2.4 Error Model for the OFDM Transmission applied in the 5 GHz Unlicensed Band						13
	synchronization			OFDM symbols
	PREAMBLE	SIGNAL		DATA
		basic		variable PHY mode
	PHY mode

Figure 2.3: 802.11 Burst. The signal field is neglected in the error calculation. See Figure 3.3 for more details.

2.4.1Interference Calculation

The numerical analysis presented in Section 2.4.3 requires a C/(ΣI+N) value for the decision about correct or erroneous reception of frames. This value is calculated as follows. The value of C/(ΣI+N) is lower than for the interference free burst when interfering (alien) bursts superpose with the original burst. The original, received burst arrives with signal level C, whereas other bursts contribute to ΣI. Figure 2.4 shows the way of calculating the cumulative interference power. The received power of the burst incoming with the carrier power level is stored in the receiver instance for the length of its transmission. Other bursts that arrive during this time will contribute to the interference power.

received frame

three

interfering

preambles

alien frame

bursts

interfering

alien frame

	power	received power "C"
		noise
	Rx

Rx power		time
	interference levels
Rx power	cumulated	time
	interference "Σ Ι"
	preamble	time
	interference

Figure 2.4: (Mangold et al., 2001f) The simplified interference model. A cumulative interference plus the noise is used for the preamble interference and frame interference calculations. A successful frame reception requires individual minimum C/(ΣI+N) values, which can be different for synchronization (preamble reception) and frame reception.

14	2. Wireless Communications in Unlicensed Bands

Overlapping bursts transmitted at the same time is a typical case when two or more stations try to access a random access channel at the same time in HiperLAN/2 scenarios.

Further, 802.11 stations will transmit at the same time because of the contentionbased protocol. Overlapping bursts occur also if 802.11a and HiperLAN/2 operate at the same frequency channel. The case where the original burst arrives later than multiple interfering bursts or signals is also considered.

Assuming the original burst’s received signal strength is high enough, compared to the signal strength of the interfering burst; it can correctly be decoded even if the receiver was currently receiving another burst.

The calculation of the received powers is based on the distance d the signal traveled, the actual transmitter power level PTx, the frequency fc it was sent at and by assuming certain OFDMand environmental conditions. The following equation can be used for the path loss calculations between the stations:

								c			2
P		g		g
			Tx		Rx
	Tx
							fc 4π
PRx =									2
								c
PTx gTx gRx
							f 4π
							c

	,d ≤1m			(free space),
		1		(2.3)
			,d >1m	(multi-path).
		d γ

In this equation, the antenna gains gTx and gRx are 1 and the path loss coefficient γ = 3.5, respectively. The cumulative interference power is calculated as the maximum interference power during one received burst by summing up the signal power of all interfering bursts, even if they do not contribute during the complete reception interval.

The interference level that is used for bit error calculations is derived from the simple model. It is not taken into account that in some cases only some of the consecutive OFDM symbols of a received burst may be distorted by a short interfering burst. Using the instantaneously updated cumulative interference level ΣI, the C/(ΣI+N) ratio at the receiver antenna is calculated. If the received C/(ΣI+N) throughout the burst duration is above a predefined level, the receiver sensitivity, then it is assumed that the frame can be decoded. It is then evaluated according to the C/(ΣI+N) for the preamble and the frame, separately.

If another burst arrives during reception of a burst then it contributes to the interference power level, which means that the C/(ΣI+N) for the original burst must be re-evaluated. In this case, a receiving station considers the new burst as interference. It waits for the rest of the original burst to arrive and then calculates

2.4 Error Model for the OFDM Transmission applied in the 5 GHz Unlicensed Band 15

the frame error probability with the error calculation discussed in Section 2.4.2. In this calculation, the value Eav/N0 relates to C/(ΣI+N).

The following assumptions are made for the error calculations explained in Section 2.4.2:

•The maximum excess delay of the radio channel is always smaller than

Tg. Tg=400 ns or Tg=800 ns => ISI=ICI=0.

•The background noise and interferences is interpreted as Additive White Gaussian Noise (AWGN), neglecting the fact that interfering signals might well be correlated to the original received signal.

•The energy of a data symbol is evenly spread over all sub-carriers all bits transmitted as part of one OFDM symbol observe the same Eav/N0.

•The radio channel behaves like the WSS channel with constant received power over the burst duration of all bursts.

2.4.2Error Probability Analysis

The rest of the error calculation is based on the Qiao-Choi transmission error probability analysis (Qiao and Choi 2001). This analysis is summarized in Appendix C, p. 225.

2.4.3Results and Discussion

The following figures show the results of the analysis presented in Appendix C. Figure 2.5 shows the Bit Error Ratio (BER) vs. Eav/N0 for the four linear modulation modes used in HiperLAN/2 and 802.11, comparing single carrier transmission and OFDM multi-carrier transmission. With OFDM, the BER is slightly larger than without, which is obviously due to the introduction of the guard interval in OFDM. With only a small degree of multi-path, there is not much gain from the guard interval, since it is assumed that the maximum excess delay of the radio channel is always smaller than Tg.

Figure 2.6 presents the results for the Packet Error Ratio (PER) vs. Eav/N0 for a frame length of 54 byte, the size of a HiperLAN/2 data frame, assuming OFDM multi-carrier transmission. It can be seen that BPSK3/4 shows roughly the same performance as QPSK1/2, indicating that selecting BPSK3/4 for operation is never the optimal choice. However, the error analysis is based on the AWGN assumption. BPSK3/4 can become more useful than QPSK1/2 in realistic scenarios with multi-path channels. Another reason to select the BPSK3/4 mode although it shows the same error performance as QPSK1/2 is the longer duration required for transmission of data frames, which can be advantageous in resource sharing scenarios.

16	2. Wireless Communications in Unlicensed Bands

In Figure 2.7, the PER is shown for different frame lengths. In 802.11, an arbitrary frame length of up to 2304 byte not including MAC and PHY headers is allowed. Ethernet feeds the MAC with a data packet of a maximum length of 1514 byte. Typically, long frames are fragmented into shorter frames before transmission in 802.11, mainly in order to increase the utilization of the radio channel.

The resulting PER vs. Eav/N0 for a frame length of 54, 512 and the maximum size of 2304 byte are shown. As expected, the PER increases with the frame length. Using the results of the analysis presented here, the simulation tool WARP2 works with an error model that models many important radio effects. However, it remains to be investigated how to model the multi-path in office and outdoor scenarios, where the maximum excess delay of the radio channel generally exceeds the guard interval of the OFDM symbols. Results presented in Khun-Jush et al. (1999) indicate that the results that have been calculated here are optimistic for typical office scenarios. In general, with ISI and ICI because of multi-path, a better C/(ΣI+N) is required, however, with the same dependencies between the different PHY modes as presented here.

The analytical approach discussed here does not perfectly represent the real life error characteristics. However, the model takes into account all relevant effects such as the channel characteristics, OFDM parameters, frame lengths, preambles, and modulation and coding schemes, where some necessary simplifications are made. Specifically, the model relies on the AWGN assumption.

Figure 2.5: (Mangold et al., 2001f) BER vs. Eav/N0 for the linear modulation modes of interest, i.e., BPSK, QPSK, 16QAM, and 64QAM. A guard interval of 800 ns is assumed for the underlying OFDM. HiperLAN/2 optionally allows the use of a guard interval of 400 ns.

2.4 Error Model for the OFDM Transmission applied in the 5 GHz Unlicensed Band 17

This way of calculating transmission errors in OFDM transmission schemes with adaptive modulation and coding rates intends to support fast calculations of frame transmission, by allowing to simulate hidden stations, interferences, possible failed synchronization, and signal capture-effects. The channel models have been implemented in the SDL-based simulation environment WARP2, which is capable of accurately modeling the two radio transmission protocols, HiperLAN/2 and 802.11a.

Figure 2.6: (Mangold et al., 2001f) PER vs. Eav/N0 for a frame length of 54 byte. The PHY mode 64QAM1/2 is not part of 802.11a or HiperLAN/2.

arrows indicate

increasing

frame size

Figure 2.7: (Mangold et al., 2001f) Influence of the frame length on PER in 802.11. The resulting PER vs. Eav/N0 for a frame length of 54 byte, 512 byte and the maximum size of 2304 byte are shown. BPSK3/4 is not shown as it overlays with QPSK1/2. The PHY mode 64QAM1/2 is not part of 802.11a or HiperLAN/2.