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Home Office Sat-Receiver
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- Server Server 2 Server 1 a) Network Level 1: Single AV-Link b) Network Level 2: Single Home Network
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Sat-Receiver Kid’s Room Living Room Kitchen DVD/VCR Receiver DSL Access Point PC PC Server Figure 5.1: Typical wireless network in a detached house Optimization can start by selection of suitable parameters for each individual link. The task becomes more demanding if several links are operated in parallel. It can be expected that such network concepts will be introduced to many households. This fact makes the competition for scarce resources more demanding, because a limited number of wireless channels has to be shared between neighboring logical networks. 5.1 Link Adaptation 167 Figure 5.2 shows the levels of system organization. The final goal of link adaptation will be the best selection of parameters for all links such that overall efficiency will be maximized. Server Server 2 Server 1 a) Network Level 1: Single AV-Link b) Network Level 2: Single Home Network c) Network Level 3: Interfering Home Networks Figure 5.2: Levels of system organization As already mentioned, OFDM based transmission is very well suited to optimize the transmission efficiency for each link. In order to discuss these aspects, some basic parameters should be defined. A parameter set given by the OFDM transmission modes of standardized wireless LAN (IEEE 802.11a/g) is a good platform. We therefore use it as a starting point for the discussion of the general aspects of link adaptation in a multiple link environment as given by our system example. Basic transmission parameters of WLAN in Europe are [1]: • channel bandwidth: 20 MHz • transmission frequencies: 2.4 - 2.483 GHz, 5.15 - 5.35 GHz, 5.47 - 5.725 GHz, 4 + 19 non-overlapping channels • total number of carriers: 53 (48 data carriers + 4 continuous pilots, center carrier set to zero amplitude) • net symbol duration: 3.2 microseconds, yielding a carrier spacing of 312.5 kHz • guard interval: 0.8 microseconds • modulation scheme: BSPK, QPSK, 16-QAM, 64-QAM • forward error correction: convolutional code, constraint length 7, rate 1/2, puncturing for rate 2/3 and 3/4 in certain standardized transmission modes Regulation with respect to allowed transmit power and power density slightly differs between frequency bands and countries. Limits for the 2.4 GHz band in Europe may serve as a reference: Maximum equivalent isotropically radiated power (EIRP) is 20 dBm, maximum power density is 10 mW/MHz. Efficient link adaptation requires knowledge about the channel, e.g., transfer func- tion or packet transmission statistics. In order to simulate and discuss the options, |