Multiuser Communications

Code Division Multiple Access: DS-CDMA, MC-CDMA, MC-DS-CDMA
Prof Lajos Hanzo, Dr. Lie-Liang Yang, Dr. Bin Hu
[1] Multiuser Detection Assisted Time- and Frequency-Domain Spread Multicarrier Code-Division Multiple-Access.
[2] Single- and Multi-Carrier DS-CDMA: Multi-User Detection, Space-Time Spreading, Synchronisation, Networking and Standards.
[3] Performance of Broadband Multicarrier DS-CDMA using Space-Time Spreading-Assisted Transmit Diversity. [More Publications]

With the substantial increase of Internet users and with the development of new services high-speed access in the future generations of wireless systems is an important requirement. Consequently, broadband systems with bandwidths much wider than that of the 3rd-generation systems are required for meeting future requirements. Hence, compatibility with both the emerging Broadband Access Networks (BRAN), which have opted for a multi-carrier, Orthogonal Frequency Division Multiplexing (OFDM) based solution and the existing 2nd- and 3rd-generation CDMA ystems is an important consideration.

A potential candidate multiple access scheme meeting these requirements has been proposed by our group. The multiple-access scheme is constituted by frequency-hopping (FH) based multicarrier DS-CDMA (FH/MC DS-CDMA), where the entire bandwidth of future systems can be divided into a number of sub-bands and each sub-band can be assigned a subcarrier. According to the prevalent service requirements, the set of legitimate subcarriers can be distributed in line with the instantaneous information rate requirements. FH techniques are employed for each user, in order to occupy the whole system bandwidth and to efficiently utilize the system's frequency resources. Specifically, slow FH, fast FH or adaptive FH techniques can be utilized depending on the system's design and the state-of-the-art.

In FH/MC DS-CDMA systems the sub-bands are not required to be of equal bandwidth. Hence existing 2nd- and 3rd-generation CDMA systems can be supported using one or more subcarriers, consequently simplifying the frequency resource management and efficiently utilizing the entire bandwidth available. This regime can also remove the spectrum segmentation of existing `legacy' systems, while ensuring compatibility with future BRAN and un-licensed systems. Furthermore, a number of sub-channels with variable processing gains can be employed, in order to support various services requiring low- to very high-rate transmissions, for example for wireless Internet access.

Figure: Transmitter diagram of the frequency-hopping multicarrier DS-CDMA system.

The objective of this research is to investigate the relevant techniques in the context of FH/MC DS-CDMA, including modulation/demodulation, error-control, synchronisation, equalisation, multiuser interference suppression, adaptive detection, etc.
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Interleave Division Multiple Access and Random Waveform Channel Code Division Multiple Access
Prof Lajos Hanzo, Dr. Rong Zhang
[1] Three Design Aspects of Multicarrier Interleave Division Multiple Access.
[2] Repeat Accumulate Code Division Multiple Access and its Hybrid Detection.
[3] Channel code-division multiple access and its multilevel-structured LPDC-based instantialtion. [More Publications]

Based on the code-spread chip-interleaved CDMA philosophy, IDMA entails reversing the classic position of DS-spreading and interleaving employed in traditional CDMA system, leading to chip-interleaving instead of bit-interleaving, where the different users are distinguished by their unique user-specific chip-interleavers. The benefits of IDMA are

Figure : The transceiver of a multiuser multiantenna IDMA system.

1. Near-capacity random coding: The user-specific chip-interleavers effectively enlarge the free-distance of the channel code employed and the resultant codewords of the different users constitute unique noise-like random signatures, since the resultant system satisfies the conditions to be met by Shannonian near-capacity systems.
2. Time diversity: Chip-interleaving is capable of increasing the achievable time diversity in time-selective channels, where the burst errors are dispersed and hence the achievable decoding capability is improved.
3. Joint coding and spreading design: IDMA is capable of striking the best tradeoff between the channel-coding rate and DS-spreading factor, hence improving the attainable error-resilience.
4. Near-capacity iterative receiver design: The chip interleaver employed is capable of substantially improving the reception quality, when an iterative receiver is used, provided that the consecutive chips of the randomly coded sequences are sufficiently uncorrelated, which
is analogous to having a sufficiently long turbo-interleaver in serially concatenated codes.

In this generalised spirit, apart from the well-known DS-CDMA scheme, two lesser-known schemes were also proposed, namely TCMA and IDMA. In the former, the separation of the users is achieved by the unique combination of user-specific Generator Polynomials (GP) combined with bit-to-symbol mapping schemes and interleavers, whilst the latter employs user-specific interleavers, which may be regarded as rate-one channel codes. These two generalised CDMA techniques are instantiations of our more general non-orthogonal random waveform based multiuser communications, where the employed interleaved codes obey the random-coding principle. Alternatively, random-coding principle may be achieved by the family of LDPC codes. Hence we will jointly refer to these schemes using the generic terminology of Interleaved Random CDMA (IR-CDMA).

Based on the random coding principle, the family LDPC codes becomes also attractive, since it has a built-in interleaver, which renders the employment of an extra user-specific interleaver after channel coding unnecessary. However, low-rate LDPC codes is hard to design. On the other hand, Repeat Accumulate codes having diverse coding rates constitute attractive candidates for employment in IR-CDMA. Similar to LDPC codes, RA codes also have an innate interleaver, while exhibiting a linearly increasing encoding complexity as a function of the codeword length. In this chapter, we propose non-systematic regular RA codes aided IR-CDMA, where different users are distinguished by their unique user-specific RA generator matrices, i.e. their interleavers. We design a Structured Embedded (SE) interleaver generation technique for the sake of reducing the interleaver’s storage requirements and investigate its correlation properties compared to those of random interleavers.

Figure : The SE interleaver design of RA coded IR-CDMA and the Latin Squre interleaver design of LDPC coded IR-CDMA.

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Ultra Wide Band Communications: Hybrid Direct Sequence and Time Hopping
Prof Lajos Hanzo, Prof. Lie-Liang Yang, Dr. Raja A Riaz, Dr. Qasim Ahmed
[1] Reduced-Rank Adaptive Multiuser Detection in Hybrid Direct-Sequence Time-Hopping Ultrawide Bandwidth Systems.
[2] EXIT-chart aided three-stage concatenated ultrawideband time-hopping spread-spectrum impulse radio design.
[3] Performance of Hybrid Direct-Sequence Time-Hopping Ultrawide Bandwidth Systems over Nakagami-m Fading Channels. [More Publications]

In recent years the UWB techniques have received wide interest in both the research and industry communities. This is because UWB techniques are capable of providing high data rate, accurate position and ranging, immunisation to multipath fading, covert communications due to low transmission power, coexistence with other communication systems because of the low power spectral density (PSD), material penetration capabilities, etc. The study has shown that UWB is one of the alternative techniques that are well suitable for short-range
communications, such as for wireless sensor networks (WSN) and personal area networks (PAN).

Initially, UWB has been implemented with the aid of timehopping pulse-position modulation (TH-PPM) techniques without carrier modulation. In the carrier-less or baseband UWB systems information is transmitted with the assistance of trains of time-shifted pulses through pulse position modulation. In the TH-PPM UWB multiple pulses are usually used to transmit a single symbol for the sake of enhancing the transmission performance. Recently, the direct-sequence spread spectrum (DS-SS) technique has also been proposed for implementation of UWB communications. In DS-UWB a data bit is transmitted associated with multiple chips and the chip-duration is usually equal to the width of the basic timedomain signal. In DS-UWB the conventional CDMA related techniques may be applied for improving the multiple-access capability.

Explicitly, both the TH-UWB and DS-UWB have their advantages and disadvantages. Due to using low-duty baseband pulses and supporting low rate, TH-UWB is more desirable when long battery life becomes important. TH-UWB systems also have the capability to mitigate the multipath interference. It has been demonstrated that the performance of the TH-UWB is slightly better than that of the DS-UWB in the presence of interference, when communicating over AWGN channels. By contrast, the performance of the DS-UWB systems degrade due to inter-chip interference (ICI) and inter-symbol interference (ISI) when single-user correlation detector is considered. It has been shown that the DS-UWB system is capable of mitigating the multiuser interference. However, it experiences severe narrowband interference. It can be shown that, when the energy per bit is constant in both the DS-UWB and TH-UWB systems, the energy per pulse in the TH-UWB is higher than that in the DS-UWB. Consequently, the peak-toaverage-power and the power-spectral density (PSD) is lower in DS-UWB in comparison with the TH-UWB. Furthermore, the DS-UWB usually causes less inband interference on the other types of systems operated in the same frequency band of the UWB systems.

In this contribution we propose and investigate a novel UWB system, namely the hybrid DS-TH UWB system, which employs both DS spreading and TH. It can be shown that the hybrid DS-TH UWB system is capable of inheriting the advantages of both the DS-UWB and TH-UWB, while avoiding their disadvantages. Furthermore, it can be shown that the hybrid DS-TH UWB is capable of providing more degrees-of-freedom for system design and reconfigurations than either the pure DS-UWB or the pure TH-UWB. In this contribution we compare the performance of the hybrid DS-TH UWB systems with that of the pure DS-UWB or pure TH-UWB, when the conventional single-user correlation detector or conventional minimum mean-square error (MMSE) multiuser detector is employed. The performance of the above-mention three types of UWB systems is investigated and compared, when communicating over typical UWB channels experiencing Nakagami-m fading. Our study and simulation results show that there exists a tradeoff between the DS and TH spreading factors. Given the channel conditions, signal-to-noise ratio (SNR) and the total spreading factor equalling to the product of the DS and TH spreading spreading factors, there exist optimum DS and TH spreading factors, which result in the lowest achievable bit-error rate (BER).
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The employment of CDMA enables multi-rate services to be provided upon involving various methods, such as multi-code transmission, multiple spreading factor transmission and modulation mode multiplexing.

Direct Sequence Code Division Multiple Access (DS-CDMA) is interference-limited due to the multiple access interference (MAI) generated by the users transmitting within the same bandwidth simultaneously. The signals from the users are separated by means of spreading sequences that are unique to each user. These spreading sequences are usually non-orthogonal. Even if they are orthogonal, the asynchronous transmission or the time-varying nature of the mobile radio channel may partially destroy this orthogonality. The non-orthogonal nature of the codes results in MAI, which degrades the performance of the system. The frequency selective mobile radio channel also gives rise to inter-symbol interference (ISI) due to multi-path propagation. This is exacerbated by the fact that the mobile radio channel is time-varying. A class of CDMA receivers known as multiuser receivers exploit the available information about the spreading sequences and mobile channel impulse responses of all the CDMA users in order to improve the performance of the CDMA users. These multiuser receivers include joint detection (JD) receivers, interference cancellation (IC) receivers, tree-search type algorithms and iterative receiver schemes. Figure 3 shows a general classification of the various types of multiuser receivers.

Figure: Classification of CDMA detector

Mobile radio signals are subject to propagation path loss as well as slow fading and fast fading. Due to the nature of the fading channel, transmission errors occur in bursts when the channel exhibits deep fades or when there is a sudden surge of multiple access interference (MAI) or inter-symbol interference (ISI). Adaptive-rate CDMA techniques can be used to overcome this phenomenon, where the information rate is varied in accordance with the channel quality. The information rate is chosen accordingly, in order to provide the best trade-off between the BER and throughput performance for a given application. The signal to interference plus noise ratio (SINR) at the output of the JD receiver is derived and this parameter is used as the criterion for adapting the information rate. There are various methods of varying the information rate are considered, including Adaptive Quadrature Amplitude Modulated (AQAM) and the Variable Spreading Factor (VSF) scheme. AQAM is an adaptive-rate technique, whereby the data modulation mode is chosen according to some criterion related to the channel quality. On the other hand, in VSF transmission, the information rate is varied by adapting the spreading factor of the CDMA codes used, while keeping the chip rate constant. Figure 4 shows the stylized amplitude variation in a fading channel and the switching of the modulation modes in a four-mode AQAM system, where the performance degrades but the throughput increases when switching from Mode 1 to 4.

Figure: Basic concept of a four-mode AQAM transmission in a narrowband channel. The variation of the modulation mode follows the fading variation of the channel over time.

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Figure : Block diagram of an OFDM based transmission system.

The problem of inter-symbol interference can be elegantly addressed by employing OFDM ( Orthogonal Frequency Division Multiplexing ), which is a multi-carrier transmission scheme conceived back in the 60's. With the availability of high-performance signal-processing devices it has been re-discovered since the operation of multi-carrier modulation can be efficiently implemented by means of a Fast Fourier Transform (FFT). As opposed to single-carrier transmission, OFDM seeks to avoid inter-symbol interference by appending a guard interval to each FFT output block - before signal interpolation and up-conversion to the HF ( High Frequency ) stage - at the cost of a slight reduction in bandwidth efficiency. The problem of channel equalization at the receiver is simplified substantially. In multiuser environments of higher mobility, the OFDM scheme suffers from inter-subcarrier interference, which is due to a loss of orthogonality between different sub-channels. OFDM is well established in a variety of services such Digital Audio Broadcasting, Asynchronous Digital Subscriber Lines and the new Hyperlan II standard.

Research is mainly concerned with the development of signal processing algorithms for tailoring OFDM to the needs of a multi-cellular environment, of which a very important aspect is the signal separation of simultaneously transmitting users. This is could be achieved with multiple-antenna assisted reception at the basestation, which requires further investigation in conjunction with OFDM. Related aspects are channel parameter estimation as well as synchronization mechanisms.
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