Tuesday, July 24, 2007

[Wireless] Avis Connect - Auto Wi-Fi

Last Saturday in San Jose's International airport, I found Avis (a rental car company) is providing a service called auto Wi-Fi. The device looks like a cable modem with two antennas. It says then you can connect to the Internet everywhere you go. The price is $10.95 per day, which is really cheap compared with other paid Wi-Fi services. I'm a little surprised how it works to provide Wi-Fi service when driving, because Wi-Fi is just a short range system and needs to connect itself to a 'real' Internet access network. Mobile WiMAX can connect to Internet direclty but as I know currently the WiMAX network is still under construction.

When I came back home, I checked the Internet and found the answer: it is a combination of a Wi-Fi router and a high-speed cellular modem. The access to the Interent uses a patent-pending, enhanced-reception technology from Autonet Mobile, which roams among existing 3G cellular networks and can switch seamlessly between neighboring networks and high/low-speed connections as needed without interrupting the session. This idea is pretty smart.

Autonet promises the service will work on 95 percent of U.S. roads in the future. It sounds pretty good. Next time for traveling by car, I will have a shot and check how the service is going.

Thursday, July 12, 2007

[WiMAX] Fundamentals of WiMAX - (5)

In this post, let's take a quick look of the standardizations of WiMAX.

IEEE 802.16 Group was formed in 1998, its inital focus is to develop an air-interface standard for a LOS-based point to mulitpoint wireless broadband system working in 10G-66GHz band. In 2001, the original 802.16 standard was completed. After that, the task group produced 802.16a, which is an amendment to 802.16. This 802.16a is for NLOS applications in the 2G-11GHz band. Further revision is IEEE 802.16-2004 (fixed WiMAX), which replaced all previous specifications and formed the first version of WiMAX standard. It targeted fixed applications. In order to support mobile services, in 2005 a new amendament to IEEE 802.16-2004 called IEEE 802.16e-2005 (Mobile WiMAX) was approved by IEEE.

In short, WiMAX uses OFDM in the physical layer. The difference of the two specifications is: IEEE 802.16-2004 uses fixed-FFT-size OFDM, but IEEE 802.16e-2005 uses scalable OFDMA. Therefore, for fixed WiMAX, because FFT size is fixed, the subcarrier spacing will increase with larger bandwidth, and then the symbol time will decrease. It implies that a larger portion of OFDM subcarriers is needed as guard time to overcome delay spread of the channel, which means spectral efficiency is lowered. In the contrast, for mobile WiMAX, the subcarrier spacing is fixed but the FFT size is adjustable. This scheme can be used to balance to requirements of delay spread and Doppler spread of the channel for different operating environments.

In addition to OFDM, in physical layer WiMAX also supports some optional advanced techniques for increasing the link performance and reliability. These techniques include powerful FEC - Forward Error Correction coding (such as Turbo coding and LDPC - Low Density Parity Check code, which is very close to the Shannon channel capacity), channel interleaving, multiple antenna techniques - MIMO (please refer to Fundamentals of WiMAX - (3) ) and adaptive modulation. From here you can see that WiMAX almost integreted the most popular and advanced techniques of digital communications together. This is an advantage of creating a new system, because you do not have too much back-compatibility issues to take into account.

Another big portion of WiMAX's standard is specifying the MAC layer functions between the lower physical layer and the upper network and transport layers. The MAC layer should be able to accommodate different traffic types, support QoS (Quality of Service), provide security and mobility and so on. Those are just common functions and are considered in almost every wireless system. The distinct part of WiMAX's MAC layer from other similar systems is on the scheduling and channel-access schemes. In downlink, the Base Station allocates dedicated or shared resources periodically to each subscriber station. Then each MS will use this allocated rresource to request bandwidth. This procedure is called Polling. Polling can be done with unicast or multicast. In uplink, WiMAX specifies a contention access and resolution scheme for multiple MSs accessing the shared resource. If one MS already has an allocation for traffic sending, it will not be polled. But it can be allowed to request more bandwidth.

Thursday, July 5, 2007

Is iPhone a phone?

The long rumored iPhone has finally arrived. Someone tore down the iPhone and found the processor is using ARM chips, the ROM and RAM are from Samsung, and the touch screen is made by Epson Imaging Device Corp. They estimated the hardware cost of 8GB iPhone will be about $270, which means the margin is about $330. Of course the margin cannot be that high, because the development cost is not taken into account. But it can be anticipated that the profit will be considerable.

So many reviews have been posted on the Internet officially by some evaluation groups or personally by Apple fans after June 29. No doubt some Apple fans shouted acclaims and some others attacked its shortcomings severely. I cannot afford buying an iPhone, but I'll try to give my two cents here based on the reviews I have read recently.

Basically, iPhone is not just a cell phone, it is even not just a smart phone. It innovates the UI (user interface) in a revolutionary way, which makes it so distinct. Although it lacks some functions that some ordinary handsets or smart phones are supporting, such as no voice dialing, no speed dialing, no games, only supports GSM and EDGE etc, its defects cannot obscure the virtues. The reason is that Apple does not have too much experience and technique accumulations on cell phone design, but it grows fast.

Shortly put, the applications of iPhone can be enhanced, the bugs can be fixed, and the services can be enriched gradually. They will not be problems as long as people have belief on it. Especially from the view of the psychology, Americans love to see a burgeoning force challenges the conventional world and finally gets the success (Google is a lively example). Of course, those big guys in handset industry will not await their dooms. They will fight back definitely. For customers, this will always be a good news. For Apple, the next generation iPhone will be critical and crucial. People can forbear the weakness even flaw of the new product, but cannot tolerate them again. I guess Apple alreay noticed this kind of issue. So, we'll see.