The standard 802.16e [1] has been standardized in 2006. The version provides the possibility of MSs with mobility that is greatly different from 802.16d [2] only provides fixed broadband wireless access. With the mobility, the system is not only the last mile solution for the end users. The system will be mode like cellular system but with higher transmission rate and bandwidth than 3G or even 3.5G system.
Therefore, on the completion of IEEE802.16-2004 [2] and IEEE 802.16-2005 [1]
standards for WMAN (Wireless Metropolitan Area Network), nowadays, WiMAX reaches maturity as a serious alternative for mobile broadband access deployment. As a continuous effort to enhance WiMAX system, 802.16j MR task group [3] has been charted in March 2006. The objective of 802.16j is to adapt the promising advantages of multi-hop access in terms of capacity, throughput, and coverage to WiMAX systems. Low-cost RSs are allocated between the 802.16 compliant MS/SS (Subscriber Station) and Base Station (MR-BS) because RSs do not require the wired backhaul and are much cheaper than MR-BS. Multi-hop networks provide the increased deployment flexibility and improve the economic viability of 802.16WiMAX system due to the cheaper cost.
Therefore, RSs are provided to be allocated in the transparent and non-transparent MR network during the transmission between BS to MS/SS. Because a RS relays the user data and control information between BS and MS, it is a significant issue to let the system throughput and performance increase under the influence of RS. According to good channel conditions in MR network, deploying RSs in excellent location to relay user data and choosing the best transmission route for MS are keys to influence the throughput of communication.
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Therefore, having a good deployment for RS to transmit data efficiently and using a great method for MS to select transmission route can achieve the services’
wanted throughputs and enhance the system performance is really crucial in the future wireless communication system. In this thesis, we will discuss these two issues and propose two algorithms of the transmission route selection for comparison.
The rest of this thesis is organized as follows: In chapter2, the review to Mobile WiMAX system and the review to WiMAX Relay System. In chapter3, we briefly introduce the network simulator because we use the network simulator to run the simulation. In chapter4, the proposed algorithms are discussed detailed. In chapter5, we describe the setting of simulation parameter and show the simulation results.
Finally in chapter6, the conclusion and future works will be provided.
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Chapter 2 Review to WiMAX System
The IEEE Standard 802.16 Family specifics the air interface of fixed and mobile WIMAX systems. The standard scope includes two layers: the MAC layer and the PHY layer. Specification of the MAC layer includes the security and convergence sublayer and common part sublayer. The system architecture, optimizations and algorithms is not in the defined scope. A reference model of IEEE 802.16 PHY/MAC is shown in Figure 2-1.
Figure2.1 IEEE 802.16 PHY/MAC Reference Model[1][2]
We briefly review the operations of MAC layer and PHY layer in the IEEE 802.16 standard. In Figure2.2, it illustrate the system architecture of the IEEE 802.16e.The CS provides any transformations or mapping of external network data which are received through the CS service access point (SAP), into MAC service data units (MSDU) that are received by the MAC layer through the MAC SAP [4].This
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sublayer contains several functions which are classifying external network SDUs and associating them to the proper MAC service flow identifier (SFID) and connection ID (CID). It also contains such the functions of payload header suppression (PHS).
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Figure2.2 The MAC simulation architecture of IEEE 802.16[4]
The MAC CPS provides the core MAC functionality of system access such as bandwidth allocation, scheduling, contention mechanism, connection establishment, and connection maintenance. It receives data from vary kind of CSs, through the MAC SAP, classified to particular MAC connection. And, in scheduling types side, the IEEE 802.16-2004 standard specially supports five quality-of-service scheduling
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types: unsolicited grant service(UGS) for the constant bit rate(CBR) service and real-time VoIP, real-time polling service(rtPS) for the variable rate (VBR) service, non-real-time polling service(nrtPS) for non-real-time VBR, and best effort service(BE) for service with no rate or delay requirements. Especially, there is an additional service type called extended real-time polling service (ertPS) for voice over IP (VoIP) service in 802.16e standard.
These quality-of-service classes are correlated with certain predefined sets of QoS-related service flow parameters. The MAC scheduler provides the appropriate data handling mechanisms for data transmission according to each QoS classes. After finishing the SFID-CID mapping, the upper-layer protocol data units (PDUs) with an assigned individual CID are passed into different level queues in the MAC layer. And then, these data packets in these queues are regards as MSDUs and will be fragmented or packed into various sizes according to the MAC scheduling operation, and be processed by a selective repeat automatic repeat request (ARQ) block mechanism if the ARQ-enabled function is opened.
For the UL traffic, each SS should perform ranging to the BS before SS entering the system. When SS performs the initial ranging, the SS will ask a request to be served in the DL via the burst profile by BS transmitting its choice of DL interval usage code (DIUC) to the SS. And then, the BS will command the SS to use an uplink burst profile with the allocated UL interval usage code (UIUC) with the grand of the SS in UL-MAP message. The DL-MAP and UL-MAP contain the channel ID and the MAP information elements (IEs), the functions of IEs are which describes the PHY specification mapping in the DL and UL specifically. The burst profile includes the DIUC, UIUC, and the type-length-value (TLV) encoded information. The TLV encoded information will report different PHY layer of the modulation type, FEC code type, and encoding parameters. The MAC data payload is packed according to
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these encoding types.
Radio link control (RLC) that the PHY layer requires equally, which is the capability of the PHY layer to transition from one burst profile to another. The RLC starts with the periodical BS broadcasting of the burst profiles which have been chosen for the downlink and uplink connections. After determining the downlink and uplink burst profiles between the BS and a particular SS, RLC will monitor the burst profile and set control to the burst profiles. The SS will do the ranging with the RNG-REQ message to request a change in downlink burst profile. The BS will use channel measurements reporting request (REP-REQ) message to request signal-to-noise ratio (SNR) channel measurements reports. And then, the SS uses the channel measurement report response (REP-RSP) message to respond the channel measurements listed in the received REP-REQ.
In the IEEE 802.16 system, the system uses the frame-based transmission architecture where the frame length is variable. Every frame is separated into two subframes: the DL subframe and UL subframe. Here, we pay attention to the frame structure on OFDMA-PHY in time division duplex (TDD) mode. A DL subframe consists of DL subframe prefix to specify the modulation and coding (in PHY mode), the length of the first DL burst, and the broadcast MAC control message. The DCD (Downlink channel descriptor) and UCD (Uplink channel descriptor) comprise of the detail information of the DL burst profile and the UL burst profile. It does not define the admission control process although IEEE 802.16 defines the connection signaling between SS and BS. By the connection classifier based on CID, all packets from the application layer are classified and are passed to the appropriate queue. At the SS, the scheduler will get the packets from the queues and forward them to the network in the appropriate time slots as defined by the UL-MAP sent by the BS. By the scheduler module based on the BW-request message that reports the current queue size of each
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connection in SS, the UL-MAP is determined. So, in the following article of this chapter 2 in my thesis, I will separate MAC layer and PHY layer structure of IEEE 802.16e WiMAX into discussion.
2.1 WiMAX Physical Layer
The physical (PHY) layer of WiMAX is based on IEEE 802.16-2004 and IEEE 802.16e-2005 standards and was designed getting much influence from Wi-Fi, especially IEEE 802.11a. Although many aspects of the two technologies are different because of the inherent difference in each of their purpose and applications, some of their basic constructs are very similar. The IEEE 802.16 suite of standards (IEEE 802.16-2004/IEEE 802.16E-2005) defines within its scope four PHY layers, any of which can be used to develop a broadband wireless system with the media access control (MAC) layer. The PHY layers defined in IEEE 802.16e are divided into several parts:
z WirelessMAN SCa, a single-carrier PHY for frequencies between 2GHZ and 11GHZ for point-to-multipoint operations.
z WirelessMAN OFDM, a 256-point FFT-based OFDM PHY layer for point-to-multipoint operations in non-LOS conditions at frequencies between 2GHz and 11GHz. This PHY layer, finalized in the IEEE 802.16-2004 specifications, has been accepted by WiMAX for fixed operations and is often referred to as fixed WiMAX. The OFDM PHY transmission chains are illustrated in Figure 2.3.
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Figure2.3 OFDM PHY transmission chain[5]
z WirelessMAN OFDMA, a 2,048-point FFT-based OFDMA PHY for point-to multipoint operations in NLOS conditions at frequencies between 2GHz and 11 GHz. In the IEEE 802.16e-2005 specifications, this PHY layer has been modified to SOFDMA (scalable OFDMA), where the FFT size is variable and can take any one of the following values: 128, 512, 1024, and 2048. The variable FFT size allows for optimum operation/implementation of the system over a wide range of channel bandwidths and radio conditions.
The PHY layer has been accepted by WiMAX for mobile and portable operations and is also referred to as mobile WiMAX. The OFDMA PHY transmission chains are illustrated in Figure 2.4.
Figure2.4 OFDMA PHY transmission chain[5]
Figure2.5 illustrates the evident functional stage of a WiMAX PHY layer. The first functional stages have relation to many functions such as forward error
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correction (FEC), channel encoding, rate matching, interleaving, and symbol mapping. The next set of functional stages is connected to the construction of the OFDM symbol in the frequency domain. During this stage period, data is individually mapped onto the appropriate subchannels and subcarriers. Pilot symbols are inserted into the pilot subcarriers allowing the receiver to estimate and get track of the channel state information. This stage is also responsible for any space/time encoding for transmit diversity or MIMO. The final functions are related to the conversion of the OFDM symbol from the frequency domain to the time domain and finally to an analog signal so we can use analog signal to transmit over the air. Although Figure2.5 shows only the logical components of a transmitter, similar components also exist at the receiver, to reconstruct the transmitted information sequence in reverse order.
Figure2.5 Functional stages of WiMAX PHY[5]
In the first of this 2.1 chapter, I describe the various components of modulation scheme and channel-coding as defined in the IEEE802.16e-2005 standard. Next, we will introduce the burst profile that is the basic tool in the 802.16 standard MAC Layer.
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2.1.1 Digital Modulation
About the recent modern communication systems, IEEE 802.16e WiMAX uses digital modulation. Now, the famous rule of a digital modulation is to modulate an analogous signal with a digital sequence in order to transport this digital sequence. It shows in Figure 2.6 [6].
Figure2.6 Digital modulation rule[6]
There are many advantages with regard to classical analogue modulation: better resistance to noise, use of high-performance digital communication and coding algorithms, etc. By adjusting the physical characteristics of a sinusoidal carrier, either the frequency, phase or amplitude, or a combination of some of these, the variants are retrieved. The IEEE 802.16 standard supports four modulations – BPSK, QPSK, 16-QAM, 64-QAM. I will give a short explanation for these modulations used in OFDM and OFDMA PHYsical layers in this section.
z Quadrature Phase Shift Keying(QPSK)
If we need a higher spectral efficiency modulation, greater modulation symbol can be used. For instance, QPSK considers two-bit symbols of modulation. Table 2.1 illustrates the possible phase values as a function of the modulation symbol. QPSK can be used many variants but QPSK
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always has a four-point constellation. Figure 2.7 shows the constellation.
The decision at the receiver, e.g. between symbol‘00’ and symbol’01’, is less easy than a decision between ‘0’ and‘1’. So, compared to BPSK, the QPSK modulation is less noise-resistant as it has a smaller immunity against interference.
Table2.1 Possible phase values for QPSK modulation [6]
Figure2.7 Example of a QPSK constellation[6]
z Quadrature Amplitude Modulation(QAM): 16-QAM and 64-QAM
The QAM changes the amplitudes of two sinusoidal carriers depending on the digital sequence that must be transmitted; the two carriers being out of phase of +π/2. So, this modulation is called quadrature. Both 16-QAM which are 4bits modulation symbols and 64-QAM which are 6bits modulation symbol modulation are included in the IEEE 802.16standard.
The 64-QAM is shown in Figure 2.8. Evidently, 6 bits are transmitted with each modulation symbol. The 64-QAM modulation is optional in some
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cases:
when the OFDM PHYsical Layer is used, License-exempt bands
for OFDMA PHY, yet the Mobile WiMAX profiles indicates that 64-QAM is mandatory in the downlink
Figure2.8 Example of 64-QAM constellation[6]
2.1.2 Channel Coding
The radio link varies very fast, it often suffering from great interference. Channel coding are mainly used to prevent and to correct the transmission errors of wireless systems, must have a very excellent performance in order to maintain high data rates.
The channel coding chain in 802.16 is composed of three steps:
z Randomizer
z Forward Error Correction z Interleaving
They are implemented in the order in transmission. So, in 2.1.2 section, I will talk about the above three function in brief.
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2.1.2.1 Randomisation
Randomisation introduces protection from information-theoretic uncertainty, to avoid long sequences of constructive ones or consecutive zeros, and it is also useful for avoiding non-centered data sequences. Data randomisation is performed on each downlink and uplink burst of data. If the situation is that the amount of data to transmit does not fit exactly the amount of data allocated, it must add padding of 0xFF to the end of the transmission block. In Figure 2.9, the Pseudo-Random Binary Sequence (PRBS) generator used for randomisation is shown. Each data byte to be transmitted enters sequentially into the randomizer, with the Most-Significant Byte (MSB) first. For Preambles, they are not randomized because the randomizer sequence is used only to information bits.
Figure2.9 PRBS generator used for data transmission in OFDM and OFDMA PHY[6]
2.1.2.2 Forward Error Correction (FEC) Codes
For OFDM PHY, the types of FEC encodings are listed below:
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z Concatenated Reed-Solomon Convolutional Code (RS-CC). It includes the concatenation of a Reed-Solomon outer code and a rate-compatible Convolutional inner code. The code is mandatory on both uplink and downlink.
z Convolutional Turbo Codes(optional) z Block Turbo Coding(BTC)(optional)
When requesting access to the network, the most robust burst profile must be used. For OFDMA PHY, the FEC encodings are:
z (Tail-biting) Convolutional Turbo Codes (CC). This code is mandatory according to the 802.16 standard. Based on WiMAX profiles, only the Zero-Tailing Convolutional Code is mandatory.
z Convolutional Turbo Codes (CTC). This code is optional according to the 802.16 standards. Yet, according to the mobile WiMAX profiles, the CTC is mandatory.
z Block Turbo Coding (BTC) (optional)
z Low Density Parity Check (LDPC) codes (optional)
2.1.2.3 Interleaving
Interleaving is a method used to protect the transmission against long sequences of consecutive errors. These long sequences of error may affect many bits in a row and can then result in many losses of transmitted burst. Interleaving can facilitate error correction. The encoded data bits are interleaved by a block interleaver with a block size corresponding to the number of bits per allocated subchannels per OFDM symbol [1]. The function of interleaver is made of two steps:
z Distribute the coded bits over subcarriers. A first permutation makes sure
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that adjacent coded bits are mapped onto nonadjacent subcarriers.
z The second permutation makes sure that adjacent coded bits are mapped alternately on to less or more significant bits of the constellation, thus avoiding long runs of bits of low reliability.
2.1.2.4 Repetition
The standard indicates that repetition for the OFDMA PHY can be used to increase the signal margin further over the modulation and FEC mechanisms.
As for repetition coding, R=2, 4, 6, the number of allocated slots will be a whole multiple of the repetition factor R for the uplink. On the other hand, for the downlink, the number of the allocated slots will be in the range of R×K, R×K+(R-1), when K is the number of required slots if we want to apply the repetition scheme.
The binary data that fits into a region which is repetition coded is reduced by a factor R compared to a nonrepeated region of the slots with the same size and FEC code type. After finishing FEC and bit-interleaving, these data are segmented into slots, and then, each group of bits designated to fit in a slot is repeated R times to form R continuous slots following the normal slot ordering that is used for data mapping. This repetition scheme applies only to QPSK modulation, and it can be applied in all coding schemes, but it cannot be used in HARQ with CTC.
2.1.3 Burst Profile
The burst profile is a basic tool and useful function in the 802.16 standard MAC Layer. The function of burst profile allocation, which changes dynamically and possibly very fast, is mainly about physical transmission. The parameters of the burst profiles of WiMAX are summarized. And, the burst profiles are used for the link
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adaption procedure.
2.1.3.1 Downlink Burst Profile Parameters
The burst profile parameters of a downlink transmission for OFDM and OFDMA PHYsical layers are proposed in Table 2.2. The parameters called FEC code is the Modulation and Coding Scheme (MCS). For OFDMA PHY, there are totally 20 MCS combinations of modulation (BPSK, QPSK, 16-QAM, 64-QAM), coding (CC, RS-CC, CTC or BTC) and coding rate (1/2, 2/3, 3/4 and 5/6). The most frequency-use efficient MCS among all MCS is 64-QAM (BTC) 5/6. For OFDMA PHY, there are 34 MCS combinations of modulation (BPSK, QPSK, 16-QAM, 64-QAM), coding rate (1/2, 2/3, 3/4, 5/6). The Downlink Interval Usage Code (DIUC) is the burst usage descriptor, which includes the burst profile.
Table2.2 Downlink burst profile parameters for OFDM and OFDMA PHYsical layers [6]
2.1.3.2 Uplink Burst Profile Parameters
In Table 2.3, I list the burst profile parameters of an uplink transmission for an
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OFDM PHT, and in Table 2.4, I list the burst profile parameters of an uplink transmission for an OFDMA PHY respectively.
Table2.3 Uplink burst profile parameters for the OFDM PHYsical Layer [6]
Table2.4 Uplink burst profile parameters for the OFDMA PHYsical Layer [6]
2.1.3.3 MCS Link Adaptation
The choice between different burst profiles or between different MCSs is a useful tool because choosing the MCS most suitable for the state of the radio channel leads to an optimal (highest) average data rate. This procedure is the so-called link adaptation procedure. The order of magnitudes of SNR thresholds can be obtained from Table 2.5, proposed in the 802.16 standard for some test conditions.
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Table2.5 Received SNR threshold assumptions [1]
2.2 WiMAX MAC Layer
In a network, the PHY layer use the physical medium, such as radio frequency, light waves, or copper wires to reliably deliver information bits from the transmitter to receiver. However, the PHY layer is not informed of quality of service (QoS) requirements and is not aware of the nature of the application, such as VoIP, HTTP, or
In a network, the PHY layer use the physical medium, such as radio frequency, light waves, or copper wires to reliably deliver information bits from the transmitter to receiver. However, the PHY layer is not informed of quality of service (QoS) requirements and is not aware of the nature of the application, such as VoIP, HTTP, or