Research

SDMA

When multiple users simultaneously perform wireless communications, inter-user interference corrupts signals and prevents us from communicating with each other any longer. Conventional cellular systems have achieved such a simultaneous communication, i.e., multiple access, by FDMA, TDMA, and recently CDMA (see below). Any of these schemes, however, exploits time or frequency, which is a limited resource, to achieve multiple access. Therefore, they have their limits to improve system efficiencies.

On the other hand, with a smart antenna at the base station (receiver side), we can spatially separate multiple users; i.e., signals from interference users are reduced and that from the desired user is enlarged and picked out. Expanding this process to all users, we can achieve multiple access with the same transmission time and carrier frequency band. The concept of the system is illustrated below. The smart antenna at the base station controls amplitudes and phases at antenna inputs, i.e., optimally weight the inputs, so that it forms array directive patterns reducing interference to each other. Consequently, we can separate and detect signals from multiple users. The technique that achieves multiple access by exploiting spatial difference in users is called "space division multiple access (SDMA)."

SDMA
SDMAの概念図 Concept of SDMA.

As aforementioned, FDMA, TDMA, and CDMA are also known as conventional multiple access techniques. In the following, we would like to briefly explain them. Note that these techniques can be combined with each other to further improve the system performance such as user capacity and transmission rate.

FDMA

Frequency division multiple access, or FDMA, achieves multiple access by dividing the given frequency band and allocating it to users. Each user transmits a signal through its own sub-band. Therefore, it is relatively easy for the base station to separate and detect the signals.

TDMA

Time division multiple access, or TDMA, achieves multiple access by allocating multiple time slots to users. Each user transmits a signal in turn through its own time slot. Therefore, there is no inter-user interference.

CDMA

Code division multiple access, or CDMA, achieves multiple access by multiplying signals by orthogonal (or low-correlated) codes to give them redundancy. The code multiplication is called "(spectral) spreading" because it results in expansion of the signal frequency bandwidth. Although the base station receives multiplexed signals, it can detect the desired signal by multiplying the received signal by the correspondent code.

[Reference]

Y. Doi, J. Kitakado, T. Ito, T. Miyata, S. Nakao, T. Ohgane, and Y. Ogawa, "Development and Evaluation of the SDMA Test Bed for PHS in the Field," IEICE Trans. Commun., vol. E86-B, no. 12, pp. 3433-3440, Dec. 2003.

Y. Doi, S. Nakao, Y. Tanaka, T. Ohgane, and Y. Ogawa, "Development and Evaluation of a Smart Antenna Test Bed for Wireless LAN," IEICE Trans. Electron., vol. E87-C, no.9, pp.1449-1454, Sept. 2004.