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As microcell wireless systems become more widespread, intercell interference among the access points will increase due to the limited frequency resource. In the overlapping cell scenario, radio resources should be shared by multiple cells. Although time and frequency resource sharing has been described in many papers, there is no detailed report on dynamic spatial resource sharing among multiple cells for microcell wireless systems. Thus, we present the effectiveness of spatial resource sharing among two access points. We introduce two scenarios based on the zero forcing method; one is the primary-secondary AP scenario and the other is the cooperative AP scenario. To evaluate the transmission performance of spatial resource sharing, channel matrices are measured in an indoor environment. The simulation results using the measured channel matrices show the potential of spatial resource sharing.

The rapid increase in data traffic has led to strong demands for large-capacity transmission systems. Wireless local area networks (WLAN) and cellular systems are two major wireless access systems that are being targeted to achieve even higher data rates. Orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO) have been recognized as effective ways to attain higher throughput for IEEE 802.11n [

The above trends bring new challenges for wireless technologies because both the larger bandwidth and the larger number of wireless devices may cause significant shortages in the frequency channels. One example is the ongoing standardization for very high throughput (VHT) wireless LANs in bands below 6 GHz, that is, IEEE802.11ac, which is expected to be the next standard after IEEE802.11n. It has been virtually agreed that IEEE802.11ac will double the maximum bandwidth of IEEE802.11n from 40 MHz to 80 MHz to achieve 1 Gbps throughput at the median access control (MAC) service access point (SAP) [

In the overlapping cell scenario, radio resources should be shared by multiple cells. There are three resources to be shared, time, frequency, and spatial resources. The first one, time resource sharing is realized by carrier sense multiple access collision avoidance (CSMA/CA) or request to send/clear to send (RTS/CTS) protocols [

Compared to time and frequency resource sharing, spatial resources sharing research is relatively immature especially for the overlapping cell scenario. MIMO technology which uses spatial signal processing is employed by the latest wireless access standards such as 3 G long term evolution (LTE) [

In this paper, two scenarios (both based on the zero forcing method) for spatial resource sharing are considered. One is the primary-secondary AP scenario and the other is the cooperative AP scenario. To evaluate the effectiveness of these methods, a channel state information measurement experiment is conducted in a large office and

This paper is organized as follows. Section

Throughout the paper, superscript * and superscript

We consider a downlink MIMO system for a single AP and STA in the overlapping cell scenario. To evaluate the effectiveness of spatial resource sharing, we compare the two overlapping cell scenario to the single cells scenario using time resource sharing. In this section, we describe the channel conditions and two scenarios of spatial domain resource sharing; the primary-secondary AP scenario and the cooperative AP scenario. Furthermore, we focus on the CSI error and show the relationship between CSI and ICI.

It is assumed that the number of APs is two, each hosts

When spatial resource sharing is active, the two APs can transmit in the same time slot. The received signal vectors at STA

As the transmission weight for the time resource sharing scenario, we employ eigenvector transmission [

When there are two APs using the same frequency and the same time slot, the interference from the other AP degrades the transmission performance. Thus, we consider the transmission weight that prevents the interference to the other AP. In AP

The CSI is obtained at the transmitter and the receiver. We assume that the STA holds the perfect CSI while the CSI at the AP has some estimation error. The channel matrix obtained at AP

When two APs transmit signals in different time slots, that is, time resource sharing systems, the mutual information between AP

We describe the channel capacity in a spatial resource sharing system with two APs as

In the cooperating mode, both APs uses the transmission weight based on ZF. This case is expected to yield the high capacity among two APs although the signal powers at the destination STAs are degraded due to ZF beamforming. The achievable transmission bit rate in CZ method is given as

In spatial resource sharing, the ICI determines the channel capacity in (

Then, the ICI power for ZF weight,

Intercell interference power versus _{.}

Experimental environment.

We measured the channel matrices in an actual indoor environment using the testbed for MIMO-OFDM transmission. The measurement environment is the

In the experiment, we used a circular array of eight antennas with element spacing of 1.0

A block diagram of the experimental testbed is shown in Figure

Parameters of MIMO-OFDM testbed.

Number of antennas | 8 (Tx), 4 (Rx) |

Radio frequency | 4.85 GHz |

Bandwidth | 20 MHz |

Total transmission power | 10 dBm |

Sensitivity | |

Sampling rate | 20 MHz (A/D), 80 MHz (D/A) |

Number of FFT points | 64 |

Number of subcarriers | 48 (Information) + 4 (Pilot) |

Short preamble length | |

OFDM symbol length | 3.2 |

Block diagram of the MIMO testbed.

SNR distribution in an experimental environment corresponding to (a) AP 1 and (b) AP 2.

In this experiment, a virtual wall is considered between the AP 1’s STAs and AP 2’s STAs. Thus, we define the interference channel matrices as

First, we consider that

Figure

Median value of achievable transmission bit rate versus INR for

Figure

Figures

Median value of achievable transmission bit rate versus INR for

The achievable transmission bit rate is calculated using 2 (AP positions)

CDF of achievable bit rate using measured

CDF of achievable bit rate using measured

Figures

Median value of achievable bit rate using measured

Median value of achievable bit rate using measured

In

Although Figures

We evaluated the transmission performance in spatial resource sharing. This paper introduced the primary-secondary AP scenario based on zero forcing (PSZ) and cooperative AP scenario based on zero forcing (CZ). Simulation results clarify the difference of applicability domain of these scenarios and the achievable transmission bit rate improvement compared to time domain resource sharing scenario. By using the channel matrices measured in an actual indoor environment, CZ and PSZ methods showed that they yielded 1.59 (1.90) times and 1.38 (1.51) times the achievable transmission bit rate of the time domain resource sharing system for

The authors thank Mr. Kazuyasu Okada, the Executive Manager of NTT Network Innovation Laboratories, for his constant encouragement. The authors would also like to thank Mr. Yoshinobu Makise and Mr. Masaaki Ida of NTT Advanced Technology Corporation for their support.