Wan Choi

Distributed antenna systems (DAS) have been widely implemented in state-of-the art cellular communication systems to cover dead spots. Recent academic studies have shown that in addition to coverage improvements, DAS can also have potential advantages such as reduced power and increased system capacity in a single cell environment. This paper analytically quantifies downlink capacity of multicell DAS for two different transmission strategies: selection diversity (where just one or two of the dis

The capacity and robustness of cellular MIMO systems is very sensitive to other-cell interference which will in practice necessitate network level interference reduction strategies. As an alternative to traditional static frequency reuse patterns, this paper investigates intercell scheduling among neighboring base stations. We show analytically that cooperatively scheduled transmission, which is well within the capability of present systems, can achieve an expanded multiuser diversity gain in te

In this paper, a novel transmission technique for the multiple-input multiple-output (MIMO) broadcast channel is proposed that allows simultaneous transmission to multiple users with limited feedback from each user. During a training phase, the base station modulates a training sequence on multiple sets of randomly chosen orthogonal beamforming vectors. Each user sends the index of the best beamforming vector and the corresponding signal-to-interference-plus-noise ratio for that set of orthogona

Some studies have been done on capacity of a code division multiple access (CDMA) system with mixed multirate sources. However, a vast majority of these studies have concentrated on the reverse-link. This trend comes from the fact that the capacity of a CDMA system is reverse-link limited. However, the forward-link can be a limiting link because emerging data services are likely to require higher data rates in the forward-link than in the reverse-link. In this paper, we analyze and simulate the

Even though spatial multiplexing provides a significant spectral efficiency advantage in a single point-to-point noise- limited link, recent studies have shown that this advantage can be lost in cellular MIMO systems unless extra diversity is provided. Spread spectrum is a likely candidate for the extra diversity because spread spectrum can simultaneously provide frequency diversity and robustness to interference. This paper investigates the effectiveness of spatial multiplexing in the forward l

This paper investigates the effects of co-channel interference on spatial diversity techniques. By analyzing distributions of post-processing signal to noise plus interference ratio (SINR) after matched filtering, we capture the performance characteristics of spatial diversity techniques in an interference limited environment. Using intuition from the theoretical analysis of a single carrier system, the performance of spatial diversity techniques in an OFDMA system with co-channel interference i

This paper proposes an adaptive multi-node incremental relaying technique in cooperative communications with amplify-and-forward (AF) relays. In order to reduce the excessive burden of MRC with all diversity paths at the destination node, the destination node decides if it combines signals over the first N(<; K) time slots/frames or over all of the K times slots, where K is the number of relay nodes. Our analytical and simulation results show that the proposed adaptive multi-node incremental rel

Distributed antenna systems (DAS) are being widely considered for state-of-the art cellular communication systems to cover dead spots. Recent academic studies have shown that in addition to coverage improvements, DAS can also allow a capacity improvement. This paper provides a generalized information theoretic analysis of downlink multicell DAS for two different transmission strategies: selection diversity (where just one of the distributed antennas are used) and blanket transmission (where all

Improving the capacity of code-division-multiple-access (CDMA) systems through advanced signal processing has been an area of intensive research for many years, with limited success. Multiantenna technologies called multiple-input multiple-output (MIMO) are an obvious candidate to increase, particularly, downlink capacity. Nearly all research on MIMO-CDMA, however, has focused on increasing the throughput achieved per user, rather than increasing the number of supportable users, which is still t

As an alternative to traditional static frequency reuse patterns, this paper investigates cooperatively scheduling among neighboring base stations in a cellular multiple antenna system, where each cell adopts dirty paper coding. It is shown that cooperatively scheduled transmission can achieve almost the same amount of interference reduction as conventional frequency reuse and achieve an extra capacity gain. We analytically quantify the capacity gain of cooperatively scheduled transmission over

We propose an automatic on-off switching (AOS) repeater that is switched off automatically when there is no active user within its coverage. With the AOS repeater, we can reduce the unnecessary noise enhancement. The reverse link capacity of a DS/CDMA system with AOS repeaters is analyzed mathematically and compared with that of a system with conventional repeaters. Also, the AOS circuit in a repeater can protect the reverse link capacity of a DS/CDMA system from excessive noise enhancement by a

In this paper, the effectiveness of spatial multiplexing in the forward link of cellular CDMA systems with linear (i.e. low complexity) receivers is investigated. General MIMO systems without spreading are a special case of our analysis when the spreading gain is unity. Through the development of new closed-form results on outage probability and capacity for MIMO-CDMA, we show that cellular MIMO outage capacity is severely degraded by the enhancement of other-cell interference by the linear spat

Direct Sequence Code Division Multiple Access (DS-CDMA) receivers typically use Maximal Ratio Combining (MRC) to favorably combine the energies of distinct multipath components from diversity branches. In previous research, compromising assumptions have been made to simplify the analysis, including a Gaussian approximation for interference or constant equal cross correlations. However, these assumptions corrupt the analysis especially in certain operating conditions such as a relatively small nu

The capacity and robustness of cellular MIMO systems is very sensitive to other-cell interference, which will in practice necessitate network level interference reduction strategies. As an alternative to traditional static frequency reuse patterns, this paper investigates cooperatively scheduled transmissions among neighboring base stations. We analytically show that cooperatively scheduled transmission, which is well within the capability of present systems, can achieve an expanded multiuser di

We analyze the forward link Erlang capacity of the 3rd generation CDMA system with voice and data users. The forward link outage condition for 2G CDMA system is extended to the case of the 3rd generation CDMA system with voice and data users. By introducing a forward power control factor, we derive the forward link blocking probability and Erlang capacity with closed form.