Subsystem design key to wireless gaming

More and more users want a portable experience that enables them to play interactively with other gamers. While multiple-player gaming has been around for years, it has been limited because of the need for a wired connection between players.

If gamers are truly to embrace wireless gaming, however, it must offer an exper ience that equals or exceeds the wired experience. A best-case scenario would support up to five players, provide rapid interactivity and have ample battery power-no gamer is going to put up with the game dying in the middle of NBA Live as he makes the winning basket.

Another challenge is the growing ubiquity of wireless connectivity. As more wireless devices enter the world, there is more and more chance for interference. Successful wireless gaming will require devices that keep on working despite the presence of another wireless device in the vicinity.

As gaming manufacturers look to add wireless functionality, they need to think about whether they should adopt a standard such as Bluetooth or build a proprietary solution. The use of a standard makes it possible to play games across platforms and offers the ability to interact with other wireless devices. For the manufacturer, however, standard technologies may not offer the speed, latency and high-resolution graphics capabilities that their users demand. More important, standard technologies may not provide them with the competitive differentiation required to keep or move their device in market leadership. And since gaming, like any market, is price-sensitive, the manufacturer must make sure that any wireless functionality comes at a reasonable price.

As a result of all these factors, many manufacturers have decided to work with vendors to develop proprietary wireless connectivity. One example of a ground-up collaboration for a custom solution took place recently between Motorola and Nintendo for the Game Boy and Game Boy Advance.

The Motorola chip set in the newly announced Nintendo wireless adapter is a total solution operating in the 2.4-GHz ISM band and consists of a custom baseband device, a transceiver and a custom printed antenna based on a new design. It was was engineered from the ground up to meet the specific needs of Nintendo and with portable gaming in mind. The primary design considerations were low cost, low lat ency and low power consumption, making it suitable for long play time on two AA batteries.

The chip set is an amplitude-modulation link implementing a proprietary time-division multiple-access (TDMA) protocol customized for Nintendo. It is not based on any industry standard.

Baseband device

Implemented in 0.18-micron CMOS, the baseband device provides Nintendo with a custom interface, general link control and a number of proprietary functions to enhance the game play experience.

At the heart of the baseband integrated circuit is a large link control block that performs much of the link control in hardware. The processor is Motorola's 32-bit M-Core RISC engine. Special attention has been paid to keep the power consumption to a minimum. This includes utilizing the M-Core's stop and doze modes as well as carefully controlling the RF IC to ensure minimal transmit and receive times. The baseband device provides the clock and enables signals and data to the RF IC.

The link is executed on a proprietary TDMA protocol for more predictable game data transmission timing. The protocol has a 16-millisecond frame, with information being transferred in a 2.8-ms subframe window. Over this structure, data is transmitted with Manchester encoding and data scrambling. For mitigation of multipath effects, a two-frequency-hopping algorithm has been implemented. The algorithm remains at 2.426 or 2.456 GHz until a packet is lost, at which time it shifts all transmissions to the alternate frequency.

Coexistence with interferers is assured by a combination of retransmission and the use of the two frequencies. Performance testing shows less than a 5 percent packet transmission degradation in the face of Bluetooth or 802.11b links performing large file transfers.

Simple, fast transceiver

The transceiver is Motorola's MC13190, a very simple, very fast AM radio. With a digital logic interface, the device receives and delivers Manchester-encoded data. The receive p ath results in a typical receiver sensitivity of -71 dBm. The transmit path results in typical power out of +4.8 dBm.

The transceiver is implemented in Motorola's 0.35-micron CDR1 RF BiCMOS technology. With an over-the-air data rate of 4.798 Mbits/second, the transceiver is able to transmit the game data packets in microseconds. It is this high-speed transmission that enables both low-latency and low-power performance. (Specific figures for latency are held by Nintendo.)

The low-latency performance is attained by combining the high-speed transmission of the transceiver with the transmission protocol of the baseband IC. Transmissions are broadcast from the "parent" game-hosting Game Boy Advance or Game Boy Advance SP wireless adapter unit to as many as four "child" wireless adapter units. The children use this broadcast to trigger return transmissions, resulting in a very short communication cycle among as many as five units.

Turning off the transceiver and related baseband IC blocks w hen not specifically transmitting or receiving data packets helps to achieve the low-power performance. With a turn-on time (off to transmitting a sync word) of 32 microseconds, the transceiver can remain off for a large percentage of the game play duration. The typical transmit current is 40.5 milliamps, and while this is roughly the same as many other radios, the time the transmitter is on is very short. The typical receiver current is 11 mA, which is significantly lower than for other radios and reflects the simplicity of the RF IC architecture.

The Inverted C Antenna, or ICA, is a horizontally polarized differential antenna that Motorola created specifically for the Nintendo wireless gaming adapter. Its footprint is 27 x 9 mm.

The antenna used in this solution is a new de sign from Motorola's Microwave Technologies Research Laboratory dubbed the Inverted C Antenna. The ICA, which was created specifically for this chip set, is a printed antenna that has a fairly small footprint at 27 x 9 mm, including the keep-out areas. The small size enabled Nintendo to create a small, unobtrusive accessory that easily connects to the Game Boy Advance.

The ICA is a horizontally polarized differential antenna. The radiated pattern is fairly omnidirectional. Efficiency is 75 percent, and gain is greater than 3 dBi in the Advance Game Boy wireless adapter.

In summary, the chip set has the optimum combination of low complexity, low latency and low power necessary for a wireless link in the mobile gaming environment.

Kurt Southworth (kurt.southworth@motorola.com) is a product manager in the radio products division of Motorola Semiconductor Products Sector's wireless and mobile systems group (Phoenix).