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Today's wireless ICs require only a handful of external components and have become a minor percentage of the total solution cost. Looking into the future and with respect to what existing wireless applications have to offer, the need for additional features and performance has placed considerable constraints on battery capacity. This, in turn, has created the need for more advanced power management solutions. In today's applications, the battery and power management cost is close to 50% of the total bill of material. To address these new challenges, designers at Micrel have developed a new type of modulation called spread spectrum ASK/OOK. This new modulation scheme enables high output power via its spread spectrum technology with just 50% of the power consumption. In this design, we show how solutions like the MICRF405 dramatically reduce current consumption and cost.

A number of license-free frequency bands have been set aside for industrial scientific and medical (ISM) use by the Federal Communications Commission (FCC) in North America. The operation of these frequency bands is specified in FCC part 15.247. They cover the frequency bands 902 MHz to 928 MHz, 2400 MHz to 2483.5 MHz and 5725 MHz to 5850 MHz.

Since the introduction of wireless local area network (WLAN), the 2.4 GHz frequency band has experienced considerable success. Today, the frequency band is crowded with a wide range of applications ranging from RFID to WiMAX. Unfortunately, some applications, including those widely adopted in residential, building and industrial environments, have created a fairly high noise floor that makes the technology unsuitable for industrial, low power and long-range applications. For these applications, the 902 MHz to 928 MHz frequency band is considered to be the better-suited ISM band. To operate in this frequency band, some sort of frequency spread spectrum is required. Historically, there have been two types of spread spectrum.

Frequency-hopping spread spectrum (FHSS) refers to a transmission method where the data signal is modulated with a narrowband carrier signal that “hops” in a random but predictable sequence from frequency to frequency, as a function of time and over a wide band of frequencies. The signal energy is spread in time domain rather than chopping each bit into small pieces in the frequency domain. This technique reduces interference because a signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency and at the same time. The transmission frequencies are determined by a spreading, or hopping, code. In order to properly receive the signal, the receiver must be set to the same hopping code and must listen to the incoming signal at the right time and correct frequency. To operate in the 902 MHz to 928 MHz ISM band, FCC regulations require manufacturers use 25 or more frequencies with a maximum dwell time (the time spent at a particular frequency during any single hop) of 400 ms. The most significant disadvantage of frequency-hopping spread spectrum transmissions is the required frequency synchronization between the transmitter and the receiver. The frequency synchronization requirement results in a slow access time and higher-power consumption as the system needs to transmit on all channels to synchronize with the receiver.

Another form of spread spectrum transmission is referred to as digital modulation or direct-sequence spread spectrum (DSSS). It is a transmission method where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern, which as each bit is transmitted, increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, then the original data can be recovered due to the redundancy of the transmission. DSSS radios have a short access time since the channel is stationary. The disadvantage of a DSSS radio lies in its fairly complex demodulation scheme since the signal that is received requires de-spreading and synchronization.

Spread spectrum ASK/OOK

Operating within the 902 MHz to 928 MHz, 2400 MHz to 2483.5 MHz and 5725 MHz to 5850 MHz ISM bands — with high output power — has historically only been allowed by using either FHSS or DSSS technology. A few years ago, the FCC changed this regulation as more and more applications were electing to use the IEEE 802.11x specifications. This is when the demand for higher data rates started to escalate. To further increase the data rate, the industry proposed to remove the requirement for the digital processing gain of a receiving unit. This was accepted and the IEEE specification was then incorporated into the protocol description, thereby allowing automatic adjustments of the spreading code, depending upon the signal-to-noise ratio (SNR). These events have now been implemented in the new frequency regulations 15.247 and are now referred to as, “frequency hopping systems “and “digital modulated systems.”

The FCC classifies “digital modulated system” with the following definition: “Systems using digital modulation techniques may operate in the 902 MHz to 928 MHz, 2400 MHz to 2483.5 MHz and 5725 MHz to 5850 MHz bands. The minimum 6 dB bandwidth shall be at least 500 kHz.”

As more and more wireless applications on the market require ever higher out-put, power becomes more important. With “digital modulated systems” the maximum radiated output power is defined as 1 W. However, the maximum output power is more affected by the definition 15.247 d. For digitally modulated systems, the peak power spectral density conducted from the intentional radiator to the antenna shall not be greater than 8 dBm in any 3 kHz band during any time interval of continuous transmission.

This modification to the 15.247 ruling opens up new and interesting modulation possiblities that enable long-range wireless links using extremely low power. Micrel has recently filed for a patent on one such new modulation solution referred to as spread spectrum on/off keying (SSOOK) or spread spectrum amplitude shift keying (SSASK).

SSASK combines the traditional known ASK/OOK with a digital modulated signal. A typical block diagram of a transmitter is shown in Figure 1 and illustrates how the ASK/FSK transmitter MICRF405 operates in SSASK/OOK mode. The SSASK/OOK modulation is created by adding “user data” to an AM modulator and creating an amplitude shift or turning “on” and “off” a FSK modulated carrier signal. The FSK signal is generated by adding a PN sequence to the FSK modulator that is programmed to give an occupied bandwidth >500 kHz as specified by the FCC. The FSK data rate and the PN sequence are selected in a ratio giving as equal a peak density within the 6 dB bandwidth as possible. The result is a spread spectrum ASK/OOK spectrum as shown in Figure 2

The radiated spectrum and the peak density of a SSASK/OOK modulated spectrum is equal to a digital modulated system and, therefore, is considered by the FCC as a digital modulated system. The main benefit of this new modulation type lies in its low power consumption since it only transmits when sending a 1 and the ability to increase the output power without the need of a FHSS.

With SSASK/OOK, the user data information is present in the variations of the amplitude of the signal and enables the use of traditional ASK/OOK super-heterodyne receivers such as the MICRF005 (Figure 3). The SSASK/OOK signal received by MICRF005 will appear as a standard OOK/ASK modulated signal and will be demodulated accordingly.

SSASK/OOK application circuit

In the 902 MHz to 928 MHz band, ASK/OOK intentional radiators are required to implement FHSS when the application requires higher output power than 50 mV/m. By using the MICRF405 in SSASK/OOK mode, transmission with an output power of 10 dBm is achieved without the need of FHSS. The application circuit in Figure 4 consists of a matching circuit, crystal and decoupling capacitors. The maximum output power allowed by the FCC when using the MICRF405 as a SSASK/OOK device and an external power amplifier is +20 dBm.

ABOUT THE AUTHOR

Staale Pettersen is product marketing manager at Micrel, San Jose, Calif. Stalle is based in the Norway design office.