WLAN Standards and System Analysis

Currently, there are a number of frequency bands allocated for WLAN applications and standards. Our target frequencies for this implementation are the bands of 2.4 GHz (ISM) and 5.2/5.8 GHz (UNII/ISM) currently being used for the majority of employed WLAN standards. It should be noted that the description of WLAN standards in this section is merely intended for a comprehensive design cycle, from specifications to implementation. Clearly, the concurrent multi-band design can also be used for other applications with different specifications.

The Institute of Electrical and Electronics Engineers (IEEE) has developed technology specifications for both frequency bands under the family of 802.11 standards. IEEE 802.11 standards are mainly intended for high-speed wireless local-area networks (WLAN) applications. Some of the heavyweights in the telecommunications industry introduced another standard at 2.4 GHz called Bluetooth which is mainly intended for short-range and low data-rate communications. This results in very low power radios that, for instance, can connect home appliances and devices in a network. The European Telecommunications Standards Institute (ETSI) has made standards under the family High Performance Radio Local Area Networks (HIPERLAN), Broadband Radio Access Networks (BRAN), at the 5 GHz frequency band intended for high-data rate multimedia LANs such as High Definition TV (HDTV). Table 5.1 summarizes some of the specifications of these standards that mainly set the requirements for radio design and architecture. As the table indicates, the radio requirements for these standards, as intended for short-range applications, is more relaxed than other standards such as the ones used in cellular phones. This will allow for low-power implementations of the radio as will be seen later in this chapter.

To gain a better understanding of standards and our target performance numbers, we will show how the numbers in the table are translated into radio specifications. In particular, in the design of radio receivers, dynamic range and selectivity are among the most important specifications.

Dynamic range is the difference between the maximum and maximum signal levels the

receiver can handle without exceeding the intended bit-error-rate (BER). The maximum input signal level, directly mentioned in standard, is limited by receiver nonlinearities. The minimum level, known as sensitivity, is mostly limited by multiple sources of noise in receiver. It can be shown (e.g., [99]) that the receiver’s overall noise-figure (NF) has to be limited to the following bound

NF[dB] ≤ sensitivity[dB] – 10log[C/(I+N)] - 10log(kBTB) (5.1) In (5.1), sensitivity is usually given as a part of radio specifications; C/(I+N) is the ratio of carrier signal power to the combined power of interference and noise that for any given modulation can be derived from the required BER specification; and kBTB is the total

Table 5.1: Summary of a few wireless networking standards at 2.4 GHz and 5 GHz

Additionally, the receiver should be able to detect the signal in the presence of out-of- band strong interferences, referred to as blockers. The ability to recover the weak narrow- band signal among all the blockers is a measure of receiver selectivity. The maximum level of blockers that might be present in the receiving environment is a function of frequency and is often specified for the given application. Receiver selectivity is limited by nonlinearities and another phenomenon known as reciprocal mixing, the result of the down- conversion of the blockers with oscillator phase-noise into the intended signal bandwidth (Chapter 3). Hence, there will be an upper bound on the local oscillator phase-noise at certain offsets determined by such requirements on the blocker rejections (e.g., [99])

Band Name ISM UNII, HIPERLAN

Frequency 2400 – 2483.5 MHz 5250 ± 75 MHz 5250 ± 100 MHz

Standards Bluetooth 802.11b HiperLAN/1 802.11a

Multiple Access FHSS,TDD DSSS, FHSS NPMA OFDM

Modulation GFSK (BT=0.5) DPSK, CCK GMSK (BT=0.3) BPSK, QPSK, QAM (16, 64) Sensitivity -70dBm -76dBm -70dBm -82 .. -65dBm

Max. Input Level -20dBm -10dBm -20dBm -30dBm

Channel BW 1MHz 5MHz 23.5MHz 16.6MHz

Max. Data Rate 1Mbit/s 11Mbit/s 23.5Mbit/s 54Mbit/s

Max. Output Power (mW) 100 (class 1) 1000 (USA) 100 (Europe) 1000 40 (5.15-5.25 GHz) 200 (5.25-5.35 GHz) Range 100m 100m 100m (?) 50m

Application Short-range cable replacement

High-speed WLAN

High-speed

multimedia LAN High-Speed WLAN

DSSS: Direct Sequence Spread Spectrum FHSS: Frequency Hopping Spread Spectrum TDD: Time Division Duplex

NPMA: Non-preemptive Priority Multiple Access OFDM: Orthogonal Frequency Division Multiple Access

BPSK: Binary Phase Shift Keying QPSK: Quadrature Phase Shift Keying QAM: Quadrature Amplitude Modulation GFSK: Gaussian Frequency Shift Keying GMSK: Gaussian Minimum Shift Keying DPSK: Differential Phase Shift Keying CCK: Complementary Clock Coding

PN(foffset)[dBc/Hz] ≤ C[dBm] – 10log[C/(I+N)] – I[dBm] – 10log(B) (5.2) In (5.2), C is the signal level that has to be recovered in the presence of interference level I at frequency foffset.

The target receiver dynamic-range, NF, and VCO phase-noise of the standards in Table 5.1 are derived in Table 5.2. For comparison purposes, the numbers for the more stringent GSM standard for mobile-phone communication are also mentioned in Table 5.2.

Table 5.2: Receiver target numbers for standards in Table 5.1

Once again, it should be noted that the implementation of the concurrent receiver is most valuable in demonstrating the new concept than in satisfying the application-specific radio requirements. Nonetheless, Table 5.2 provides realistic performance goals in designing the radio.