WiMAX Technology for Broadband Wireless Access 2007 phần 4 ppsx

6 The Physical Layer of WiMAX 6.1 The 802.16 Physical Transmission Chains The modulation and OFDM transmission aspects, described in the previous chapter, are the major building blocks of the WiMAX PHYsical Layer. In this chapter, some elements of the transmission chains of WiMAX are described for both OFDM and OFDMA PHYs. 6.1.1 The Global Chains The PHY transmission chains of OFDM and OFDMA are illustrated in Figures 6.1 and 6.2. The blocks are the same with the small difference that OFDMA PHY includes a repeti- tion block. The modulation is one of the four digital modulations described in the previous chapter: BPSK, QPSK, 16-QAM or 64-QAM. The modulated symbols are then transmitted on the OFDM orthogonal subcarriers. In the following, WiMAX channel coding building blocks are described. The building blocks of channel coding are described in Section 6.2. A possible FEC code is the Turbo Code. Turbo Code theory and the basic elements of its use in WiMAX can be found in Section 6.3. The Transmission Convergence Sublayer (TCS), which can be applied in OFDM PHY, is described in Section 6.4. Finally, the burst profi les of OFDM and OFDMA PHY, an important building block of IEEE 802.16 MAC layer, are described in Section 6.5. 6.2 Channel Coding The radio link is a quickly varying link, often suffering from great interference. Channel coding, whose main tasks are to prevent and to correct the transmission errors of wireless systems, must have a very good performance in order to maintain high data rates. The 802.16 channel coding chain is composed of three steps: randomiser, Forward Error Correction (FEC) and interleaving. They are applied in this order at transmission. The corresponding operations at the receiver are applied in reverse order. Error detection is realised with HCS and CRC (see Chapter 8). WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd. ISBN: 0-470-02808-4 70 WiMAX: Technology for Broadband Wireless Access 6.2.1 Randomisation Randomisation introduces protection through information-theoretic uncertainty, avoiding long sequences of consecutive ones or consecutive zeros. It is also useful for avoiding non- centred data sequenes. Data randomisation is performed on each downlink and uplink burst of data. If the amount of data to transmit does not fi t exactly the amount of data allocated, padding of 0ϫFF (‘ones’ only) is added to the end of the transmission block. The Pseudo- Random Binary Sequence (PRBS) generator used for randomisation is shown in Figure 6.3. Each data byte to be transmitted enters sequentially into the randomiser, with the Most Sig- nifi cant Byte (MSB) fi rst. Preambles are not randomised. The randomiser sequence is applied only to information bits. The shift-register of the randomiser is initialised for each new burst allocation. For OFDM PHY, on the downlink, the randomiser is reinitialised at the start of each frame with the se- quence: 1 0 0 1 0 1 0 1 0 0 0 0 0 0 0. The randomiser is not reset at the start of burst 1. At the start of subsequent bursts (starting from burst 2), the randomiser is initialised with the vector PHY -sical PDU to be transmitted Rando- misation FEC encoder (CC, Turbo Code, …) Inter- leaving Modulation To OFDMA Part: IFFT, CP, Subchannels etc. (see Chapter 5) Repe- tition Figure 6.2 OFDMA PHY transmission chain 151413121110987 6 5 4 3 2 1 data out data in MSBLSB Figure 6.3 PRBS generator used for data randomisation in OFDM and OFDMA PHY. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) PHYsical PDU to be transmitted Rando- misation FEC encoder (CC, Turbo Code or other) Interleaving Modulation To OFDM Part: IFFT, CP, etc. (see Chapter 5) Figure 6.1 OFDM PHY transmission chain The Physical Layer of WiMAX 71 shown in Figure 6.4. This PRBS generates a Pseudo-Noise (PN) sequence of length 2 15 Ϫ 1. The frame number used for initialisation refers to the frame in which the downlink burst is transmitted. BSID is the BS identity and DIUC the burst profi le indicator (see Chapter 9). For other cases (uplink, OFDMA), the details can be found in the standard. The bits issued from the randomiser are then applied to the FEC encoder. 6.2.2 Forward Error Correction (FEC) Codes For OFDM PHY, the FEC encodings are: • Concatenated Reed–Solomon Convolutional Code (RS-CC). This code is mandatory on both the uplink and downlink. It consists of the concatenation of a Reed–Solomon outer code and a rate-compatible convolutional inner code (see below). • Convolutional Turbo Codes (CTC) (optional). • Block Turbo Coding (BTC) (optional). For Turbo Coding, see Section 6.3 below. The most robust burst profi le or, equivalently, the most robust coding mode must be used when requesting access to the network and in the FCH burst (see Chapter 9 for FCH burst). For OFDMA PHY, the FEC encodings are: • (Tail-biting) Convolutional Code (CC). This code is mandatory according to the 802.16 standard. According to WiMAX profi les, only the Zero-Tailing Convolutional Code (ZT CC) is mandatory. • Convolutional Turbo Codes (CTC). This code is optional according to the 802.16 standards [1,2]. Yet, according to mobile WiMAX profi les, the CTC is mandatory. • Block Turbo Coding (BTC) (optional). • Low Density Parity Check (LDPC) codes (optional). RS-CC encoding will now be described. WiMAX Turbo coding, BTC and CTC, will be described in the following section. 6.2.2.1 RS-CC (Reed–Solomon Convolution Code) For OFDM PHY, the RS-CC encoding is performed by fi rst passing the data in block format through the RS encoder and then passing it through a convolutional encoder (see Figure 6.5). b3 b14 b13 b12 b11 b8 b7 b6 b5 11 1 BSID OFDM randomizer DL initialization vector DIUC Frame Number LSBMSB LSBMSB b2 b1 b0 b3 b3 b2 b2 b1 b1 b0 b0 b3 b2 b1 b0 Figure 6.4 OFDM randomiser downlink initialisation vector for burst 2,…,N. (From IEEE Std 802.16- 2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) 72 WiMAX: Technology for Broadband Wireless Access Reed–Solomon codes are used in many communications systems and other applications. The RS error correction works by adding some redundant bits to a digital data sequence. This is done by oversampling a polynomial constructed from the uncoded data. The polynomial is evaluated at several points and then these values are sent (or recorded). By sampling the polynomial more often than needed, the receiver can recover the original polynomial in the presence of a relatively low number of errors. A Reed–Solomon code is specifi ed as RS(N,K) with T-bit symbols. The data points are sent as encoded blocks. The total number of T-bit symbols in an encoded block is N ϭ 2 T Ϫ1. Thus a Reed–Solomon code operating on 8-bit symbols has N ϭ 2 8 Ϫ1 ϭ 255 symbols per coded block. The number K, K Ͻ N, of uncoded data symbols in the block is a design parameter. Then, the number of parity symbols added is N Ϫ K symbols (of T-bits each). The RS decoder can correct up to (N Ϫ K)͞2 symbols that contain an error in the encoded block. The RS encoder of OFDM PHY is denoted as an (N, K) ϭ (255, 239) code, which is capable of correcting up to eight symbol errors per block. This Reed–Solomon encoding uses GF(2 8 ), where GF is the Galois Field operator. The Reed–Solomon encoder and decoder require Galois fi eld arithmethics. The following polynomials are used for the OFDM RS systematic code, an RS code that leaves the data unchanged before adding the parity bits: Code generator polynomial: g(x) ϭ (x ϩ m 0 ) (x ϩ m 1 ) (x ϩ m 2 ) … (x ϩ m 2TϪ1 ), m ϭ 02 HEX ; Field generator polynomial: p(x) ϭ x 8 ϩ x 4 ϩ x 3 ϩ x 2 ϩ 1. The coding rate of the OFDM PHY RS encoder is then 239/255 (very close to one). The standard indicates that this code can be shortened and punctured to enable variable block sizes and variable error-correction capabilities. The convolution code has an original coding rate of 1/2, as shown in Figure 6.6. The convolu- tional encoder is a zero-terminating convolutional encoder. A single 0 ϫ 00 tail byte is appended to the end of each burst, needed for decoding algorithm normal operation. Puncturing patterns defi ned in the standard can be used to realise the following different code rates: 2͞3, 3͞4 and 5͞6. For OFDMA, the convolutional encoder is also the one shown in Figure 6.6. The HARQ procedure (described in Chapter 8), in its IR (Incremental Redundancy) variant, uses four different FEC blocks for each uncoded FEC block. This is realised using different puncture patterns. Each FEC block is identifi ed by an SPID (SubPacket IDentifi er). The tail-biting convolutional code encoder of OFDMA (simply known as CC) works as fol- lows: the convolutional encoder memories are initialised by the (six) last data bits of the FEC block being encoded (the packet data bits numbered b n Ϫ 5,…,b n ). This OFDMA PHY con- volutional encoder may employ the Zero-Tailing Convolutional Coding (ZT CC) technique. In this case, a single 0 ϫ 00 tail byte is appended at the end of each burst. This tail byte is appended after randomisation. Outer code : Reed-Solomon (RS) encoder Inner code : Convolutional code (CC) encoder Data from randomiser Encoded data Figure 6.5 Illustration of the RS-CC encoder of OFDM PHY The Physical Layer of WiMAX 73 6.2.3 Interleaving Interleaving is used to protect the transmission against long sequences of consecutive errors, which are very diffi cult to correct. These long sequences of error may affect a lot of bits in a row and can then cause many transmitted burst losses. Interleaving, by including some di- versity, can facilitate error correction. The encoded data bits are interleaved by a block inter- leaver with a block size corresponding to the number of coded bits per allocated subchannels per OFDM symbol [1]. The interleaver is made of two steps: • Distribute the coded bits over subcarriers. A fi rst permutation ensures that adjacent coded bits are mapped on to nonadjacent subcarriers. • The second permutation insures that adjacent coded bits are mapped alternately on to less or more signifi cant bits of the constellation, thus avoiding long runs of bits of low reliability. 6.2.4 Repetition Repetition was added by the 16e amendment for OFDMA PHY. The standard indicates that it can be used to increase the signal margin further over the modulation and FEC mechanisms. In the case of repetition coding, R ϭ 2, 4 or 6, the number of allocated slots (Ns) will be a whole multiple of the repetition factor R for the uplink. For the downlink, the number of the allocated slots (Ns) will be in the range of R ϫ K, R ϫ K ϩ (R Ϫ 1), where K is the number of required slots before applying the repetition scheme. For example, when the required number of slots before the repetition is 10 (ϭ K) and the repetition of R ϭ 6 is applied for the burst transmission, then the number of the allocated slots (Ns) for the burst can be from 60 slots to 65 slots. X output Y output 1 bit delay 1 bit delay 1 bit delay 1 bit delay 1 bit delay 1 bit delay Data in Figure 6.6 Convolutional encoder of rate 1/2. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) 74 WiMAX: Technology for Broadband Wireless Access The binary data that fi ts into a region that is repetition coded is reduced by a factor R com- pared to a nonrepeated region of the slots with the same size and FEC code type. After FEC and bit-interleaving, the data are segmented into slots, and each group of bits designated to fi t in a slot is repeated R times to form R contiguous slots following the normal slot ordering that is used for data mapping. This repetition scheme applies only to QPSK modulation. It can be applied in all coding schemes except HARQ with CTC. 6.3 Turbo Coding Turbo codes are one of the few FEC codes to come close to the Shannon limit, the theoretical limit of the maximum information transfer rate over a noisy channel. The turbo codes were proposed by Berrou and Glavieux (from ENST Bretagne, France) in 1993. The main feature of turbo codes that make them different from the traditional FEC codes are the use of two error-correcting codes and an interleaver. Decoding is then made iteratively taking advantage of the two sources of information. Data transmission is coded as follows (see Figure 6.7). Three blocks of bits are sent. The fi rst block is the m-bit block of uncoded data. The second block is n͞2 parity bits added in sequence for the payload data, computed using a convolutional code. The third subblock is another n͞2 parity bits added in sequence for a known permutation of the payload data, also computed using a convolutional code. Hence, two different redundant blocks of parity bits are added to the sent payload. The complete block has m ϩ n bits of data with a code rate of m͞(m ϩ n), as shown in the fi gure. The data decoding process is the major innovation of turbo codes. The likelihood is used in order to take advantage of the differences between the two decoders. The turbo code inven- tors like to make the parallel with solving crosswords through both vertical and horizontal approaches. Each of the two convolutional decoders generates an hypothesis, with derived likelihoods, for the m-bits sequence, called the a posteriori probability (APP). The hypothesis and the received sequence (recalculated) parity bits are compared and, if they differ, the decoder exchanges the derived likelihoods it has for each bit in the hypotheses. An iterative process is run until the two convolutional decoders come up with the same hypothesis for the m-bits sequence. The number of steps is usually of the order of 10. Coded Sequence (m+n bits) Convolutional Code 1 Convolutional Code 2 Data parity bits n/2 parity bits n/2 e r ut c nuP ).tp o ( r e x e l pitluM Interleaver m Figure 6.7 Turbo coded sequence generation The Physical Layer of WiMAX 75 6.3.1 Convolutional Turbo Codes (CTC) Different classes of turbo codes exist. Convolutional Turbo Codes (CTC) are defi ned as op- tional FEC for OFDM and OFDMA PHY. For OFDMA PHY, the CTC can be used for the support of the optional Hybrid ARQ (HARQ, see Chapter 8). According to mobile WiMAX profi les, the CTC is mandatory for OFDMA PHY. A brief overview of the CTC defi ned for OFDMA PHY is proposed here. The CTC encoder, including its constituent encoder, is depicted in Figure 6.8. It uses a double binary Circular Recursive Systematic Convolutional Code. The bits of the data to be encoded are alternatively fed to A and B, starting with the MSB of the fi rst byte being fed to A. The encoder is fed by blocks of k bits or N couples (k ϭ 2N bits). For all the frame sizes, k is a multiple of 8 and N is a multiple of 4. Further, N is limited to 8 Յ N͞4 Յ 1024. A CTC interleaver Constituent encoder C1 1 2 switch Y1W1 Y2W2 Systematic par t C2 B A S1 S2 Constituent encoder Parity part 1 2 S3 B Figure 6.8 OFDMA PHY Convolutional Turbo Code (CTC) encoder. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) 76 WiMAX: Technology for Broadband Wireless Access The encoding block size depends on the number of subchannels allocated and the modula- tion specifi ed for the transmission. Concatenation of a number of subchannels must be per- formed in order to make larger blocks of coding where it is possible, with the limitation of not passing the largest block under the same coding rate. The concatenation rule should not be used when using HARQ. A table providing the number of subchannels concatenated as a function of the number of subchannels is given in the standard. Figure 6.9 shows a block diagram of CTC subpacket generation. The CTC encoded code- word with a coding rate of 1͞3 goes through the interleaving block and puncturing is per- formed. FEC structures proposed in the standard [1] puncture the mother codeword to gener- ate a subpacket with various coding rates: 1͞2, 2͞3, 3͞4 and 5͞6. The subpacket may also be used as HARQ packet generation (with different SPIDs). The length of the subpacket is chosen according to the needed coding rate, refl ecting the channel condition (this is link adaptation). 6.3.2 Block Turbo Codes (BTC) Block Turbo Codes (BTC) are defi ned as an optional FEC for OFDM and OFDMA PHY. The BTC is also optional in WiMAX profi les. For OFDM and OFDMA PHY, the BTC is based on the product of two simple component codes, which are binary extended Hamming codes or parity check codes. The codes are not the same for the two PHYs. BTC component codes of OFDM are shown in Table 6.1. The Table 6.1 BTC component codes of OFDM PHY. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved) Component code (n,k)Code type (64,57) Extended Hamming code (32,26) Extended Hamming code (16,11) Extended Hamming code (64,63) Parity check code (32,31) Parity check code (16,15) Parity check code (8,7) Parity check code Figure 6.9 Block diagram of subpacket generation. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) 1/3 CTC Encoder Interleaver Puncturing block The Physical Layer of WiMAX 77 component codes are used in a two-dimensional matrix form, which is depicted in Figure 6.10. The k x information bits in the rows are encoded into n x bits by using the component block (n x , k x ) code specifi ed in the standards for the respective composite code. After encod- ing the rows, the columns are encoded using a block code (n y , k y ), where the check bits of the fi rst code are also encoded. The overall block size of such a product code is n ϭ n x n y , the total number of information bits k ϭ k x k y and the code rate is R ϭ R x R y , where R i ϭ k i /n i , i ϭ x, y. Data bit ordering for the composite BTC matrix is defi ned such that the fi rst bit in the fi rst row is the LSB (Least Signifi cant Byte) and the last data bit in the last data row is the MSB. To match a required packet size, BTCs may be shortened by removing symbols from the BTC array. In the two-dimensional case, rows, columns, or parts thereof, can be removed until the appropriate size is reached. 6.4 Transmission Convergence Sublayer (TCS) The Transmission Convergence Sublayer (TCS) is defi ned in the OFDM PHY Layer and the Non-WiMAX SC PHY Layer. The TCS is located between the MAC and PHY Layers. If the TCS is enabled, the TCS converts MAC PDUs of variable size into proper-length FEC blocks, called TC PDU. An illustration of a TC PDU is shown in Figure 6.11. A pointer byte is added at the beginning of each TC PDU, as illustrated in the fi gure. This pointer indicates the header of the fi rst MAC PDU. The TCS is an optional mechanism for the OFDM PHY. It can be enabled on a pre- burst basis for both the uplink and downlink through the burst profi le defi nitions in the uplink and downlink channel descriptor (UCD and DCD) messages respectively. The TCS_ENABLE parameter is coded as a TLV tuple in the DCD and UCD burst profi le encodings (see Chapters 8 and 9 for TLV and UCD/DCD). At SS initialisation, the TCS capability is negotiated between the BS and SS through SBC-REQ/SBC-RSP MAC mes- sages as an OFDM PHY specifi c parameter. The TCS is not included in the OFDMA PHY Layer. Figure 6.10 BTC and shortened BTC structure. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) Information bits s k c ehC Checks checks on checks n x n y k x k y 78 WiMAX: Technology for Broadband Wireless Access 6.5 Burst Profi le The burst profi le is a basic tool in the 802.16 standard MAC Layer. The burst profi le alloca- tion, which changes dynamically and possibly very fast, is about physical transmission. Here the parameters of the burst profi les of WiMAX are summarised. The burst profi les are used for the link adaptation procedure. The use of burst profi les and the link adaptation procedure will be seen in more detail in Chapters 9 and 10. 6.5.1 Downlink Burst Profi le Parameters The burst profi le parameters of a downlink transmission for OFDM and OFDMA PHYsical layers are proposed in Table 6.2. The parameter called FEC code is in fact the Modulation and Coding Scheme (MCS). For OFDM PHY, there are 20 MCS combinations of modula- tion (BPSK, QPSK, 16-QAM or 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 effi cient (and then less robust) MCS Figure 6.11 Format of the downlink Transmission Convergence sublayer PDU. (From IEEE Std 802.16-2004 [1]. Copyright IEEE 2004, IEEE. All rights reserved.) Transmission Convergence sublayer (TC) PDU P MAC PDU that has started in the previous TC packet First MAC PDU that starts in this TC packet Second MAC PDU that starts in this TC packet P = 1 Byte pointer field Table 6.2 Downlink burst profi le parameters for OFDM and OFDMA PHYsical layers Burst profi le parameter Description Frequency (in kHz) Downlink frequency FEC code type Modulation and Coding Scheme (MCS); there are 20 MCSs in OFDM PHY and 34 MCSs in OFDMA PHY (as updated in 802.16e) DIUC mandatory exit threshold The CINR at or below where this burst profi le can no longer be used and where a change to a more robust (but also less frequency-use effi cient) burst profi le is required. Expressed in 0.25 dB units. See Chapter 9 for DIUC DIUC minimum entry threshold The minimum CINR required to start using this burst profi le when changing from a more robust burst profi le. Expressed in 0.25 dB units TCS_ enable (OFDM PHY only) Enables or disables TCS [...]... 266 (From IEEE Std 802.16-20 04 [1] Copyright IEEE 20 04, IEEE All rights reserved.) Modulation BPSK QPSK QPSK QAM-16 QAM-16 QAM- 64 QAM- 64 Coding rate Receiver SNR threshold (dB) 1/2 1/2 3 /4 1/2 3 /4 1/2 3 /4 6 .4 9 .4 11.2 16 .4 18.2 22.7 24. 4 80 WiMAX: Technology for Broadband Wireless Access adaptation procedure In the following chapters the MAC procedures that can be used for the implementation of link... any transformation or mapping of external network data received through the CS Service Access Point (SAP) into MAC SDUs received by the MAC Common Part Sublayer (CPS) through the MAC SAP (see Figure 7.1) This includes classifying external WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd ISBN: 0 -47 0-02808 -4 84 WiMAX: Technology for Broadband Wireless Access External... address is used When authorised to access, a candidate SS node receives a 16-bit Node IDentifier (Node ID) upon a request to an SS identified as the Mesh BS The Node ID is the basis of node identification in the Mesh mode WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd ISBN: 0 -47 0-02808 -4 96 WiMAX: Technology for Broadband Wireless Access MAC Header (6 bytes) Payload... obtained from Table 6.5, proposed in the standard [1] for some test conditions These SNR thresholds are for a BER, Bit-Error Rate, measured after the FEC, that is smaller than 10 –6 Part Three WiMAX Multiple Access (MAC Layer) and QoS Management WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd ISBN: 0 -47 0-02808 -4 7 Convergence Sublayer (CS) 7.1 CS in 802.16 Protocol... Copyright IEEE 20 04, IEEE All rights reserved.) 94 WiMAX: Technology for Broadband Wireless Access 7.5.3 Header Compression in WiMAX The PHS is a header suppression mechanism There are also header compression algorithms that compress packet headers by other means than repetitive header suppression The 802.16e amendment mentions that the Convergence Sublayer (CS) supports SDUs in two formats that facilitate... are now given 90 WiMAX: Technology for Broadband Wireless Access 7 .4 CS and QoS During the creation of a service flow, the CS specification that the connection being set up will use is defined Possible choices of CS specification are No CS, Packet IPv4, Packet IPv6, Packet 802.3/Ethernet, Packet 802.1Q VLAN, Packet IPv4 over 802.3/Ethernet, Packet IPv6 over 802.3/ Ethernet, Packet IPv4 over 802.1Q VLAN,... Figure 7 .4 shows the possible transitions between these different service flows A BS may choose to activate a provisioned service flow directly, or may choose to take the path to active service flows by passing the 88 WiMAX: Technology for Broadband Wireless Access Provisioned service flows (Initial state) Admitted service flows Active service flows (Intermediate state) (Final state) Figure 7 .4 Possible... SS may request uplink bandwidth allocations and the expected behaviour 86 WiMAX: Technology for Broadband Wireless Access Table 7.1 CID ranges as defined in Reference [1] Values are between 0000 (the 16 bits are equal to zero) and FFFF (the 16 bits are equal to one) It seems probable that the BS decides for a number m of CIDs for each of the basic and primary management connections that may be requested,... DL-MAP to denote bursts for transmission of downlink broadcast information to normal mode SS Used in DL-MAP to denote bursts for transmission of downlink broadcast information to sleep mode SS May also be used in MOB_TRF-IND messages Used in DL-MAP to denote bursts for transmission of downlink broadcast information to idle mode SS May also be used in MOB_PAG-ADV messages Used by the BS for transmission of... header before properly using the received packet (Figure 7.8) Receiving entity MAC layer Layer 3 (ATM or IP) … CS sublayer Payload PHS … CPS sublayer Payload header * Payload … Privacy sublayer PHYsical layer Air interface Figure 7.8 Header suppression mechanism at the receiving entity The receiver has to restore the header before properly using the received packet 92 WiMAX: Technology for Broadband Wireless . classifying external WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd. ISBN: 0 -47 0-02808 -4 84 WiMAX: Technology for Broadband Wireless Access network Service. (see Chapter 8). WiMAX: Technology for Broadband Wireless Access Loutfi Nuaymi © 2007 John Wiley & Sons, Ltd. ISBN: 0 -47 0-02808 -4 70 WiMAX: Technology for Broadband Wireless Access 6.2.1 Randomisation Randomisation. encoder of rate 1/2. (From IEEE Std 802.16-20 04 [1]. Copyright IEEE 20 04, IEEE. All rights reserved.) 74 WiMAX: Technology for Broadband Wireless Access The binary data that fi ts into a region