Showing papers in "IEICE Transactions on Communications in 1994"

Multi-Carrier CDMA in Indoor Wireless Radio Networks

Abstract: This paper examines a novel digital modulation/multiple access technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) where each data symbol is transmitted at multiple narrowband subcarriers. Each subcarrier is encoded with a phase offset of 0 or π based on a spreading code. Analytical results are presented on the performance of this modulation scheme in an indoor wireless multipath radio channel. Introduction This paper examines the performance of a new spread spectrum transmission method called “MCCDMA” in an indoor wireless environment. MC-CDMA may be a suitable modulation technique in the indoor environment where the dispersive character of indoor propagation [1] allows for the exploitation of this technique. With MC-CDMA, each data symbol is transmitted over N narrowband subcarriers where each subcarrier is encoded with a 0 or π phase offset. If the number of and spacing between subcarriers is appropriately chosen, it is unlikely that all of the subcarriers will be located in a deep fade and consequently frequency diversity is achieved. As an MC-CDMA signal is composed of N narrowband subcarrier signals [2] each with a symbol duration, Tb, much larger than the delay spread, Td, an MC-CDMA signal will not experience significant intersymbol interference (ISI). Multiple access is achieved with different users transmitting at the same set of subcarriers but with spreading codes that are orthogonal to the codes of other users. Basic Principles of MC-CDMA The generation of an MC-CDMA signal can be described as follows. As shown in Fig. 1, a single data symbol is replicated intoN parallel copies. Each branch of the parallel stream is multiplied by one chip of a spreading code of length N and then binary phase-shift keying (BPSK) modulated to a subcarrier spaced apart from its neighboring subcarriers by F/Tb Hz whereF is an integer number. The transmitted signal consists of the sum of the outputs of these branches. For F = 1, this scheme is similar to performing Orthogonal Frequency Division Multiplexing (OFDM) [3] on a Direct-Sequence spread-spectrum signal [4]. Recently, there has been a growing interest on idea of combining OFDM and DS-CDMA [5] [6] [7] [8] [9]. Modern DSP methods make the implementation of MC-CDMA feasible and attractive. With F = 1, the transmit bandwidth is minimized. However, larger values of F may be desired to further increase the transmit bandwidth, i.e., to achieve a larger frequency diversity gain, without increasing the complexity in signal processing due to large spreading factors,N. The transmitted signal corresponding to the kth data bit of themth user ( am[k] ) is (1) sm t ( ) cm i [ ] am k [ ] 2πfct 2πi F Tb t + ( ) Tb t kTb − ( ) cos i 0 = N 1 − ∑ = cm i [ ] 1 1 , − { } ∈

Spectral efficiency improvement by base station antenna pattern control for land mobile cellular systems

Abstract: The paper proposes use of an adaptive array base station in order to increase spectral efficiency without decreasing cell radius. The base station antenna controls its directivity pattern to reduce co-channel interference by applying an adaptive array technique. Computer simulation results show that the cluster size can be reduced to one, indicating that one can reuse the same frequency group at all cells. Thus, the improvement in spectral efficiency is as much as 16 times as that of an omni-antenna. Moreover, load sharing, which is expected to improve channel utilization for unbalanced load situations, is available by cell overlapping. >

Buffer Sharing in Conflict-Free WDMA Networks

Abstract: A wavelength division multiaccess network with buffer sharing among stations is studied. All stations in the network are connected to a passive optical star coupler and each station has a different fixed wavelength laser for transmitting packets. Each station in the network reports its packet backlog to a scheduler which computes and then broadcasts a transmission schedule to all the stations through a control channel in each time slot. A transmission schedule includes two types of assignments: (1) to assign a maximum number of stations for conflict-free transmissions, and (2) to assign to relocation of packets from congested stations to uncongested relaying stations through idling transceivers for distributed buffer sharing. The major benefit is the reduction of packet loss due to buffer overflow. Results show that as much as 75% of the buffers can be saved with buffer sharing. >