Doc.: IEEE 802.22-05/0093r0
IEEE P802.22
Wireless RANs
|System description and operation principles for IEEE 802.22 WRANs |
|Date: 2005-11-07 |
|Author(s): |
|Name |Company |Address |Phone |email |
|Ying-Chang Liang |Institute for Infocomm |21 Heng Mui Keng Terrace, |65-68748225 |ycliang@i2r.a-star.edu.sg |
| |Research (I2R) |Singapore 119613 | | |
|Wing Seng Leon | | | |wsleon@i2r.a-star.edu.sg |
|Yonghong Zeng | | | |yhzeng@i2r.a-star.edu.sg |
|Changlong Xu | | | |clxu@i2r.a-star.edu.sg |
|Ashok Kumar Marath | | | |ashok@i2r.a-star.edu.sg |
|Anh Tuan Hoang | | | |athoang@i2r.a-star.edu.sg |
|Francois Chin | | | |chinfrancois@i2r.a-star.edu.sg |
|Lei Zhongding Lei | | | |leizd@i2r.a-star.edu.sg |
Table of Contents
List of Tables 4
List of Figures 5
Abbreviations and Acronyms 6
1 Introduction 8
2 Two-layer OFDMA 8
3 System design 10
3.1 Scalable design to support 1.25, 2.5, 5 to 7.5 MHz bandwidths 10
3.2 Scalable design to support 6, 7 and 8MHz TV channels 11
3.2.1 Option A: Fixed sampling frequency for different TV bandwidths 11
3.2.2 Option B: Variable sampling rate for different TV bandwidths 12
4 Transmitter structures 12
4.1 Randomizer 14
4.2 FEC encoder 15
4.2.1 Convolutional codes (CC) 15
4.2.2 Shortened block turbo codes (SBTC) 17
4.3 Interleaver 22
4.4 Modulations 23
5 Pre-transforms 25
6 TDD as the duplex mode and channel sensing 26
6.1 Adaptive guard time control for BS 29
6.2 Frame structure for option B 31
7 Preamble and pilot structures 34
7.1 Downlink preamble 34
7.2 Uplink preamble 34
7.3 Downlink pilot 35
7.4 Uplink pilot 36
7.5 Preamble for multiple antennas 37
8 Ranging 37
9 Multiple antenna technologies 37
9.1 Transmit diversity schemes 38
9.1.1 Cyclic delay transmission (CDT) 38
9.1.2 Space-frequency coding (SFC) 40
9.1.3 Switched beam combined with CDT / Space time block coding (STBC) 41
9.2 Adaptive antennas 42
9.2.1 Interference avoidance 43
9.2.1.1 Interference avoidance for downlink 43
9.2.1.2 Interference avoidance for downlink 44
9.2.1.2.1 CPE with single transmit antenna 44
9.2.1.2.2 CPE with multiple transmit antennas 44
9.2.2 Delay spread reduction 44
9.2.2.1 Basic transmit beamforming (BTB) 45
9.2.2.2 Advanced transmit beamforming (ATB) 45
9.2.2.3 ATB for systems with repeaters 48
9.3 Virtual multiple antenna system 48
10 Sectorization 48
10.1 Transmitter structure for CDT 50
10.2 Preamble and pilot design for sectorization 52
11 Cellular deployment structure 54
12 Multiuser diversity and scheduling 55
13 Random beamforming 55
14 Adaptive modulation and coding selection (MCS) and transmit power control 57
14.1 Receiver requirement for different data rates 58
14.2 Transmit power control (TPC) 59
14.2.1 Range and granularity 59
14.2.2 TPC mechanisms 59
References 60
List of Tables
Table 1: Parameters used to define an OFDMA symbol 8
Table 2: System parameters for a CPE with an oversampling factor of 8/7 10
Table 3: Variable CP lengths for OFDMA 10
Table 4: Minimum peak rates for mandatory mode 11
Table 5: Parameter reconfiguration for a CPE to support variable TV bandwidths 11
Table 6: Variable sampling rate supporting variable TV bandwidths 12
Table 7: Detailed parameters for variable TV bandwidths 13
Table 8: The puncturing patterns for different code rates 16
Table 9: Data payload for a subchannel 17
Table 10: Parity check matrix for the Hamming codes in OFDMA 18
Table 11: Possible data payload for one subchannel 22
Table 12: Code parameters for different coded block sizes 22
Table 13: Downlink burst specification. 28
Table 14: Pilot subcarrier locations for N=256. 35
Table 15: Pilot subcarrier locations for N=512. 36
Table 16: Space-frequency coding for two transmit antenna case 40
Table 17: Maximum access delay for four channel models proposed for WRAN 44
Table 18: Downlink data rate for different modulation/coding schemes (MCSs) and CP factors 57
Table 19: Uplink data rate for different modulation/coding schemes (MCSs) and CP factors 58
Table 20: Receiver SNR assumptions (CC used, for BER = 10-6) 58
Table 21: Receiver minimum input level sensitivity (dBm) 59
|Table 1: Parameters used to define an OFDMA symbol…………………………............. |8 |
|Table 2: System parameters for a CPE with an oversampling factor of 8/7……………… |10 |
|Table 3: Variable CP lengths for OFDMA……………………………………..………….. |10 |
|Table 4: Minimum peak rates for mandatory mode…………………………..……………. |11 |
|Table 5: Parameter reconfiguration for a CPE to support variable TV bandwidths ………...……………………………………………………………….. |11 |
| |11 |
|Table 6: Variable sampling rate supporting variable TV bandwidths ……….…………… |12 |
|Table 7: Detailed parameters for variable TV bandwidths…………………...................... |12 |
|Table 8: The puncturing patterns for different code rates………………………………… |16 |
|Table 9: Data payload for a subchannel………………………………………..…………... |16 |
|Table 10: Parity check matrix for the Hamming codes in OFDMA………….…………… |17 |
|Table 11: Possible data payload for one subchannel …………………………..………….. |20 |
|Table 12: Optional channel coding per modulation…………………………..................... |20 |
|Table 13: Downlinkg burst specification…………………………………………………... |25 |
|Table 14: Pilot subcarrier locations for N=256………………………………………........ |32 |
|Table 15: Pilot subcarrier locations for N=768………………………………………….... |32 |
|Table 16: Space-frequency coding for two transmit antenna case ………………………. |37 |
|Table 17: Maximum access delay for four channel models proposed for WRAN………..……………………………………………………………………………. | |
| |40 |
|Table 18: Downlink data rate for different modulation/coding schemes (MCSs) and CP |53 |
|factors…………………………………………………………..…………………………... | |
|Table 19: Uplink data rate for different modulation/coding schemes (MCSs) and CP |54 |
|factors……………………………………………………………………………………... | |
|Table 20: Receiver SNR assumptions (CC used, for BER = 10-6) …………….…………. |54 |
|Table 21: Receiver minimum input level sensitivity (dBm)………………….................... |55 |
List of Figures
Figure 1: Time domain representation for one OFDMA symbol 9
Figure 2: Example of an OFDMA symbol with localized subchannels and guard bands 9
Figure 3: Block diagram for downlink transmitter at BS 13
Figure 4: Block diagram for uplink transmitter at CPE 14
Figure 5: Randomizer 15
Figure 6: OFDMA randomizer initial sequence 15
Figure 7: Convolutional encoder of rate ½ 16
Figure 8: Block turbo code (BTC) structure 20
Figure 9: Shortened BTC (SBTC) structure 21
Figure 10: BPSK, QPSK, 8PSK, 16-QAM constellations 24
Figure 11: 64-QAM constellation 24
Figure 12: 256 QAM constellation 25
Figure 13: TDD frame structure 27
Figure 14: Adaptive TDD with channel sensing 29
Figure 15: TDD frame structure with adaptive guard time 30
Figure 16: Uplink bursts received in the BS 31
Figure 17: TDD and sensing frame structure: FFT = 1024, Pattern 1 32
Figure 18: TDD and sensing frame structure: FFT = 1024, Pattern 2 32
Figure 19: TDD and sensing frame structure: FFT = 2048 33
Figure 20: Sensing symbol patterns 33
Figure 21: Uplink pilot pattern 36
Figure 22: Calculating the propagation delay 37
Figure 23: Block diagram of CDT with 2 transmit antennas 39
Figure 24: Transmission model for CDT 39
Figure 25: Equivalence of the composite channel 40
Figure 26: Block diagram of SFC transmitter with 2 transmit antennas 41
Figure 27: Combined beamforming and transmit diversity 41
Figure 28: Transmit beamforming for interference avoidance and frequency reuse 43
Figure 29: Downlink transmitter block diagram for BTB with NT antennas 45
Figure 30: Downlink transmitter block diagram for ATB with 2 beamformers per user 47
Figure 31: Channel lengthening and shortening using ATB 47
Figure 32: Virtual multiple antenna system for uplink transmission 48
Figure 33: An example of sectorization by dividing one cell into three sectors 49
Figure 34: Inter-sector diversity 49
Figure 35: CDT for Preamble/pilot channel and sector edge users: Time domain implementation 50
Figure 36: CDT for Preamble/pilot channel and sector edge users: Frequency domain implementation 51
Figure 37: CDT for Preamble/pilot channel and sector edge users: Frequency domain implementation using different scrambling codes 52
Figure 38: Preamble/Pilot patterns for three sectors within the same cell 53
Figure 39: Scattered pilot patterns for three sectors within the same cell 53
Figure 40: Scrambling code generation for the three sectors within the same cell 54
Figure 41: Cellular deployment structure of WRANs 55
Figure 42: Procedure of random transmit beamforming for MIMO scenario 56
|Figure 1: Time domain representation for one OFDMA symbol………………………… |9 |
|Figure 2: Example of an OFDMA symbol with localized subchannelization and guard |9 |
|bands………………………………………………………………….…………………… | |
|Figure 3: Block diagram for downlink transmitter at BS……………………..…………… |13 |
|Figure 4: Block diagram for uplink transmitter at CPE…………………………………... |13 |
|Figure 5: Randomizer…………………………………………………………………….. |14 |
|Figure 6: OFDMA randomizer initial sequence………………………………..………….. |15 |
|Figure 7: Convolutional encoder of rate ½……………………………………..………….. |15 |
|Figure 8: Block turbo code (BTC) structure………………………………….................... |18 |
|Figure 9: Shortened BTC (SBTC) structure………………………………….................... |19 |
|Figure 10: BPSK, QPSK, 8PSK, 16-QAM constellations……………………..………….. |21 |
|Figure 11: 64-QAM constellation……………………………………………….………… |22 |
|Figure 12: 256-QAM constellation…………………………………………….................. |22 |
|Figure 13: TDD Frame Structure……………………………………………..................... |24 |
|Figure 14: Adaptive TDD with channel sensing…………………………………………. |26 |
|Figure 15: TDD frame structure with adaptive guard time………………………………. |27 |
|Figure 16: Uplink bursts received in the BS……………………………………………… |28 |
|Figure 17: TDD and sensing frame structure: FFT = 1024, Pattern 1……………………. |29 |
|Figure 18: TDD frame structure: FFT = 1024, Pattern 2…………………….……………. |29 |
|Figure 19: TDD frame structure: FFT = 2048………………………………..................... |30 |
|Figure 20: Sensing symbol pattern……………………………………………………….. |30 |
|Figure 21: Uplink pilot pattern………………………………………………………….... |33 |
|Figure 22: Calculating the propagation delay ……………………………………………. |34 |
|Figure 23: Block diagram of CDT with 2 transmit antennas…………………………….. |35 |
|Figure 24: Transmission model for CDT……………………………………….………… |36 |
|Figure 25: Equivalence of the composite channel……………………………..………….. |36 |
|Figure 26: Block diagram of SFC transmitter with 2 transmit antennas………………… |37 |
|Figure 27: Combined beamforming and transmit diversity…………………..……………. |37 |
|Figure 28: Transmit beamforming for interference avoidance and frequency reuse …….…………………………………………………………………………. |39 |
|Figure 29: Downlink transmitter block diagram for BTB with NT antennas…….............. |41 |
|Figure 30: Downlink transmitter block diagram for ATB with 2 beamformers per user. ..………………………………………………………………………………… |43 |
| |43 |
|Figure 31: Channel lengthening and shortening using ATB…………………………….. |43 |
|Figure 32: Virtual multiple antenna system for uplink transmission…………………….. |44 |
|Figure 33: An example of sectorization by dividing one cell into three sectors ..………… |45 |
|Figure 34: Inter-sector diversity ………………………………………………................. |45 |
|Figure 35: CDT for Preamble/pilot channel and sector edge users: Time domain |46 |
|implementation…………………………………………………………..………………… | |
|Figure 36: CDT for Preamble/pilot channel and sector edge users: Frequency domain |47 |
|implementation…………………………………………………………………………… | |
|Figure 37: CDT for Preamble/pilot channel and sector edge users: Frequency domain implementation using different |48 |
|scrambling codes………..………………………………... | |
|Figure 38: Preamble/Pilot patterns for three sectors within the same cell……………….. |49 |
|Figure 39: Scattered pilot patterns for three sectors within the same cell………………... |49 |
|Figure 40: Scrambling code generation for three sectors with the same cell ……………. |50 |
|Figure 41: Cellular deployment structure of WRANs…………………………………..…. |51 |
|Figure 42: Procedure of random transmit beamforming for MIMO scenario……………. |52 |
Abbreviations and Acronyms
|Term |Description |
|AMC |Adaptive modulation and coding |
|AST |Allocation start time |
|ATB |Advanced transmit beamforming |
|BER |Bit-error rate |
|BPSK |Binary phase shift keying |
|BS |Base station |
|BTB |Basic transmit beamforming |
|BTC |Block turbo code |
|CC |Convolutional code |
|CDT |Cyclic delay transmission |
|CID |Connection identifier |
|CINR |Carrier-to-interference-plus-noise ratio |
|CP |Cyclic prefix |
|CPE |Customer premise equipment |
|CQI |Channel quality information |
|DIUC |Downlink interval usage code |
|DL |Downlink |
|DLFP |Downlink frame prefix |
|DL-MAP |Downlink map |
|DOA |Direction of arrival |
|FCH |Frame control header |
|FEC |Forward error correction |
|FFT |Fast Fourier transform |
|IBI |Inter-block interference |
|LSB |Least significant bit |
|MCS |Modulation and coding scheme |
|MIMO |Multiple input multiple output |
|MSB |Most significant bit |
|OFDM |Orthogonal frequency division multiplexing |
|OFDMA |Orthogonal frequency division multiple access |
|PAPR |Peak-to-average power ratio |
|PD |Propagation delay |
|PN |Pseudo noise |
|PRBS |Pseudo random binary sequence |
|PSK |Phase shift keying |
|QAM |Quadrature amplitude modulation |
|QoS |Quality of service |
|QPSK |Quadraturertenary phase shift keying |
|RTD |Round trip delay |
|SBTC |Shortened block turbo code |
|SFC |Space frequency coding |
|SINR |Signal-to-interference-plus-noise ratio |
|SNR |Signal-to-noise ratio |
|STBC |Space time block coding |
|TDD |Time division duplex |
|TPC |Transmit power control |
|TTG |Transmit/receive transition gap |
|UIUC |Uplink interval usage code |
|UL |Uplink |
|UL-MAP |Uplink map |
|WRAN |Wireless regional area network |
1. Introduction
The IEEE 802.22 WRAN operates in the VHF/UHF TV bands using cognitive radio technology. It coexists with other license-exempt devices, such as wireless microphones, which are called primary users here. The proposal documents the physical (PHY) layer specifications and operation principles for IEEE 802.22 WRANs, with key attributes as follows:
• OFDMA as the multiple-access scheme for both uplink and downlink, with pre-transform for uplink to reduce the peak-to-average power ratio (PAPR)
• TDD as the duplex mode, with adaptive guard time control to maximize the system throughput
• The CPEs support the usage of single TV channel with variable channel bandwidth uptoup to 8MHz; the base station (BS) supports the usage of multiple TV channels, either contiguous or discontiguous
• Scalable bandwidth ranging from 1.25 MHz to 7.5 MHz for each CPE
• Preamble and pilot design to avoid interference to primary users
• Shortened block Turbo codes (SBTC) with special parity check matrix design
• Supporting transmit power control (TPC) and adaptive modulation and coding (AMC)
• Adaptive antennas for interference avoidance, range extension and delay spread reduction
• Sectorization for enhanced channel capacity
• Distributed channel sensing using guard interval between downlink subframe and uplink subframe
2. Two-lLayer OFDMA
The uplink (from CPE to BS) and downlink (from BS to CPE) use two layer orthogonal frequency division multiple access (OFDMA) as the modulation scheme. The first layer is referred to as inter-frequency band multiple access. The basic frequency division unit is one TV channel or frequency band with BW = 6, 7, or 8 MHz. When multiple frequency bands (n*BW) are available, they bands may can be allocated to different users. It is preferable that each user is allocated with one TV channel. However, it is optional that each user may occupy multiple bands can also be allocated to a single user to achieve higher data rate (frequency band multiplexing) or better link performance (frequency band diversity). In this case, frequency band hopping is not allowed since band hopping may cause difficulties with dynamic change of available bands and significant sensing overheads. The second layer is referred to multiple access within one TVfrequency band, i.e, one frequency band may be shared by multiple users.
Table 1: Parameters used to define an OFDMA symbol
Table 1: Parameters used to define an OFDMA symbol
|Parameter |Description |
|B |ChanelChannel bandwidth |
|N |FFT size |
|[pic] |Number of used subcarriers, including DC |
| |subcarrier |
|[pic] |Oversampling factor |
|[pic] |Subcarrier sapcingspacing |
|Tb |Useful symbol duration |
|Tc |Cyclic prefix duration |
|Ts =Tc+Tb |OFDMA symbol duration |
|[pic] |Cyclic prefix factor |
Table 1 shows the parameters used to define one OFDMA symbol. The time domain OFDMA signal with duration Tb is generated by an inverse FFT of the frequency domain OFDMA symbol. To prevent interblock inteferenceinterference (IBI) and to maintain orthogonality among the subcarriers, a cyclic prefix (CP), which is a copy of the tail end of the OFDMA symbol of duration Tc, is appended to the beginning of the symbol. Figure 1 illustrates the time domain representation of the CP extended OFDMA symbol.
For each OFDMA symbol, the transmitted signal can be represented as
[pic],
for [pic], where [pic] is the modulated symbol allocated to the kth subcarrier, [pic]is the carrier frequency, and the DC subcarrier (k = 0) is not used.
[pic]
Figure 1: Time domain representation for one OFDMA symbolFigure 1: Time domain representation for one OFDMA symbol
In the frequency domain, the useful subcarriers of a given OFDMA symbol are divided into groups of subchannels. Subcarriers belonging to a given subchannel may or may not be adjacent to one another. When the subchannel contains contiguous subcarriers, this subchannel is called localized subchannel (see Figure 2). Otherwise, it is referred to as distributed subchannel. The distribution of subcarriers of a given subchannel over the entire transmission bandwidth achieves frequency diversity for the transmission. When a primary user is active and if the bandwidth of the primary user consistsit occupies the bandwidth of of multiple subcarriers, the use of distributed subchannel allocation would affect multiple subchannels. If the number of affected subcarriers is small, channel coding can be used to recover the transmitted bits.
[pic]
Figure 2: Example of an OFDMA symbol with localized subchannels and guard bands
Figure 2: Example of an OFDMA symbol with localized subchannels and guard bands
3. System dDesign
Scalability is athe fundamental feature in the proposed system design of the proposal. Each CPE operates within one TV channel, while the BS can operate over multiple TV channels. The BS will decide which TV channel and which subchannel(s) of that TV channel each CPE will be allocated to.
3.1 Scalable design to support bandwidths from 1.25, 2.5, 5 to 7.5 MHz bandwidths
Table 2 shows the system parameters for CPEs operating in channel bandwidths of 1.25, 2.5, 5 and 7.5 MHz with an oversampling factor of 8/7. The subcarrier spacing is 5.5804 kHzthe same for all cases.
Table 2: System parameters for a CPE with an oversampling factor of 8/7
Table 2: System parameters for a CPE with an oversampling factor of 8/7
|Parameters |Values |
|Channel bandwidth |1.25 MHz |
|Useful OFDMA symbol |179.2 μs |
|interval | |
The CP insertion reduces the system throughput, thus variable CP length is designed to support different propagation conditions, in order to minimize the throughput loss. Table 3 illustrates four different CP factors supportable by the proposed WRAN. Note CP fact of 3/8 is used to take care of systems with repeaters.
Table 3: Table 3: Variable CP lengths for OFDMA
Variable CP lengths for OFDMA
|CP factor |1/16 |1/8 |
|Channel bandwidth |1.25 MHz |
|Total number of subcarriers (N) |256 |
|Number of used subcarriers ([pic]) |209 |
|Number of subchannels |4 |
|Number of data subcarriers per subchannel |48 |
|Number of pilot subcarriers per subchannel |4 |
|Number of subchannels per user |4 |2 |
|Minimum peak rates |1.513 Mbps |504 kbps |
| |(QPSK, ¾ coding rate) |(QPSK, ½ coding rate) |
3.2 Scalable design to supporting 6, 7 and 8MHz TV channels
Two options are proposed to adapt to different TV channel bandwidths of 6, 7 and 8 MHz. Option A is based on fixed sampling frequency and variable number of useful subcarriers. Option B is based on variable sampling frequency and fixed number of useful subcarriers.
3.2.1 Option A: Fixed sampling frequency for different TV bandwidths
Using the sampling frequency corresponding to channel bandwidth of 7.5 MHz, as shown in Table 5, the proposal supports TV channels with a bandwidth of 6, 7 or 8 MHz in a scalable manner.
Table 5: Table 5: Parameter reconfiguration for a CPE to support variable TV bandwidths
Parameter reconfiguration for a CPE to support variable TV bandwidths
|TV channel bandwidth ([pic]) |8 MHz |7 MHz |6 MHz |
|Sampling frequency |8.5714 MHz |
|FFT size |1536 |
|Number of useful subcarriers |1249 |1145 |937 |
|([pic]) | | | |
|Spectrum efficiency | | | |
|[pic] |87.2% |91.4% |87.2% |
|Number of subchannels |24 |22 |18 |
3.2.2 Option B: Variable sampling rate for different TV bandwidths
In this option, a variable sampling rate is employed for different TV channel with BW = 6, 7, or 8 MHz. Different sampling rates give rise to different carrier spacings, symbol lengths, and useful bit rates. However, the same frame structure, FFT size, guard interval, rules for coding, mapping, interleaving, and AMC are kept for different BW. The basic clock frequency is 8/7 MHz. There are two choices of FFT size for this option and there are three choice of CP length approximately ¼, 1/8, and 1/16 of symbol duration. The basic parameters supporting variable TV bandwidth is listed in Table 6.
Table 6: Variable sampling rate supporting variable TV bandwidths
Table 6: Variable sampling rate supporting variable TV bandwidths
|TV channel bandwidth |8 MHz |
|Number of useful subcarriers |864 / 1728 |
|CP length |(28 / 14 / 7 us) / (56 / 28 / 14 us) |
|Spectrum efficiency |78% |
|(With ~1/16 CP factor) | |
|Number of subchannels |27 (32 / 64 subcarriers per subchannels) |
Table 7 shows the detailed parameters for variable TV bandwidth, including subcarrier spacing, sampling frequency, number of occupied subcarriers, CP length, symbol duration, frame duration, symbol rate, bandwidth efficiency, etc. It should be noted that the subcarrier spacing is around 4 kHz which does not require expensive oscillaters.. The maximum CP length is 56 μs to combat large delay spread experienced in the WRAN channel.
4. Transmitter structures
Figure 3 shows the block diagram of downlink transmitter for BS. The data bits from MAC layer is first randomized, then passed through the FEC encoder, followed by the bit interleaver, then by the symbol mapper. The mapped symbols, together with preamble and pilots symbols, and mapped symbols from other users, are then passed to the OFDMA formulator, the outputs of which are then passed to the windowing and pulse shaping operation, and finally transmitted out through out the transmit antenna. When multiple antennas are equipped, the transmitter block diagram will be modified to incorporate various transmit processing, such as transmit beamforming and space time coding.
Table 6: Variable sampling rate supporting variable TV bandwidths
|TV channel bandwidth |8 MHz |7 MHz |6 MHz |
|Sampling Frequency |8/7*8 = |8/7*7 = |8/7*6 = |
| |9.14MHz |8 MHz |6.86 MHz |
|FFT size |1024 / 2048 |
|Number of useful subcarriers |864 / 1728 |
|CP length |(28 / 14 / 7 us) / (56 / 28 / 14 us) |
|Spectrum efficiency |78% |
|(With ~1/16 CP factor) | |
|Number of subchannels |27 (32 / 64 subcarriers per subchannels) |
Table 7 shows the detailed parameters for variable TV bandwidth, including subcarrier spacing, sampling frequency, number of occupied subcarriers, CP length, symbol duration, frame duration, symbol rate, bandwidth efficiency, etc. It should be noted that the subcarrier spacing is around 4 kHz which does not require expensive oscillaters. The maximum CP length is 56 μs to combat large delay spread experienced in the WRAN channel.
Table 7: Table 7: Detailed parameters for variable TV bandwidths
Detailed parameters for variable TV bandwidths
4. Transmitter structures
Figure 3 shows the block diagram of downlink transmitter for BS. The data bits from MAC layer is first randomized, then passed through the FEC encoder, followed by the bit interleaver, then by the symbol mapper. The mapped symbols, together with preamble and pilots symbols, and mapped symbols from other users, are then passed to the OFDMA formulator, the outputs of which are then passed to the windowing and pulse shaping operation, and finally transmitted out through out the transmit antenna. When multiple antennas are equipped, the transmitter block diagram will be modified to incorporate various transmit processing, such as transmit beamforming and space time coding.
[pic][pic]
Figure 3: Block diagram for downlink transmitter at BS
Figure 3: Block diagram for downlink transmitter at BS
[pic]
Figure 4: Block diagram for uplink transmitter at CPEFigure 4: Block diagram for uplink transmitter at CPE
Figure 4 shows the block diagram of uplink transmitter for CPE. The data bits from MAC layer is first randomized, then passed through the FEC encoder, followed by the bit interleaver, then by the symbol mapper. The mapped symbols are then passed to a pre-transformer, the output of which, together with preamble and pilots symbols, are then passed to the OFDMA formulator, the outputs of which are then passed to the windowing and pulse shaping operation, and finally transmitted out through out the transmit antenna. Typical examples of pre-transforms include the Fourier transform and the Walsh-Hadamard transform. When the transform matrix is an identity matrix, the modulated symbols are just mapped to the subchannels. Again, when multiple antennas are equipped at the CPE, the transmitter block diagram will be modified to incorporate various transmit processing, such as transmit beamforming or space time coding.
4.1 Modulation (symbol mapper) schemes:
Downlink supports the modulation schemes of QPSK, 16QAM, 64 QAM and 256QAM. Uplink supports BPSK, QPSK, 8PSK, 16QAM and 64QAM.
4.12 Randomizer
Prior to FEC encoding, the data of both downlink and uplink will be randomized to ensure adequate bit transitions for supporting clock recovery. The randomizer shall be used independently for each allocation of a data block (subchannels on the frequency domain and OFDM symbols on the time domain). If the amount of data to transmit does not fit exactly the amount of data allocated, a whole 1’s sequence shall be added to the end of the transmission block, up to the amount of data allocated. This randomization is performed by modulo-2 addition the data with a Pseudo Random Binary Sequence (PRBS). The PRBS generator polynomial is 1+X14+X15 and illustrated in Figure 5. The shift-register of the randomizer shall be initialized for each FEC block. Each data byte to be transmitted shall enter sequentially into the randomizer, MSB first. The randomizer sequence is applied only to information bits and preambles are not randomized. The randomizer is initialized for each FEC block with the sequences generated as shown in Figure 6.
[pic]
Figure 5: RandomizerFigure 5: Randomizer
[pic][pic]
Figure 6: OFDMA randomizer initial sequenceFigure 6: OFDMA randomizer initial sequence
4.23 FEC eEncoder
After randomization, the bits shall be applied to the encoder. Two types of FEC encoders are proposed: convolutional codes (CC) and block Turbo codes (BTC).
4.23.1 Convolutional codes (CC)
The basic FEC block consists of a binary convolutional encoder with native rate of ½ and constraint length of 7. The generator polynomials are as follows.
[pic]
The generator is depicted in Figure 7 in detail.
[pic]
Figure 7: Convolutional encoder of rate ½Figure 7: Convolutional encoder of rate ½
Puncturing operation is used for native code to obtain different code rates. The corresponding puncturing patterns and serialization orders for different code rates are defined in Table 8. In this table, “1” means a transmitted bit and “0” means a removed bit, whereas X and Y are referred to Figure 7.
Table 8: Table 8: The puncturing patterns for different code rates
The puncturing patterns for different code rates
| |Code Rates |
|Rate |1/2 |2/3 |3/4 |5/6 |
|dfree |10 |6 |5 |4 |
|X |1 |10 |101 |10101 |
|Y |1 |11 |110 |11010 |
|XY |X1Y1 |X1Y1Y2 |X1Y1Y2X3 |X1Y1Y2X3Y4X5 |
A tail-biting convolutional encoder is used to encode each FEC block. This implies that the memory of the encoder is initialized by the last 6 data bits of the currently encoded FEC block. The basic sizes of the useful data payloads for different modulation type and encoding rate are displayed in Table 9.
Table 9: Table 9: Data payload for a subchannel
Data payload for a subchannel
|Modulation |QPSK |8PSK |16 QAM |64 QAM |Coded Bytes|
|Encoding |1/2 |3/4 |
|rate | | |
|15 |11 |[pic] |
|31 |26 |[pic] |
|63 |57 |[pic] |
With the aid of Figure 8, the procedure to construct product code is listed as follows:
1) Place (ky kx) information bits in information area (the blank area in Figure 8). The information bits may be placed in columns with indexes from 1 to nx-1, except for columns 2i with i = 0, 1, 2, …, nx-kx-2 (nx-kx-1 parity check bits). Similarly, information bits may be located in rows with indexes 1 to ny except for rows with indexes 2j with j = 0, 1, 2, …, ny-ky-2 (ny-ky-1 parity check bits).
2) Compute the parity check bits of ky rows using the corresponding parity check matrix in Table 8 and inserting them in the corresponding positions signed by [pic];
3) Compute the parity check bits of kx columns using the correpondingcorresponding parity check matrix in Table 8 and inserting them in the corresponding positions signed by [pic] and [pic];
4) Calculate and append the extended parity check bits to the corresponding rows and columns.
5) The overall block size of such a product code is n = nx × ny, the total number of information bits k = kx × ky, and the code rate is R = Rx × Ry,, where Ri = ki/ni, i = x, y. The Hamming distance of the product code is d = dx × dy,. Data bit ordering for the composite BTC block is the first bit in the first row is the LSB and the last data bit in the last data row is the MSB.
Transmission of the block over the channel shall occur in a linear fashion, with all bits of the first row transmitted left to right followed by the second row, etc.
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. There are three steps in the process of shortening product codes:
Step 1) Remove Ix rows and Iy columns from the two-dimensional code. This is equivalent to shortening the constituent codes that make up the product code.
Step 2) Remove D individual bits from the first row of the two-dimensional code starting with the LSB.
Step 3) Use if the product code specified from Step 1) and Step 2) of this subclause has a non-integerintegral number of data bytes. In this case, the Q right LSBs are zero-filled by the encoder. After decoding at the receive end, the decoder shall strip off these unused bits and only the specified data payload is passed to the next higher level in the PHY. The same general method is used for shortening the last code word in a message where the available data bytes do not fill the available data bytes in a code block.
These three processes of code shortening are depicted in Figure 9. The new coded block length of the code is (nx – Ix)(ny – Iy) – D. The corresponding information length is given as (kx – Ix)(ky – Iy) – D – Q. Consequently, the code rate is given by the following equation:
[pic]
[pic]
Figure 8: Block turbo code (BTC) structureFigure 8: Block turbo code (BTC) structure
[pic]
Figure 9: Shortened BTC (SBTC) structureFigure 9: Shortened BTC (SBTC) structure
Table 11 gives the block sizes for the optional modulation and coding schemes using BTC. Table 12 gives the code parameters for each of the possible data and coded block size.
Table 11: Table 11: Possible data payload for one subchannel
Possible data payload for one subchannel
|Modulation |QPSK |8PSK |16-QAM |
|scheme | | | |
| | | |Ix |Iy |D |Q |
|6 |12 |(8,7) (32,26) |
|DL_Burst_Allocation{ | | |
| DIUC |4 bits |Coding/modulation scheme index |
| N_CID |8 bits |Number of associated connections |
| for(n = 0; n ................
................
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