the sleep mode, the SS maintains frame synchronization (this is one of the few processes that are maintained during the sleep mode). Further, the SS decodes DL information periodically to check whether the SS is being "paged" by the BS (see the description of the "wakeup" subsequently). The period (e.g., once every five frames) is predefined by the network operator and hard-wired in the SS. Battery energy is conserved as the SS decodes data occasionally.

Note that from the perspective of the BS, the SS is treated just the same as if it were not in the sleep mode, with the exception that the BS does not transmit data to a sleeping SS without first ensuring that the SS has been waken up. The session parameters associated with a sleeping SS are retained.

The sleep period is not indefinite, but continues only for a finite number of frames, after which the SS wakes up by default if it has not already been waken up by a paging message sent from the BS, or by the arrival of data at the SS intended for transmission on the UL. This maximum sleep duration is also predetermined, fixed, and set by the network operator and assumed to be known to both BS and SS.

Message Exchanges to Wakeup a Sleeping SS
1. SS Wakes Up on Its Own
If the SS has any data to send on the UL to the BS, it simply wakes up by reviving all processes that were running before it entered the sleep mode, and then transmitting the data just as it would have if it had never entered the sleep mode. As the BS retained all session parameters when the SS first entered the sleep mode, the BS is ready to receive the data and does so. The arrival of this data from the SS alerts the BS to the fact that the "sleeping" SS has now woken up by its UL transmission. The BS then treats the SS as if it is no more in sleep mode.

2. BS Wakes Up the SS via a Paging Message
The BS wakes up a sleeping SS by transmitting a "paging" message during one of the periodic frames that is received and decoded by the SS. A specific "paging" message is not supported by the current 802.16-2004 standard. However, the standard permits network operators to use several UIUCs to define modulation and coding rates (burst profiles) for both the orthogonal frequency division multiplexing (OFDM) and OFDMA modes of operation. The network operator may therefore reserve one of these UIUCs for "paging." The steps involved in waking up a sleeping SS to receive DL data is given next.

In the UL-MAPs transmitted by the BS over a number of consecutive frames, the BS specifies the CID of the SS scheduled for wakeup, employing the UIUC designated for "paging" purposes with the most robust modulation and coding scheme available. The reason for this is that the SS is only receiving and decoding periodic frames during sleep mode. Further, as the sleeping SS does not transmit anything on the UL, the BS has no information about DL channel quality, and cannot tailor its coding scheme accordingly. Thus, if the DL channel suffers degradation, the SS may not receive the BS transmission, so robust modulation/coding with repetition maximizes the chance of the SS receiving the paging "message."

Upon receiving the UIUC information in the UL-MAP, the SS exits the sleep mode. To confirm with the BS that the SS has indeed exited the sleep mode, the SS transmits another bandwidth request message with 0 bytes in the BR field. The receipt of this message by the BS is interpreted by the BS as an acknowledgment by the SS that it has now "woken up" and resumed normal operation.