HSDPA Antenna Fine Tuning

In previous blog entries on HSDPA (for links see below), I’ve been looking at HSDPA performance in operational networks from several different angles. Todays entry focuses on how sensitive an HSDPA card reacts to changes in antenna orientation.

For my tests, I’ve been using a Sierra Wireless 850 PCMCIA notebook HSDPA adapter. The card has a maximum throughput of 1.8 MBit/s and has an external antenna which can be swiveled back and forth at the side of the notebook and also tilted 180 degrees to and from the notebook. This design is not new and all Sierra Wireless cards I’ve seen so far use this kind of antenna.

While antenna adjustments seem to have no big impact on GPRS performance, a huge difference can be observed with HSDPA during medium reception conditions (3 out of 5 bars on the RX meter). As shown in the graph produced with Wireshark on the left, throughput during a file download varied dramatically between 50 kbyte/s and 150 kbyte/s depending just on the antenna position and orientation. I have to admit that I was quite surprised by this result. I tried again a couple of days later just to ensure that this was not a freak occurrence but I could easily reproduce the behavior again.

Users will probably not spend a lot of time experimenting with antenna position and orientation whenever they log on to the Internet. Thus, I think HSDPA card manufacturers have to work hard on antenna and receiver design in order to reduce such effects in the future. 3GPP standards already give some guidance for the work ahead as the standard describes optional HSDPA terminal features such as multiple antenna designs and advanced receiver algorithms for more throughput and to counter these effects. For people using the HSDPA network in a stationary or semi-stationary setup, for example with a 3.5G to Wifi bridge, I highly recommend using an external antenna or at least to place the bridge close to a window.

Designing notebooks with built in HSDPA chips might also help to reduce this effect. In such a setup the antenna can use space inside the notebook which means antennas can be bigger and thus more sensitive and less susceptible to radio interference effects.

In the next part of this article series on HSDPA, I will discuss the packet inter-spacing graph for the scenario presented here and what can be deducted from it about layer 1 air interface MAC behavior and performance.

More on my recent HSDPA experiences:

• A real live comparison of UMTS and HSDPA
• A peek into the UMTS air interface with Wireshark
• A peek into the HSDPA air interface with Wireshark