MAC SUBLAYER | OFDM-BASED WiMAX

Resource allocation is one of the major tasks of MAC because the medium access mechanisms of MAC directly affect the spectrum and power utilization. Accordingly, the MAC sublayer specifications in the IEEE 802.16 standard, relevant to our discussion in the next section, are introduced in the following.

Despite the advantages of OFDM in mitigating the channels’s impairments as mentioned before, underutilization of transmitter power and network subcarriers is its disadvantage. When an OFDM transmitter accesses the channel in a time division manner, e.g., time division multiple access (TDMA), the transmitter is forced to transmit on all available subcarriers N_sc, although it may require a less number of subcarriers to satisfy its transmission rate requirement. Consequently, the transmitter power consumption increases as the number of subcarriers increases. This disadvantage motivates the development of a PHY technology where transmitters are multiplexed in time and frequency, i.e., OFDMA. In such a technology, the users are exclusively assigned a subset of the network available subcarriers in each time slot. The number of both time slots and subcarriers can be dynamically assigned to each user; this is referred to as dynamic subcarrier assignment which introduces multiuser diversity. The multiuser diversity gain arises from the fact that the utilization of given resources varies from one user to another. A subcarrier may be in deep fading for one user  while it is not for another user (e.g., the same subcarrier for user O). Allocating this particular subcarrier to the user with higher channel gain permits higher transmission rate. To achieve multiuser diversity gain, a scheduler at the MAC sublayer is required to schedule users in appropriate frequency and time slots.

Point-to-multipoint (PMP) as well as mesh topologies are supported in the IEEE 802.16 standard. PMP mode operations are centrally controlled by the BS, but mesh mode can be either centralized or decentralized, i.e., distributed. In a centralized mesh, a mesh BS (a node that is directly connected to the backbone) coordinates communications among the nodes. Decentralized mesh is similar to multihop adhoc networks in the sense that the nodes should coordinate among themselves to avoid collision or reduce the transmission interference.

In PMP mode, the uplink (UL) channel, transmissions from users to BS, is shared by all users, i.e., UL is a multiple access channel. On the other hand, downlink (DL) channel, transmissions from BS to subscriber stations (SSs), is a broadcast channel. The duplexing methods of UL and DL include time division duplexing (TDD), frequency division duplexing (FDD), and half-duplex FDD (HFDD). Unlike PMP mode, there is no clear UL and DL channel defined for mesh mode.

An outstanding feature of WiMAX is heterogeneous traffic support over wireless channels. IEEE 802.16 provides service for four traffic types known as service flows. The mechanism of bandwidth assignment to each SS depends on the QoS requirements of its service flows. The service flows and their corresponding bandwidth request mechanisms are as follows:

• Unsolicited grant service (UGS): This service supports constant bit rate traffic. Bandwidth is granted to this service periodically or in case of traffic presence by the BS.
  
  
• Real-time polling service (rtPS): This service has been provided for real-time service flows with variable-size data packets issued periodically. rtPS flows can send their bandwidth request to the BS after being polled.
  
  
• Nonreal-time polling service (nrtPS): This service is for nonreal-time traffic with variable-size data packets. nrtPS can gain access to the channel using monocast or multicast polling mechanisms. Upon receiving a multicast polling, the nrtPS service can take part in a contention in the bandwidth contention range.
  
  
• Best effort (BE) service: This service provides the minimum required QoS for nonreal-time traffic. The channel access mechanism of this service is based on contention.

WIMAX HARQ

HARQ is supported in WiMAX that uses OFDMA physical layer. As mentioned before, WiMAX uses a basic stop-and-wait HARQ protocol. Using multiple HARQ channels can compensate the propagation delay of the stop-and-wait scheme, that is, one channel transmits data while others are waiting for feedbacks. Therefore, using a small number of HARQ channels (e.g., using 6 channels), multichannel stop-and-wait HARQ is a simple, efficient protocol that simultaneously achieves high throughput and low memory requirement.

In this scheme, each HARQ channel is independent of each other; that is, a data burst can only be retransmitted by the HARQ channel that initially sent the data burst. Each HARQ channel is distinguished by a HARQ channel identifier (ACID – ARQ channel identifier). Data reordering in an SS is done by referring to Protocol Data Unit (PDU) sequence numbers that are enabled when the HARQ operation is used. The HARQ Downlink Map Information Element (DL MAP IE) contains the information about the DL HARQ Chase sub-burst IE. It specifies the location of HARQ sub bursts, the ACID, the HARQ Identifier Sequence Number (AI_SN), etc. By referring to MAP IE, an SS can correctly retrieve a given data burst. HARQ feedbacks are sent by the SS after a fixed delay (this is called synchronous feedbacks). To specify the start of a new transmission on each HARQ channel, one-bit HARQ AI_SN is toggled on each successful transmission.

When CC is used in WiMAX HARQ, the total buffer capability needed in a specific SS is determined by the maximum data size and may be transmitted by a single channel at a time, and the total number of HARQ channels provided for the SS (i.e., the product of multiplying these two values). In the IEEE 802.16 Standard, this buffer capacity is used by the BS as a rate control mechanism when sending data to an SS.

1. A Simple Example for WiMAX HARQ

In this section, the multiple-channel stop-and-wait HARQ used by WiMAX is illustrated by a simple example. As mentioned above, these HARQ channels are independent of each other; retransmissions of a data burst can only be done by its initial sending HARQ channel. Thus, a negative acknowledged (NAKed) data burst can only be resent via the initial sending channel until it is successfully received.

Figure 1 shows the channel activities versus time frames of a HARQ scheme, from a BS to a particular SS, assuming that four channels are available for the SS. The first row shows the consecutive time frames. The next four rows show the data bursts sent and the ACK or NAK received from/by each channel at the corresponding time frame. The last row shows the data bursts storing in the SS buffer at the corresponding time frame (waiting to be forwarded to the upper layer). It is assumed that data bursts have either been received correctly or erroneously. Erroneously received data bursts are placed in the receiver’s buffer waiting for the correct copies. On the other hand, correctly received data bursts are placed in the receiver’s buffer waiting to be forwarded further (either to the next hop or to the upper layer) in the right sequence, that is, the receiver is still waiting for a correct copy of some earlier data bursts.

 

Figure 1: A simple example of the WiMAX HARQ scheme.

Figure 1 may be explained below. Data bursts 1 through 4 are sent via channels 1 through 4, at time frames 1 through 4, respectively. A corresponding ACK or NAK will be received by the BS by the end of the 4th time frame after the BS sent out the data burst. So, at the end of 4th time frame, a NAK for data burst 1 is received, and thus the second copy of data burst 1 is sent at time frame 5, via channel 1 (the same channel it was initially sent). Similarly the second copy of data burst 2 is sent at time frame 6 via channel 2. Since an ACK for data burst 3 is received at time frame 6, a new data burst (5) is sent via channel 3 at time frame 7.

The data bursts stored at the receiver’s buffer may be explained as follows. At time frame 4, data bursts 1, 2, and 3 are in the receiver’s buffer. Data bursts 1 and 2 have been erroneously received (and are waiting for correct copies) while data burst 3, even though has been correctly received, is also waiting so that it may be forwarded further in the correct order. Similarly, at time frame 5, data bursts 1 through 4 are in the buffer. At time frame 6, only data bursts 2 through 4 are left in the buffer since data burst 1 has been correctly forwarded. At time 7, after data burst 2 is correctly forwarded, data bursts 3 and 4 are also forwarded, so nothing is left in the buffer. The rest of the time frames may be explained similarly.

ARQ AND HARQ

ARQ has long been used in wireless communications to ensure a reliable, in-sequence data transmission by retransmitting corrupted data. There are three types of basic ARQ schemes: stop-and-wait, go-back-N, and selective repeat. The stop-and-wait scheme is the simplest to implement, yet it has the lowest throughput especially when the propagation delay is long. To take its advantage of simple implementation while avoiding the long, wasteful delays, the multiple-channel (or parallel) stop-and-wait ARQ has been proposed. One famous example is the Advanced Research Projects Agency Network (ARPAnet) that supports multiplexing of eight logical channels over a single link and runs stop-and-wait ARQ on each logical channel.

The performance of ARQ schemes suffers quickly because more retransmissions are required. To reduce the frequency of retransmissions, a system can adopt a forward error correction (FEC) functionality to correct errors that occur during transmissions. Combining an ARQ scheme with the FEC functionality is called HARQ.

In general, there are three types of HARQ, described below. In type I HARQ, the receiver simply discards erroneously received packets after it fails to correct errors, and sends a negative acknowledgement (NAK) back to the transmitter to request a retransmission. There is thus no need to have a buffer storing erroneous received packets. A fixed code rate is used for error correction, type I HARQ therefore cannot effectively adapt to changing channel conditions. (Note that code rate is defined as the ratio of the total number of information bits over the total bits transmitted. Thus, the higher the code rate, the lower the redundancy.) Using a code rate too high may cause too many retransmissions in high packet error rate (PER) conditions; on the other hand, using a code rate too low may cause too much redundancy in low PER conditions. The throughput may therefore be degraded by either a high frequency of retransmissions or too many redundant data in transmissions. Accordingly, choosing a suitable code rate is crucial for type I HARQ. Type I HARQ is therefore best suited for a channel that has a consistent level of signal-to-noise ratio (SNR).

In type II HARQ, in addition to packets being coded with ARQ and FEC as in type I HARQ, each receiver also keeps the erroneously received packets in the buffer in order to combine them with the retransmitted packets. There are two major FEC categories of coding for type II HARQ: chase combining (CC) and incremental redundancy (IR), introduced below.

In CC, the receiver combines received copies of the same packet to get diversity gain. All the redundant bits for each retransmitted packet are the same as the first transmission; therefore, it is relatively simple but less adaptive to the channel condition because the decoder may just need a smaller number of redundant bits to correct errors. The buffer needed is the number of coded symbols of one coded packet.

In IR, it adapts to changing channel conditions by retransmitting redundant bits gradually to a receiver. This is done while waiting for a positive ACK until all redundancies are sent. At the beginning, a transmitter using IR sends coded packets with a small number of FEC redundant bits or even without any. If a retransmission is needed, different redundant bits, derived from different puncturing patterns, are retransmitted depending on the base coding rate. The information data will not be retransmitted unless a receiver still cannot successfully decode the packet after a transmitter has sent all the redundant bits. This approach increases the receiver’s coding gain one retransmission at a time. The IR scheme therefore allows the system to adjust the channel encoder rates to the channel quality. Comparing with CC, a bigger buffer is required to store all retransmitted data, including the first transmitted data.

Type II HARQ needs a larger buffer size than type I HARQ, but it has higher performance in terms of throughput. The drawback of type II IR HARQ is that a receiver has to receive the first transmitted data to combine it with subsequently received redundant bits. To overcome this drawback, a type III HARQ has been proposed [7]. It may be viewed as a special case of type II HARQ such that each retransmitted packet is self-decodable. A receiver can correctly decode information data by either combining the first transmitted packet with a retransmitted packet or use only one of the retransmitted packets.