In this thesis, we have investigated various means to cope with the design challenge for future broadband wireless packet access.
We proposed a new concept of using block spread (BS) to deal with multi-user interference for high speed transmission. This achieves near single user performance without complex multi-user detection (MUD) techniques. The code orthogonality among BS can be maintained when the channel variation across the consecutive blocks is negligible. This BS concept can be applied to new research areas on the design of the broadband wireless packet access air interface. Specifically, a block spreading CDMA (BS-CDMA) scheme has been proposed to combat MAI for uplink transmission, giving rise to a significant improved multi-user performance without using complex MUD techniques over a broadband wireless channel. The block despreading is performed before equalization at the receiver, reducing the frequency domain processing to a symbol-wise with significant power saving. A cell-specific scrambling code to suppress OCI effectively is proposed for uplink transmission in a multi-cell system. The simulated results validate the theoretical BER analysis and show that its multi-user performance approaches the single user performance when the channel is time-invariant. In comparison, the proposed BS-CDMA scheme achieves the best multi-user performance among the existing schemes such as DS-CDMA and CP-CDMA for uplink transmission. In addition, two interference cancellation
154 methods based on users’ mobility have been proposed to enhance its performance significantly when the channel variation across the consecutive blocks is not negligible.
Extending the BS-CDMA scheme to a two-layer spreading CDMA (TLS-CDMA) scheme, a two-layer spreading with an additional two-layer cell-specific scrambling has been investigated to tackle OCI and MAI more effectively for uplink transmission in a multi-cell system. With the flexible adaptation of the two-layer spreading factors, the TLS-CDMA scheme is capable of supporting variable data rates among multiple users according to the cell structure, channel conditions and the number of active users. Combined with the additional two-layer cell-specific scrambling, the TLS-CDMA scheme can achieve one cell frequency reuse because the larger spreading factor offers better OCI suppression in a multi-cell system. The two layer code generation enables the flexible and independent selection of spreading codes for different layer spreading: the 1st layer spreading is used for better OCI suppression while the 2nd layer spreading is prioritized for multi-user allocation. With analytical and simulation results showing its superiority, the TLS-CDMA scheme is capable of combating MAI and OCI more effectively than the BS-CDMA scheme in a multi-cell system. Specifically, in a hot-spot system where OCI is almost negligible, BS-CDMA is superior to support more users because of its capability to combat MAI (in this case, the 1st layer spreading factor becomes one for TLS-CDMA). However, in a multi-cell environment where OCI is significant, TLS-CDMA is superior to BS-CDMA due to its better OCI suppression capability. TLS-CDMA is superior to BS-CDMA in broadband wireless packet access, since former provides a seamless deployment between multi-cell and
155 hot spot environment using the same air interface by changing the two-layer spreading factors. In addition, compared to BS-CDMA, TLS-CDMA has more flexible accommodation of the lower data rate with higher-quality transmission by utilizing the frequency diversity effect in the 1st layer spreading.
Furthermore, the BS concept has been viewed as providing an additional domain for more flexible user allocation in multi-user systems. In particular, a block-spread interleaved frequency division multiple access (BS-IFDMA) has been formulated to achieve significant performance improvement over the conventional IFDMA system, by allocating multiple users in the additional block-level code spreading process on top of the IFDMA domain over time invariant channel. This is because the same number of supported users and bandwidth, more subcarriers can be allocated for individual user in the frequency domain compared to the conventional IFDMA scheme. In addition, two MAI cancellation methods have been investigated for BS-IFDMA over mobile wireless channel and the order of detection and interference cancellation is based on users’ mobility in the system. The SMC scheme, which is the simpler of the two schemes, is able to maintain the system performance if the percentage of high mobility users is small. If there is significant percentage of high mobility users up to 50% present in the system, the HMC method is more suitable. Next, a two-dimensional code spreading for IFDMA (TCS-IFDMA) has been proposed to combat MAI and OCI more efficiently. The two-dimensional code spreading of TCS-IFDMA is not a straightforward extension to frequency domain spreading of VSCRF-CDMA. The time domain spreading which is equivalent to the block spreading is incorporated into code spreading on top of the frequency domain
156 spreading as in VSCRF-CDMA system to provide an additional domain for multi-user allocation. The analytical and simulated results show that the proposed TCS-IFDMA scheme enhances the VSCRF-CDMA scheme significantly by prioritizing users in the time domain spreading according to the system configuration. It can be considered as a promising candidate for future broadband wireless packet access to achieve seamless handover between the cellular system and hot-spot system using the same air interface deployed.
The relationship among the proposed schemes, such as BS-CDMA, TLS-CDMA, BS-IFDMA and TCS-IFDMA, and the existing schemes, such as CP-CDMA, VSCRF-IFDMA, is summarized in Figure 7-1. As shown, TLS-CDMA, BS-CDMA and CP-CDMA belong to CDMA scheme. TLS-CDMA is a generalized scheme for both BS-CDMA and CP-CDMA by changing the spreading factors in both-layer. TLS-CDMA is superior to BS-CDMA in a multi-cell environment, where OCI is significant, due to its better OCI suppression capability while BS-CDMA is superior to support more users in a hot-spot environment, where OCI is almost negligible, because of its capability to combat MAI (in this case, TLS-CDMA becomes BS-CDMA by setting the 1st layer spreading factor (G1) to one). In addition, TLS-CDMA has more flexible accommodation of the lower data rate with higher-quality transmission by utilizing the frequency diversity effect in the 1st layer spreading. Therefore, TLS-CDMA is a promising candidate for future broadband wireless packet access to provide a seamless deployment between multi-cell and hot spot environment using the same air interface by changing the two-layer spreading factors. In particular, TLS-CDMA is a potential scheme for future evolvement from
157 CDMA system.
On the other hand, TCS-IFDMA, BS-IFDMA and VSCRF-CDMA belong to IFDMA scheme. TCS-IFDMA is a generalized form for both BS-IFDMA and VSCRF-CDMA by changing the spreading factors of {Gt, Ui, G}. In IFDMA scheme, BS concept has been viewed as providing an additional domain for more flexible user allocation in multi-user system. Similarly, TCS-IFDMA is superior to BS-IFDMA in a multi-cell environment, due to its better OCI suppression capability while BS-IFDMA is superior to support more users in a hot-spot environment because of its capability to combat MAI. Enhancing the performance of DoCoMo’s VSCRF-CDMA scheme, TCS-IFDMA can be considered as a promising candidate for future broadband wireless packet access to achieve seamless handover between the cellular system and hot spot system. Compared to TLS-CDMA, TCS-IFDMA has more flexibility to provide multi-user allocation, therefore showing its robustness over mobile wireless channel with high mobility.
Figure 7-1 Relationship among the proposed schemes
TLS-CDMA (G1, G2) BS-CDMA (1, G2) G1=1 CP-CDMA (G1,1) G2=1 BS-IFDMA (Gt, Ui) Ui=1 VSCRF-CDMA (Ui, G) Ui=1 TCS-IFDMA (Gt, Ui, G) G=1 Gt=1 CDMA IFDMA
158
7.2 Future Research
There are a variety of fruitful areas for future research on the design of new air interface for future broadband wireless packet access. Our current work on the performance analysis of the proposed new air interface is based on the assumption of time invariant channel across the BS period, i.e., the channel variation across the consecutive blocks is negligible. The performance analysis can be extended to the work where the channel variation across the consecutive blocks is not negligible.
Furthermore, our current work on the performance studies of the proposed air interface schemes is based on the assumption of perfect channel estimation. In practice, the system performance will be degraded due to the channel estimation error. Studying the effect of channel estimation and pilot design for the proposed new air interface scheme is another aspect for future research. In addition, due to the severe multi-user interference in the conventional CDMA system, the MIMO detection for multi-user CDMA system becomes complicated. However, with the help of BS, the proposed new air interface has removed MAI effectively. It is interesting to investigate the system performance employing MIMO techniques, such as space-frequency scheme for future evolution of CDMA system.
Lastly, our current work focuses on error probability analysis of the proposed air interface schemes. Another important measure that has not been investigated is the capacity of the proposed schemes. It is essential to determine the capacity of the systems under realistic broadband wireless channel.
Last but not least, we hope the work done in this thesis can spark more ideas on the new air interface for future broadband wireless packet access.
159
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