DECENTRALIZED SUBCARRIER AND POWER ALLOCATION SCHEMES

The distributed infrastructure of WiMAX mesh and relay networks, and the need for reducing communication overhead between the BS and network nodes are the motivations behind proposing decentralized resource allocation schemes. Potentially, either no central controller exists or it does not influence the allocation decision in a decentralized resource allocation. These facts make the decentralized schemes more scalable.

Resource allocation in decentralized networks is essentially different from centralized one. In centralized schemes, the BS collects CSI from all users, allocates the subcarriers or power to the users, and informs the users of allocated resources. On the other hand, users may not need to know the CSI of the other parties in decentralized schemes. Besides, the parties may not be aware of the decision of each other, so a collision is probable. Accordingly, in each proposed distributed resource allocation scheme for OFDM-based networks, the following questions should be answered:

• How does a user achieve the required CSI? The hidden terminal and exposed node problems are common problems in self-organized and decentralized networks, because it is assumed that no central controller exists to assist in signaling. Besides, the signaling overhead should be reasonable for a practical implementation.
  
  
• How should the node coordinate or compete with the other nodes to attain the resources? A node does not know the requirements of the other nodes, at each instant, so competence or coordination is a “must” for a node which wants to start capturing the resources.

An interference aware subchannel allocation scheme that overcomes the drawbacks of decentralized schemes. 

As the scheme uses updated CSI at the beginning of each MAC frame, the channel does not need to be assumed time-invariant over multiple MAC frames. The scheme is appropriate for OFDM-TDD networks. The MAC frame is divided into mini-slots; at the first mini-slot of each MAC frame the ongoing receiver nodes broadcast a busy signal to inform the respected transmitters of the quality of the allocated subchannel. The inactive nodes does not send any busy signal. The ongoing transmitter nodes listen to the busy signal and adapt their subcarrier allocation to their specific receiver nodes according to the information on the busy signal. Also, a node that wants to start transmission listens to the busy signal and chooses the subcarriers that are not interfered by ongoing transmissions and their interference. In other words, it selects those subcarriers with a received busy signal power less than a threshold. The advantages of the scheme are as follows:

• Signaling overhead is low compared to other radio resource allocation schemes
• The co-channel interference is reduced significantly since the scheme is interference aware
• Full frequency reuse is possible

A decentralized power allocation problem for a cooperative transmission is formulated in Ref. [43]. The objective is to minimize the total transmission power of all users subject to providing minimum rate requirement of each user. The network model uses a time division multiple access with the OFDM multiplexing (TDMA-OFDM), so only one user accesses the total OFDM subcarriers in each time slot. The user may allocate some of the subcarriers to transmit its traffic and the rest of subcarriers to relay the other users’ traffic to minimize total transmission power. In other words, the resource allocation problem determines if a user should cooperate with other users, and in case of cooperation it determines the subcarriers and power allocation. A cooperative user may be allocated more power than a noncooperative user, because its location and channel gain allows it to cooperate with other users. However, the total network power is minimized by cooperation among users. A decoupled subcarrier and power allocation scheme is proposed to solve the problem.

Assuming an access point in the network proposes a distributed decision-making scheme for the resource allocation. Each user measures its CSI upon receiving a beacon signal from the access point. The subcarriers are divided into several groups (equal to the number of users), and the approximate channel gain of each group for each user is estimated. Then, the users contend with each other to achieve the group with the best channel quality of their own. A backoff mechanism is proposed to avoid collision. Each user start contending for the best group of subcarriers after a backoff time which is proportional to the best group gain for that user. After contention, the access point informs the users of the winners which can transmit in the next transmission interval. A frame is divided into three subframes, contention, acknowledge, and transmission subframes. This scheme is actually a distributed resource allocation scheme for a centralized network that aims to reduce the signaling overhead and processing task of the access point.

A collaborative subcarrier allocation using the swarm intelligent. The scheme relies on the central controller to achieve the updated information of available subcarriers and the highest and lowest demands for each subcarrier. In other words, the nodes negotiate with the central controller iteratively in a negotiation phase. In each iteration, the nodes inform the central controller of their demand for a specific subcarrier. Then, the central controller broadcast a message indicating the highest and lowest demands for each subcarrier. Based on the feedback messages that occur several times in each negotiation phase, the nodes intelligently decide upon subcarriers.

A comparison between the capacity performance of OFDM-TDMA and OFDM-FDMA in a two-hop distributed network. The time frame is divided by two in OFDM-TDMA. In each half of the time frame, all subcarriers are devoted for transmission over one hop. A power allocation algorithm is proposed to maximize the end-to-end capacity subject to the overall transmit power constraint of the two hops. For OFDM-FDMA, the subcarriers are assigned to the two hops without overlapping and a joint subcarrier and power allocation algorithm is proposed. The simulation performance results of the algorithms show that OFDM-FDMA achieves a higher end-to-end capacity than OFDM-TDMA.

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.

What OFDM Means to WiMAX

To the telecommunications industry, an WiMAX OFDM-based system can squeeze a 72 Mbps uncoded data rate (~100 Mbps coded) out of 20 MHz of channel spectrum. This translates into a spectrum efficiency of 3.6 bps per Hz. If five of these 20 MHz channels are contained within the 5.725 to 5.825 GHz band, giving a total band capacity of 360 Mbps (all channels added together with 1 ×frequency reuse). With channel reuse and through sectorization, the total capacity from one BS site could potentially exceed 1 Gbps.

OFDM has manifold advantages in WiMAX, but among the more notable advantages is greater spectral efficiency. This is especially important in licensed spectrum use, where bandwidth and spectrum can be expensive. Here, OFDM delivers more data per spectrum dollar. In unlicensed spectrum applications, OFDM mitigates interference from other broadcasters due to its tighter beam width (less than 28 Mhz) and guardbands, as well as its dispersal of the data across different frequencies so that if one flow is "stepped on" by an interfering signal, the rest of the data is delivered on other frequencies.

QoS: Error Correction and Interleaving

Error correcting coding builds redundancy into the transmitted data stream. This redundancy allows bits that are in error or even missing to be corrected. The simplest example would be to simply repeat the information bits. This is known as a repetition code. Although the repetition code is simple in structure, more sophisticated forms of redundancy are typically used because they can achieve a higher level of error correction. For OFDM, error correction coding means that a portion of each information bit is carried on a number of subcarriers; thus, if any of these subcarriers has been weakened, the information bit can still arrive intact.

Interleaving is the other mechanism used in OFDM systems to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time; that is, a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors.

Legacy QoS Mechanisms

The following paragraphs describe legacy mechanisms.

FDD/TDD/OFDM

WiMAX incorporates a number of time-proven mechanisms to ensure good QoS. Most notable are TDD, FDD, FEC, FFT, and OFDM. The WiMAX standard provides flexibility in spectrum usage by supporting both FDD and TDD. Thus, it can operate in both FDD/OFDM and TDD/OFDM modes. It supports two types of FDD: continuous FDD and burst FDD.

In continuous FDD, the upstream and downstream channels are located on separate frequencies, and all CPE stations can transmit and receive simultaneously. The downstream channel is always on, and all stations are always listening to it. Traffic is sent on this channel in a broadcast manner using TDM. The upstream channel is shared using TDMA, and the BS is responsible for allocating bandwidth to the stations.

In burst FDD, the upstream and downstream channels are located on separate frequencies. In contrast to continuous FDD, not all stations can transmit and receive simultaneously. Those that can transmit and receive simultaneously are referred to as full-duplex capable stations while those that cannot are referred to as half-duplex capable stations.

A TDD frame has a fixed duration and contains one downstream subframe and one upstream subframe. The two subframes are separated by a guard time called transition gap (TG), and the bandwidth that is allocated to each subframe is adaptive. The TDD subframe is illustrated in Figure 1.

(Source: IEEE)

Figure 1: TDD subframe

Within a TDD downlink subframe, transmissions coming from the BS are organized into different modulation and FEC groups. The subframe header, called the FCH, consists of a preamble field, a PHY control field, and a MAC control field. The PHY control field is used for physical information, such as the slot boundaries, destined for all stations. It contains a map that defines where the physical slots for the different modulation/FEC groups begin.

The groups are listed in ascending modulation order, with QPSK first, followed by 16-QAM and then 64-QAM. Each CPE station receives the entire DL frame, decodes the subframe, and looks for MAC headers indicating data for the station. The DL data is always FEC coded. Payload data is encrypted, but message headers are unencrypted. The MAC control is used for MAC messages destined for multiple stations.

This variation uses burst single-carrier modulation with adaptive burst profiling in which transmission parameters, including the modulation and coding schemes, may be adjusted individually to each SS on a frame-by-frame basis. Channel bandwidths of 20 or 25 MHz (typical United States allocation) or 28 MHz (typical European allocation) are specified. Randomization is performed for spectral shaping and to ensure bit transitions for clock recovery.

Forward Error Correction (FEC)

WiMAX utilizes FEC, a technique that doesn't require the transmitter to retransmit any information that a receiver uses for correcting errors incurred in transmission over a communication channel. The transmitter usually uses a common algorithm and embeds sufficient redundant information in the data block to allow the receiver to correct. Without FEC, error correction would require the retransmission of whole blocks or frames of data, resulting in added latency and a subsequent decline in QoS.

Need QoS? Throw more bandwidth at it! Throughput and latency are two essentials for network performance. Taken together, these elements define the "speed" of a network. Whereas throughput is the quantity of data that can pass from source to destination in a specific time, round-trip latency is the time it takes for a single data transaction to occur (the time between requesting data and receiving it). Latency can also be thought of as the time it takes from data send-off on one end to data retrieval on the other (from one user to the other). Therefore, the better throughput (bandwidth) management, the better the QoS

OFDM Variants 2–11 GHz

The need for NLOS operation drives the design of the 2—11 GHz PHY. Because residential applications are expected, rooftops may be too low (possibly due to obstruction by trees or other buildings) for a clear sight line to a BS antenna. Therefore, significant multipath propagation must be expected. Furthermore, outdoor-mounted antennas are expensive, due to both hardware and installation costs. The four 2—11 GHz air interface specifications are described in the following paragraphs.

WirelessMAN-OFDM This air interface uses OFDM with a 256-point transform (see OFDM description later in this chapter). Access is by TDMA. This air interface is mandatory for license-exempt bands.

The WirelessMAN-OFDM PHY is based on OFDM modulation. It is intended mainly for fixed access deployments where SSs are residential gateways deployed within homes and businesses. The OFDM PHY supports subchannelization in the UL. There are 16 subchannels in the UL. The OFDM PHY supports TDD and FDD operations, with support for both FDD and H-FDD SSs. The standard supports multiple modulation levels including Binary Phase Shift Keying (BPSK), QPSK, 16-QAM, and 64-QAM. Finally, the PHY supports (as options) transmit diversity in the DL using Space Time Coding (STC) and AAS with Spatial Division Multiple Access (SDMA).

The transmit diversity scheme uses two antennas at the BS to transmit an STC-encoded signal to provide the gains that result from second-order diversity. Each of two antennas transmits a different symbol (two different symbols) in the first symbol time. The two antennas then transmit the complex conjugate of the same two symbols in the second symbol time. The resulting data rate is the same as without transmit diversity.

Figure 1 illustrates the frame structure for a TDD system. The frame is divided into DL and UL subframes. The DL subframe is made up of a preamble, Frame Control Header (FCH), and a number of data bursts. The FCH specifies the burst profile and the length of one or more DL bursts that immediately follow the FCH. The downlink map (DL-MAP), uplink map (UL-MAP), DL Channel Descriptor (DCD), UL Channel Descriptor (UCD), and other broadcast mes-sages that describe the content of the frame are sent at the beginning of these first bursts. The remainder of the DL subframe is made up of data bursts to individual SSs.

Figure 1: Frame structure for a TDD system (Source: IEEE)

Each data burst consists of an integer number of OFDM symbols and is assigned a burst profile that specifies the code algorithm, code rate, and modulation level that are used for those data transmitted within the burst. The UL subframe contains a contention interval for initial ranging and bandwidth allocation purposes and UL PHY protocol data units (PDUs) from different SSs. The DL-MAP and UL-MAP completely describe the contents of the DL and UL subframes. They specify the SSs that are receiving and/or transmitting in each burst, the subchannels on which each SS is transmitting (in the UL), and the coding and modulation used in each burst and in each sub-channel.

If transmit diversity is used, a portion of the DL frame (called a zone) can be designated to be a transmit diversity zone. All data bursts within the transmit diversity zone are transmitted using STC coding. Finally, if AAS is used, a portion of the DL subframe can be designated as the AAS zone. Within this part of the subframe, AAS is used to communicate to AAS-capable SSs. AAS is also supported in the UL.

WirelessMAN-OFDMA This variant uses orthogonal frequency division multiple access (OFDMA) with a 2048-point transform. In this system, addressing a subset of the multiple carriers to individual receivers provides multiple access. Because of the propagation requirements, the use of AASs is supported.

The WirelessMAN-OFDMA PHY is based on OFDM modulation. It supports subchannelization in both the UL and DL. The standard supports five different subchannelization schemes. The OFDMA PHY supports both TDD and FDD operations. The same modulation levels are also supported. STC and AAS with SDMA are supported, as is multiple input, multiple output (MIMO). MIMO encompasses a number of techniques for utilizing multiple antennas at the BS and SS in order to increase the capacity and range of the channel.

The frame structure in the OFDMA PHY is similar to the structure of the OFDM PHY. The notable exceptions are that subchannelization is defined in the DL as well as in the UL, so broadcast messages are sometimes transmitted at the same time (on different subchannels) as data. Also, because a number of different subchan-nelization schemes are defined, the frame is divided into a number of zones that each use a different subchannelization scheme. The MAC layer is responsible for dividing the frame into zones and communicating this structure to the SSs in the DL-MAP and UL-MAP. As in the OFDM PHY, there are optional transmit diversity and AAS zones, as well as a MIMO zone.

Wireless High Speed Unlicensed Metro Area Network (WirelessHUMAN) WirelessHUMAN is similar to the aforementioned OFDM-based schemes and is focused on Unlicensed National Information Infrastructure (UNII) devices and other unlicensed bands.

The Function of the PHY - WiMAX

As the name might imply, the purpose of the PHY is the physical transport of data. The following paragraphs will describe different methods to ensure the most efficient delivery in terms of bandwidth (volume and time in Mbps) and frequency spectrum (MHz/GHz). A number of legacy technologies are used to get the maximum performance out of the PHY. These technologies, including OFDM, TDD, FDD, QAM, and Adaptive Antenna System (AAS), will be described in the following pages or chapters.

OFDM: The "Big So What?!" of WiMAX

OFDM is what puts the max in WiMAX. OFDM is not new. Bell Labs originally patented it in 1970, and it became incorporated in various digital subscriber line (DSL) technologies as well as in 802.11a. OFDM is based on a mathematical process called Fast Fourier Transform (FFT), which enables 52 channels to overlap without losing their individual characteristics (orthogonality). This is a more efficient use of the spectrum and enables the channels to be processed at the receiver more efficiently. OFDM is especially popular in wireless applications because of its resistance to forms of interference and degradation. In short, OFDM delivers a wireless signal much farther with less interference than competing technologies. Figure 1 provides an illustration of how OFDM works.

Figure 1: The significance of OFDM: A focused beam delivering maximum bandwidth over maximum distance with minimum interference

TDD and FDD

WiMAX supports both time division duplex (TDD) and frequency division duplex (FDD) operation. TDD is a technique in which the system transmits and receives within the same frequency channel, assigning time slices for transmit and receive modes. FDD requires two separate frequencies generally separated by 50 to 100 MHz within the operating band. TDD provides an advantage where a regulator allocates the spectrum in an adjacent block. With TDD, band separation is not needed, as is shown in Figure 2. Thus, the entire spectrum allocation is used efficiently both upstream and downstream and where traffic patterns are variable or asymmetrical.

Figure 2: A TDD subframe

In FDD systems, the downlink (DL) and uplink (UL) frame structures are similar except that the DL and UL are transmitted on separate channels. When half-duplex FDD (H-FDD) subscriber stations (SSs) are present, the base station (BS) must ensure that it does not schedule an H-FDD SS to transmit and receive at the same time.

Figure 3 illustrates this relationship.

Figure 3: ULs and DLs between BSs and SSs

Adaptive Antenna System (AAS)

AAS is used in the WiMAX specification to describe beam-forming techniques where an array of antennas is used at the BS to increase gain to the intended SS while nulling out interference to and from other SSs and interference sources. AAS techniques can be used to enable Spatial Division Multiple Access (SDMA), so multiple SSs that are separated in space can receive and transmit on the same subchannel at the same time. By using beam forming, the BS is able to direct the desired signal to the different SSs and can distinguish between the signals of different SSs, even though they are operating on the same subchannel(s), as shown in Figure 4.

Figure 4: AAS uses beam forming to increase gain (energy) to the intended SS.

Orthogonal Frequency Division Multiplexing (OFDM)

Some of the key technologies used in WiMAX systems include orthogonal frequency division multiplexing, frequency reuse, adaptive modulation, diversity transmission and adaptive antennas.

Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a process of transmitting several high speed communication channels through a single communication channel using separate sub-carriers ( frequencies) for each radio channel. The use of OFDM reduces the effects of multi-path and delay spread, which is especially important for lower frequencies and near line of sight (NLOS) transmission.

Multi-path propagation is the transmission of a radio signal which travels over two or more paths from a transmitter to a receiver. Multi-path transmission can cause changes in the received signal level as delayed signals can either add or subtract from the received signal level. Multi-path is not usually a challenge on systems that use higher frequencies as these systems tend to use highly directional (high-gain) antennas for direct line of sight transmission.

Multi-path propagation is frequency dependent meaning that the multiple paths radio signals travel will vary depending on its’ frequency.

Figure 1illustrates how a transmitted signal may travel through multiple paths before reaching its destination. In this example, the same signal is reflected off an office building where it is received by the subscriber device. The reflected signal is delayed (travels a longer path) and subtracts from the direct signal resulting in a dead spot (fade) at the receiver. Furthermore, mutli-path propagation is sensitive to frequency and that distortion occurs at different points when other frequencies are used. When a different frequency is used, the reflected signal is redirected and it does not subtract from the direct signal.

Figure 1: Multi-path Propagation

For a wide radio channel that is divided into several sub-carriers, each subcarrier channel operates at a different frequency and can have different transmission characteristics than other sub-carriers. Because multiple sub-carriers are typically combined for a single subscriber, this can reduce the effects of multi-path fading.

The use of multiple sub-carriers also has the effect of reducing the symbol rate, which can reduce the effects of delay spread. Delay spread is a product of multi-path propagation where symbols become distorted and eventually overlap due to the same signal being received at a different time. It becomes a significant problem in mountainous areas where signals are reflected at great distances. Delay spread can be minimized by either using an equalizer to adjust for the multi-path distortions or to divide a communication channel into sub-carriers (e.g. OFDM) where each sub-carrier transfers data at a much slower data transmission rate thereby reducing the effects of delay spread.

Figure 2 demonstrates how OFDM divides a single radio channel into multiple coded sub-channels. A high-speed digital signal is divided into multiple lower-speed sub channels that are independently from each other and can be individually controlled. The OFDM process allows bits to be sent on multiple sub channels. The channels selected can be varied based on the quality of the sub channel. In this figure, a portion of a sub channel is lost due to a frequency fade. As a result of the OFDM encoding process, the missing bits from one channel can be transmitted on other channels.

Figure 2: Orthogonal Frequency Division Multiplexing (OFDM)