Antenna Diversity

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Antenna diversity has long been promoted as a means of improving overall signal reception. Diversity is a technique that takes multiple observations of a signal coming off a transmit station, in order to recover that signal with greater accuracy.

Not all forms of diversity are the same. For example, dynamic diversity is optimized for low power consumption and high performance; this technique requires close attention to link-quality on the receive side for an effective implementation.

A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. A radio in "receive" mode will continually switch between antennae listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio evaluates the sync signal of the packet, on one antenna, then switches to the other antenna and evaluates. The radio then selects the best antenna for the remaining portion of that packet.

On "transmit," the radio selects the same antenna it used the last time it communicated to that given radio. If a packet fails, it switches to the other antenna and retries the packet. Thus a diversity antenna system can improve signal strength, by dissipating the negative effects of multipath loss

One caution concerning diversity—it is not designed to cover two different cells. If antenna #1 communicates to device #1, while device #2 (which is in the antenna #2 cell) tries to communicate, antenna #2 is not receptive (because of the position of the switch), and the communication fails. Diversity antennae should cover the same area, but from slightly different locations.

However, through antenna diversity, a receiving station obtains multiple observations of the same signal sent off a transmitting station. The redundancy built into the multiple observations can be used to recover the transmitted signal with a higher degree of accuracy at the receiver. It should also be noted that antenna diversity offers substantial benefits to a WLAN implementation, providing the luxury of more than one antenna and the ability to select the best antenna for usage.

While antenna diversity has many advantages, it also has its drawbacks. Antenna diversity requires more extensive signal processing, which in turn leads to increased power dissipation. Hardware overhead also increases, although a fair amount of circuits and building blocks can be shared among multiple signal paths.

Fortunately, advanced techniques such as dynamic diversity can optimize the power/performance trade-off and permit the technology to further penetrate into WLAN applications. But to properly implement dynamic diversity and achieve its full benefits, designers must perform a full and accurate characterization of the receive-side channel.

Dynamic diversity delivers several advantages: it provides a high throughput rate without excessive overall power consumption, it does not interfere with traditional transmitter-side link-enhancing techniques, such as transmitter power control and packet retransmission; and it maintains backward compatibility and interoperability with currently deployed wireless communication equipment, since it is a receiver-side-only enhancement. Moreover, the receive-side link-quality assessment techniques play a key role in realizing the dynamic-diversity concept. The dynamic-mode selection can be driven by various Physical Layer as well as Data Link Layer's MAC sublayer parameters that can indicate a change in the quality of the communication link.

Receive Modes

To understand dynamic diversity, you must understand that there are a number of receive modes.

• No diversity (single-antenna mode): This is an obvious case where there is only one receive antenna. It allows the simplest implementation and results in the lowest power consumption of all cases.

• Switched diversity: Only one receive antenna is chosen at any given time during reception, based on some prescribed selection criterion. The antenna connection is switched when the perceived link quality falls below a certain prescribed threshold.

• Selection diversity: One antenna is chosen, whose receive path yields the larger signal-to-noise ratio (SNR) or signal power. The SNR or signal-strength measurement can take place during a preamble period at the beginning of the received packet. So, a single antenna connection is maintained most times, but during the measurement of the SNR/signal strength, both antennae connections need to be established. The actual selection/switching process can also take place in between packet receptions, and can be done on a packet-by-packet basis or can take place once in a number of receptions or prescribed time period.

• Full diversity: Both antennae are connected at all times. Since both received paths must be powered up, this mode consumes the largest amount of power, but it also offers the largest performance gain compared with other configurations, especially in severe fading environments with large delay spread. The digital front-end techniques—signal detection, frame synchronization, and carrier frequency offset estimation/correction, for instance—can also benefit from the availability of multiple receive paths.

Link Assessment

The next item to consider when contemplating a diversity antenna set-up is the "link assessment." In a conventional indoor WLAN, the data rate of a given communications link is typically adjusted at the transmitter side, based on some measure of the successful packet transmission rate. As the channel condition worsens (for example, as the receiving station moves away from the transmitting station, or the antenna orientation changes in a mobile station), the link data rate is adjusted downward, since reliable communication at the initial rate is no longer feasible.

Dynamic diversity enables a higher-link rate in more adverse channel conditions than is possible in conventional systems, while keeping the transceiver/ modem components from consuming excessive overall power. In contrast to the conventional WLAN system based on transmitter-side link-quality assessment, dynamic diversity requires a receiver-side link-quality measure.

The state transition diagram in Fig. 19.4 illustrates one particular strategy that allows dynamic selection between full diversity and no diversity. As the link quality deteriorates, the receiver transitions from a single-antenna connection to the full-diversity mode. The transition back to a single-antenna connection is triggered by an indication of significantly improved link quality. When going back to a single-antenna configuration, a comparison of the received-signal strengths in the two antenna paths can easily lead to a preferable antenna connection. In this sense, the scheme of Fig. 19.4 also incorporates a "slow" form of selection diversity.

Figure 19.4: Dynamic diversity enables a higher link rate in more adverse channel conditions than is possible in conventional systems, while avoiding excessive overall power consumption by the transceiver/modem components.

A transition between diversity modes and antenna connections is signaled by a change in the perceived quality level of the link. The transmit-side link-quality assessment is typically based on the estimated dropped-packet rate, via the observation of the acknowledgement packet and the number of retries attempted. However, the link-quality assessment at the receiver side, as required for dynamic diversity, is based on any combination of three factors: SNR, modem-detection quality measure, and MAC-layer link-quality measure.

Another useful PHY Layer parameter that can lead to a good link-quality measure is a detection-quality measure (DQM). For example, the detection quality is reflected in an averaged magnitude of the soft decisions captured at the Viterbi detector input. Since the functional relationship between such a DQM and the bit error rate or the packet error rate can be obtained empirically, the DQM can drive the mode selection, the antenna selection, or both. No matter which method is adopted, the MAC Layer functions must always verify the Receiver Address field in the MAC header, to ensure the packets are intended for the receiving station under consideration.

Note 

The requirements for a Viterbi detector (also known as a Viterbi decoder) is a processor that implements the Viterbi algorithm (which is commonly used in a wide range of communications and data applications) depend on the applications where the detector is used. For instance, in addition to antennae, Viterbi detectors are used in cellular phones with low data rates (e.g. below 1 Mbps) and very low energy dissipation requirement. They also are used for trellis code demodulation in telephone line modems, where the throughput is in the range of tens of Kbps, with restrictive limits in power dissipation and the area/cost of the chip.

Another class of approaches to assessing link quality is that of MAC Layer parameters. One method applicable to WLANs is to examine the Retry Subfields in the MAC headers of the received packets, and observe the frequency of retried packets. As the frequency reaches a certain threshold, the antenna connection or the diversity mode can be changed in hopes of establishing a better link. The MAC Layer parameters are convenient, since the link quality can be assessed independent of the data rate or multipath effects.

A particularly efficient way of implementing dynamic diversity is to utilize both PHY and MAC Layer parameters. For example, the receiver can rely on the inspection of the Retry Subfields to sense degradation in the link quality, and signal a transition to the full-diversity mode. On the other hand, the reverse transition from the full-diversity mode to the simpler antenna setting can be triggered when the Received Signal Strength Indication (RSSI) level increases by some prescribed amount (which could be a data-rate-dependent value).

Diversity antenna systems are standard in many APs, because a diversity antenna system is the best way to reduce multipath loss. But although an antenna or even two antennae may be provided with an AP, as the reader now understands, different antennae provide different coverage patterns and detailed implementations vary widely. Thus the AP(s), antennae, and implementation techniques should be selected according to site coverage requirements. Furthermore, it is vital that the decision to use antenna diversity be made upfront, since that decision has an impact on how a site survey is performed.

For more information on antenna diversity read "Antenna Diversity Strengthens Wireless LANs," published in the January 2003 edition of CMP Media LLP's publication, Communications System Design. The article can also be found on the CommsDesign website at www.commsdesign.com/design_library/OEG20030103SOOS3.

For a good tutorial on antenna basics check out this website: www.ictp.trieste.it/^~radionet/1997_workshop/wireless/notes/sld027.htm.

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