Chapter 1 Introduction
1.3 Thesis Organization
The rest of this dissertation is organized as follows. Chapter 2 introduces the concepts of OFDM, OFDMA and MIMO techniques and also briefly overviews the IEEE 802.16e OFDMA specification. Chapter 3 presents the architecture design and the performance simulation of the proposed STBC-OFDM system with two transmit antennas and one receive antenna. The proposed system aims to provide high performance for WMAN communication in fixed and mobile environments. It provides the simple and robust symbol boundary detection and carrier frequency offset estimation schemes and an accurate but hardware affordable two-stage channel estimation strategy to overcome the challenge of outdoor fading channels. In Chapter 4, the robust two-stage channel estimator for STBC-OFDM systems is proposed and analyzed. An efficient architecture of the proposed channel estimator is provided for
low-complexity hardware implementation while keeping the high performance.
Chapter 5 discusses a novel programmable FIR filter based on higher radix recoding for low-power and high-performance applications. In order to verify the feasibility, the hardware implementation and the experimental results of the proposed receiver are presented in Chapter 6. Finally, Chapter 7 gives the conclusions of the proposed receiver. In addition, Appendix A presents a study of a DC-balanced low-jitter transmission code for 4-PAM signaling.
Chapter 2
Overview of IEEE 802.16e
OFDMA and MIMO Systems
2.1 Introduction
IEEE 802.16-2004 [7] specifies the air interface for fixed BWA systems and has been updated and extended to IEEE 802.16e-2005 [8] for mobile BWA systems with the concept of scalable OFDMA which is an effective technique for high bit rate applications over multipath channels. There are several specifications of IEEE 802.16e-2005 PHY layer for different applications and frequency range. For example, SC operates from 10 GHz to 66 GHz in LOS; besides, SCa, OFDM and OFDMA operate below 11 GHz in NLOS.
In this dissertation, we focus on the IEEE 802.16e OFDMA specification that supports the multi-antenna technology. We will briefly introduce the concepts of OFDM and OFDMA in this chapter. MIMO technologies will also be introduced.
Finally, we give an overview of the IEEE802.16e OFDMA standard.
2.2 Overview of OFDMA
OFDM modulation offers an attractive technique for high-speed data transmission in mobile communication since it can effectively combat frequency-selective multipath fading using relatively simply frequency-domain equalization. OFDM has been widely adopted in several standards, e.g. digital video broadcasting for terrestrial (DVB-T) [22], high-speed WLAN and WMAN such as IEEE 802.11b/g (Wi-Fi) and IEEE 802.16 (WiMAX). Moreover, the high computational complexity of OFDM implementation has became possible due to the success development of digital signal processing (DSP) and very large scale integrated circuit (VLSI).
The concept of OFDM is coming from frequency division multiplexing (FDM).
In FDM, the entire signal bandwidth is divided into several non-overlapping sub-bands as shown in Fig. 2.1 (a). The conventional parallel data transmission modulates each independent data on different sub-bands, and then these sub-bands are frequency-multiplexed. In order to prevent from the adjacent channel interference, the spectrum spacing allocated between sub-bands leads to inefficient utilization of the signal bandwidth. This problem is overcome by employing overlapping sub-bands as shown in Fig. 2.1 (b), and the signal bandwidth can be effectively utilized.
Furthermore, OFDM technology is invented to divide the entire signal bandwidth is into mutual orthogonal overlapping subcarriers, and the orthogonality guarantees subcarrier transmission without interference from each other. As a result, the OFDM technology can achieve high spectral efficiency. Each transmitted signal in each narrow band subcarrier experiences flat channel fading; thus, the channel equalization can be performed by a simple one-tap frequency-domain equalizer. However, the channel delay spread causes the inter-symbol interference (ISI) which destroys the orthogonality of subcarriers in OFDM. In order to avoid ISI effect, a guard interval with the length longer than the maximum channel delay spread is inserted to the frond of an OFDM symbol. Although the guard interval can be used to resolve ISI problem, the loss of orthogonality among subcarriers causes inter-channel interference (ICI). A cyclic prefix (CP), which is equal to a part of the OFDM symbol tail, is widely adopted in current standards to maintain the subcarrier orthogonality and avoid ICI effect as shown in Fig. 2.2.
OFDMA technology is a multi-user version of OFDM modulation, and it is a major multiple access method considered for future wireless systems. IEEE 802.16e-2005 PHY layer provides OFDMA specification for multi-user communication. Multiple access is achieved in OFDMA by assigning mutually exclusive subsets of subcarriers to individual users, which allows simultaneous low data rate transmission from several users. The all available subcarriers of uplink and downlink in OFDMA are divided into several subsets of subcarriers termed as sub-channels as shown in Fig. 2.3.
OFDMA can be seen as an alternative to combining OFDM with time division Fig. 2.2 OFDM symbol with cyclic prefix.
(a)
(b)
Fig. 2.1 (a) Conventional non-overlapping sub-bands. (b) Overlapping sub-bands.
multiple access (TDMA) or time-domain statistical multiplexing, i.e. packet mode communication. As shown in Fig. 2.4, the resources of OFDMA transmission are partitioned in the time-frequency space, and time slots are assigned along the OFDM symbol index as well as OFDM subcarrier index. OFDMA is considered as highly suitable for broadband wireless networks due to the advantages including scalability, MIMO easy applying, and the multi-user diversity ability to take advantage of channel frequency selectivity. OFDMA has another advantage of that low-data-rate users can send continuously with low transmission power. Constant delay and shorter delay can be achieved. In IEEE 802.16e OFDMA specification, there are two types of data allocation for sub-channelization, contiguous and diversity. As shown in Fig. 2.5, the
Fig. 2.4 Two-dimensional resources of OFDMA transmission.
Fig. 2.3 OFDMA subcarrier allocation.
contiguous permutation collects contiguous subcarriers to form a sub-channel. On the contrary, the diversity permutation pseudo-randomly spread out the subcarriers of sub-channel over the entire bandwidth and brings the benefit of frequency diversity and robustness against the frequency select fading channel.
A concept of scalable OFDMA (S-OFDMA) [23] is also introduced to the IEEE 802.16e OFDMA specification, which supports for 128, 512, 1K, and 2K fast Fourier transform (FFT) sizes to address a variable bandwidth sizes from 1.25MHz to 20MHz for NLOS operations as shown in Table 2.1.
Several basic term definitions should be established to help us to follow IEEE 802.16e OFDMA specification.
● Subcarrier: An OFDM symbol is made up of subcarriers as shown in Fig.
2.6. There are three subcarrier types: data subcarriers for data transmission, pilot subcarriers for channel estimation purposes and null subcarriers for no transmission, guard bands, non-active subcarriers and
Fig. 2.5 Adjacent and distributed subcarrier allocations.
the DC-subcarrier. Subcarrier spacing is 10.9375 KHz.
● Sub-channel: It is a set of subcarriers. IEEE 802.16 OFDMA systems define two modes of sub-channel building method. In the distributed subcarrier permutation mode, subcarriers of a sub-channel are not contiguous, and their distribution is determined by the permutation types of Partial Usage of Sub-channels (PUSC) and Full Usage of Sub-channels (FUSC). In adjacent subcarrier mode, sub-channels are constituted of bins and determined the distribution by the permutation type of AMC.
IEEE 802.16e OFDMA specification supports multiple schemes for dividing the frequency-domain and time-domain resources between users, which is called sub-channelization. The relationship between the basic terms of the two-dimensional units is shown in Fig. 2.7 and brief introduced as follows.
Fig. 2.6 An OFDM symbol.
TABLE 2.1
SCALABLE OFDMA PARAMETERS
Parameters Values
System channel bandwidth (MHZ) 1.25 5 10 20
FFT Size 128 512 1024 2048
Sampling Factor 28/25
Sampling Frequency 1.4 5.6 11.2 22.4
CP Ratio 1/32, 1/16, 1/8, and 1/4
Modulation Mode QPSK, 16QAM, and 64QAM
Subcarrier Frequency Spacing 10.94 kHz
Frame Duration 5 ms
● Frame: It is an essential packet format of transmitted data sequence. A frame may contain both an uplink sub-frame and a downlink sub-frame.
● Sub-frame: It is a component to make up a frame and identified as an uplink sub-frame and a downlink sub-frame.
● Zone: A zone is the region of contiguous OFDMA symbols which are applied with the same permutation scheme (i.e., PUSC, FUSC or AMC).
It is allowed to have different zones in a sub-frame.
● Segment: The set of available sub-channels form a segment which is applied with the same MAC definition. There can be three segments in a zone.
● Burst: It is a region which includes the contiguous sub-channel and OFDMA symbol to transmit the broadcast or unique data for Fig. 2.7 Two-dimensional basic terms in the OFDMA data structure.
corresponding users.
● Slot: It is the minimum possible data allocation unit and spans both the time domain (OFDM symbol) and frequency domain (sub-channel). It contains 48 data subcarriers for all sub-channelization schemes, but their arrangement is different in different schemes [24].
● Cluster: It contains 14 adjacent subcarriers over 2 contiguous symbols with 4 pilot subcarriers in PUSC permutation scheme.
2.3 MIMO Systems
MIMO technology is the use of multiple antennas at both the transmitter and receiver to improve communication performance. IEEE 802.16e standard incorporates MIMO-OFDMA specification. MIMO is a current theme in wireless communication research since it can offer significant increases in data throughput, coverage range, and system reliability without additional bandwidth or transmit power. Nevertheless, these advantages usually conflict with one another, for example, increasing the data throughput will restrict the reliability improvement. Depending on application requirements, the MIMO technology can be divided into three main schemes, beamforming [25], spatial multiplexing (SM) [26] and space-time coding (STC) [27]
as introduced below.
Beamforming technology achieves the spatial selectivity by using adaptive or fixed receive/transmit directional patterns for directional signal transmission or reception. The same signal is emitted from each transmit antennas with appropriate weighting and phasing such that the signal power is maximized at the receiver input.
Beamforming can bring the advantages of improving the signal gain from constructive combining and reducing the multipath fading effect. Conventional beamforming employs a fixed directional pattern to filter the signals. In contrast, adaptive beamforming uses an adaptive directional pattern to filter the signals with properties of the signals actually received, and it can effectively reject the unwanted noise from other directions. This process may be carried out in the time or frequency domains.
Note that beamforming requires knowledge of CSI at the transmitter.
SM technology is used to increase the transmission rate by using multiple antennas at both transmitter and receiver. It transmits a high rate data stream by dividing the stream into multiple lower rate sub-streams and transmitting these sub-streams from different transmit antennas in the same frequency channel. If these signals arrive at the receiver antennas with sufficiently different spatial signatures, the receiver can separate these streams. Spatial multiplexing can efficiently increase channel capacity at higher signal to noise ratio (SNR).
STC technology is to use multiple transmit antennas to improve the reliability of data transmission in wireless communication systems. It relies on transmitting multiple and redundant copies of the transmitted data stream to the receiver in the hope of the survival physical path between transmission and reception in a good enough channel state to allow reliable decoding. Space time codes can be separated into two main types, STBC and space-time trellis code (STTC). STBC is similarly to block codes and provides only diversity gain but has much less complexity of implementation than STTC. STTC distributes a trellis code over multiple antennas and multiple time slots and provides both coding gain and diversity gain.
More details on these schemes can be found in [27]. Herein, we focus in Alamouti's STBC in two transmit-antenna systems which will be introduced in Chapter 3.
2.4 IEEE 802.16e OFDMA Specification
IEEE 802.16e includes multiple PHY specifications such as, SC, SCa, OFDM, and OFDMA, for providing different channel conditions and applications. Herein, the OFDMA specification that supports the multi-antenna technology is followed in this dissertation.
2.4.1 Frame Structure
In the licensed bands, IEEE 802.16 systems can support time-division duplexing (TDD) and frequency-division duplexing (FDD). The other license-exempt bands, the duplexing method shall be TDD. The frame can be composed of several zones that are
divided according to subcarrier allocation methods or MIMO modes. In an FDD frame structure, the downlink (DL) and uplink (UL) sub-frames are allocated in a different frequency band without the transmit transition gap (TTG) and receive transition gap (RTG). In a TDD frame structure, the DL and UL sub-frames are allocated respectively on the same frequency band but at different time. Fig. 2.8 shows an example of an IEEE 802.16e OFDMA frame structure in TDD mode, which is a model frequently referred.
The frame structure consists of the following elements. The first symbol of a transmitted frame is a preamble symbol which is the known pattern in the receiver for the use of cell search, synchronization and channel estimation. Following the preamble symbol, the frame control header (FCH) with fixed size is transmitted for resource allocation such as the subcarriers used, length of DL-MAP and DL_Frame_Prefix. The quadrature phase shift keying (QPSK) modulation with code rate 1/2 and four repetitions is used for FCH transmission to ensure the robustness and reliable performance. DL_MAP and UL_MAP following FCH message for resource allocation of the various users in DL and UL data bursts. TTG is used to give BS and subscriber station (SS) enough time to change from downlink mode to uplink mode.
Fig. 2.8 An OFDMA frame in TDD mode [8].
For the same reason, RTG is inserted at the end of each frame. Besides, the ranging sub-channel specified in the UL_MAP message is used for contention-based bandwidth request in UL transmission.
For supporting various physical channel conditions, IEEE 802.16 OFDMA systems define two modes of sub-channel building method: the distributed subcarrier permutation mode, including PUSC and FUSC types, and the adjacent subcarrier permutation mode, including AMC type. In this dissertation, PUSC is mainly supported and will be described for the subcarrier allocation scheme in Section 3.3.3.
The ratio of these modes can be flexible in the IEEE 802.16 standard. However, one burst of data transmission consists of several slots, and one slot is the minimum possible data allocation unit. In addition, the definition of this slot depends on the OFDMA symbol structure, which varies for DL and UL, for FUSC and PUSC, and for distributed subcarrier permutations and adjacent subcarrier permutation. Fig. 2.9 shows an OFDMA frame with multiple zones. Due to no information about the permutation scheme, the first zone in the DL sub-frame is essentially PUSC to ensure the FCH and DL_MAP can be received successfully. Depending on the requirements, the following zones can be applied in PUSC, FUSC, AMC, or Tile Usage of Sub-channels (TUSC). The information of zone transition is indicated in the DL_MAP and UL_MAP. The maximum number of DL zones in one DL sub-frame is eight. The maximum number of bursts to be decoded in one DL sub-frame is 64.
Fig. 2.9 An OFDMA frame with multiple zones [8].
2.4.2 Preamble Format
The preamble symbol consists of a specific pattern known to the receiver and occupies the duration of an OFDM symbol time. It is used for frame detection, synchronization and initial channel estimation. IEEE 802.16e standard provides three types of carrier sets for different segments in the preamble symbol which can be expressed as
3
PreambleCarrierSets = + × (2.1) s k
where s=0,1,2 is the index of the carrier set, and k denotes a running subcarrier index.
These subcarriers in the preamble symbol are modulated by binary phase shift keying (BPSK) with a specific Pseudo-Noise (PN) code. The PN series modulating the pilots in the preamble can be found in [7]. Each segment in a zone uses one type of preamble carrier sets. For different FFT size, there are total 114 PN series to be chosen by the ID cell parameter and the segment index. The guard band subcarriers are contained both on the left and right side of the spectrum. The DC subcarrier is always be zeroed even if the type of carry set is 0. The power of the preamble subcarriers is boosted by a factor, 2 2 , to increase the reliability of preamble. The pilot subcarriers pk in the preamble symbol are modulated as
{ } 1 { }
Re 4 2 , Im 0
k 2 k k
p = ×æçè -w ö÷ø p = (2.2)
where wk denotes PN series, and Re{.} and Im{.} stand for the real part and the imaginary part of {.}.
2.4.3 Pilot Modulation
The OFDM symbol structure is constructed using pilots, data, and null subcarriers. The symbol is first divided into basic clusters and null carriers are allocated. In DL PUSC mode, pilots and data carriers are allocated within each cluster as shown in Fig. 2.10. For the proposed system with two transmit antennas, when the pilot subcarrier is transmitted from one antenna, the other antenna will not transmit a pilot on the same subcarrier to avoid the inter-antenna interference. The pilot location schemes periodically change every four OFDMA symbols.
The pseudo-random binary sequence (PRBS) generator depicted in Fig. 2.11 is used to produce a sequence, wk. Each pilot is boosted 2.5 dB over the average non-boosted power of each data subcarriers. The value of the pilot modulation, on subcarrier k, shall be derived from wk. The pilot subcarriers pk are modulated as
Fig. 2.10 Cluster structure.
Fig. 2.11 PRBS generator for pilot modulation.
{ } 8 1 { }
Re , Im 0.
k 3 2 k k
p = æç -w ö÷ p =
è ø (2.3)
2.4.4 Basic Specification in IEEE 802.16e OFDMA
IEEE 802.16e OFDMA specification defines different transmission types according to different purposes and applications. The modulation schemes of QPSK, 16-QAM, and 64-QAM are supported for data subcarrier. The transmission types are constructed by different modulation schemes with different code rates such as QPSK 1/2, QPSK 3/4, 16-QAM 1/2, 16-QAM 3/4, 64-QAM 1/2, 64-QAM 2/3, and 64-QAM 3/4. The major parameters can be derived and described in Table 2.2.
TABLE 2.2
MAJOR PARAMETERS OF IEEE802.16E OFDMA SPECIFICATION
Parameters Deriving formulas
FFT Size (N) 2048, 1024, 512, and 128
Channel Bandwidth (BW) 1.25-20MHz
Sampling Factor (n)
n=28/25 if BW is a multiple of 1.25, 1.5, 2, and 2.75 MHz
n=8/7 for the other cases Ratio of CP to Useful Symbol Time (G) 1/32, 1/16, 1/8, and 1/4 Sampling Frequency (FS) floor n BW( × 8000)´8000
Sampling Time T N b
Subcarrier Spacing (Δf) F N S
Useful Symbol Time (Tb) 1 fD
Guard Time (Tg) G T× b
OFDMA Symbol Duration (TS) Tb +Tg
Frame Duration (TF)
Number of OFDMA Symbols floor T T ( F S)
Number of Null Subcarriers (NNull)
Number of Clusters (NC) (N N- Null) 14
Number of Sub-channels (NSC) NC 2 Number of Pilot Subcarriers (NPilot) NC´ 2 DL
PUSC
Number of Data Subcarriers (NData) NC´ 12
Chapter 3
Downlink Baseband STBC-OFDM System Architecture
3.1 Introduction
In this chapter, the proposed downlink baseband STBC-OFDM system architecture will be described. This architecture can provide high transmission rate in IEEE 802.16e downlink communication as an alternative solution for WMAN in fixed and mobile wireless communication.
Recently, STBC-OFDM systems have received a lot of attention [28], [29] and are also adopted in IEEE 802.16e systems. Although STBC-OFDM systems with multiple antennas can provide diversity gains to improve transmission efficiency and quality of mobile wireless systems, accurate CSI is required for diversity combining, coherent detection, and decoding. Moreover, the system performance is also susceptible to the synchronization error. Therefore, synchronization and channel estimation are two crucial challenges for realizing a successful STBC-OFDM system.
Hence, a downlink baseband receiver scheme for STBC-OFDM systems is proposed and can be applied in IEEE 802.16e specification. In the proposed receiver, two main tasks, synchronization and channel estimation, are implemented. The synchronization includes symbol timing detection and carrier frequency recovery. A novel match filter is proposed to precisely detect symbol boundary, and a ping-pong algorithm is presented to effectively improve the performance of carrier frequency
synchronization [30]. Moreover, a refined two-stage channel estimation method with
synchronization [30]. Moreover, a refined two-stage channel estimation method with