Monday, January 10, 2011


802.16e WiMAX system profiles will cover 5, 7, 8.75, and 10MHz channel bandwidths for licensed spectrum allocations in the 2.3, 2.5, 3.3, and 3.5 GHz frequency bands. Targeting a worldwide coverage, the WiMAX forum intends to promote the allocation of frequency bands inferior to 6 GHz for civilian applications while considering the available spectrum all over the world. For instance, the 3.5 GHz frequency band is already assigned to fixed services in many countries, the 2.3 GHz band was reserved for the deployment of the WiBro solution in South Korea, while the 2.5 and 2.7 GHz bands have been assigned by the United States for fixed and mobile WiMAX deployment. 


Considering the modulation technique, mobile WiMAX is based on the orthogonal frequency division multiple access (OFDMA) technique and particularly on the SOFDMA variant. The frequency division multiplexing (FDM) principle consists in using multiple frequencies to transmit different signals in parallel. This is achieved by assigning a frequency range or subcarrier to each signal then modulating it by data. The multiple subcarriers used for transmitting different signals should be separated by guard bands to prevent interferences. 


Orthogonal FDM (OFDM) eliminates the guard bands by using overlapping subcarriers that are spaced apart at precise frequencies; thus achieving orthogonality. With OFDM, the center of the modulated carrier coincides with the edge of the adjacent carrier so that the independent demodulators performing a discrete Fourier transform see only their own frequencies. Redundancy may be guaranteed by scattering some bits over some sets of distant subcarriers. The OFDMA is a multiple access and multiplexing scheme that multiplexes data streams generated by multiple users onto the downlink (DL) subchannels and fulfils uplink (UL) multiple access by means of UL subchannels. The OFDMA symbol structure is made up of data subcarriers for data transmission, pilot subcarriers for estimation and synchronisation purposes, and null subcarriers used for guard bands and DC carriers. Data and pilot subcarriers are organized in a subset of subcarriers called subchannels. The minimum frequency–time resource unit of subchannelization is one slot equal to 48 data tones. 


It is worth noticing that there exist two types of subcarrier permutations for subchannelization which are the diversity permutation and the contiguous permutation. The diversity permutation organizes the subcarriers in a pseudorandom fashion to form a subchannel while the contiguous permutation organizes a block of contiguous subcarriers forming a bin. The bin is formed by nine contiguous subcarriers in a symbol with eight assigned for data, and one assigned for a pilot. Generally speaking, diversity subcarriers permutation achieves a high performance with the mobile applications while the contiguous subcarrier permutation fits well to fixed, portable, or low mobility environments. 


The SOFDMA technology optimizes the mobile access by assigning a set of subcarriers to particular users. For instance, subcarriers 1, 3, and 7 may be assigned to user 1 while subchannels 2, 5, and 9 to user 2 and so on. Users close to the base station (BS) will benefit from a larger throughput by getting an important number of subchannels with a high modulation scheme. SOFDMA offers multiple bandwidths to allow multiple spectrum allocation and fulfil different usage-model requirements. In fact, the scalability is guaranteed by adjusting the fast Fourier transform (FFT) size with respect to the available bandwidth while fixing the subcarrier spacing at 10.94 kHz. As the resource unit-subcarrier bandwidth and the symbol duration are fixed, scaling bandwidth will not affect higher layers. 


The first release of mobile WiMAX will adopt a system channel bandwidth of, respectively, 5 and 10 MHz, with a sampling frequency of, respectively, 5.6 and 11.2 MHz. The FFT size will be either 512 or 1024 while the number of subchannels will be either 8 or 16 and the useful symbol time will be fixed to 91.4 ms. The OFDMA symbol duration is 102.9 ms while the number of OFDMA symbol within a 5 ms frame is 48. The modulation scheme will gradually vary from 16 quadrature amplitude modulation (QAM) to quaternary phase shift keying (QPSK) (four channels) and even binary phase shift keying (BPSK) (two channels) at longer ranges while the power allotted to each channel will be increased. The adaptation of multiple-input multiple-output (MIMO) antenna technique along with advanced coding and modulation achieve peak DL data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.


IEEE 802.16e supports both time division duplex (TDD) and frequency division duplex (FDD) duplexing modes. TDD is adopted when the license-exempt spectrum is used because a unique channel is shared between the UL and the DL traffics which occupy different time slots. Contrarily to TDD, FDD operates with two channels, one is dedicated to the UL traffic and the second is reserved to the DL traffic. Mobile WiMAX in its first release supports only the TDD duplexing mode although the WiMAX forum intends to address particular market opportunities by supporting FDD in future releases. With TDD, it becomes feasible to adjust the DL/UL ratio with respect to the nature of the ongoing traffic so that the DL/UL asymmetric traffic is efficiently supported. Besides, TDD guarantees channel reciprocity for better support of link adaptation, MIMO, and other advanced antenna technologies. Meanwhile, TDD offers a greater flexibility for adaptation to varied spectrum allocations as it uses the same channel for both UL and DL traffics. Last but not least, transceivers designed for TDD implementations are less complex and less expensive. An OFDM frame structure for a TDD implementation is divided into DL and UL subframes separated by transmit/receive and receive/transmit transition gaps (TTG and RTG) to eliminate DL and UL transmission collisions. The frame control information is carried by the preamble, the frame control header (FCH), the DL-MAP, and UL-MAP, the UL ranging, and the UL fast channel feedback (UL CQICH) fields. More specifically, the preamble field appears as the first OFDM symbol in the frame and it is used for synchronization. The FCH field identifies the frame configuration information including the MAP message length and coding scheme and usable subchannels. DL-MAP and UL-MAP provide subchannel allocation for the DL and the UL subframes while the UL ranging subchannel, which is allocated for the mobile stations (MSs), is used for closed-loop time, frequency, and power adjustments as well as bandwidth requests. Finally, the UL CQICH enables the MSs feedbacking the channel-state information to the managing entities.

The WiMAX forum included advanced physical layer features to enhance the mobile WiMAX network coverage and capacity. For instance, mobile WiMAX supports the mandatory QPSK, 16 quadrature amplitude modulation (QAM), 64 QAM schemes in the DL, and the optional 64 QAM in the DL. Meanwhile, both convolutional code (CC) and convolutional turbo code (CTC) coding schemes are supported along with two other optional coding schemes, which are the block turbo code (BTC) and low density parity check code (LDPC). The combination of various modulation and code rates results in a fine resolution of data rates so that the BS scheduler may determine the best suited data rate for each burst allocation, based on the buffer size and the channel propagation conditions at the receiver. Moreover, a channel quality indicator (CQI) channel is used for providing channel-state information from the MSs to the base station scheduler while other channel-state information can be provided to the BS by the CQICH, which includes the physical carrier to interference plus noise ratio (CINR), the effective CINR, the MIMO mode selection, and the frequency selective subchannel selection. Hybrid auto repeat request (HARQ) is supported to provide fast response to packet errors and improve the cell edge coverage through using channel Stop and Wait protocol. The previously stated adaptive modulation and coding, CQICH and HARQ, guarantee robust link adaptation in mobile environments at vehicular speeds exceeding 120 km/h.
Related Posts with Thumbnails