Today AT&T launched its 4G LTE network in five cities. While this is a big step for AT&T, its main competitor, Verizon Wireless, already launched its 4G LTE network almost ten months ago. Verizon’s 4G LTE network now has service in 143 markets and covers over half the US population. Wireless carriers that utilize EVDO technology for 3G services such as Sprint-Nextel and Verizon Wireless have been the first to move to 4G technologies as EVDO offers slower theoretical speeds than the competing HSDPA+ technology. A single EVDO rev. A channel’s theoretical speed is 3.1Mbps in the downlink and 1.8Mbps in the uplink while a single HSDPA+ (not utilizing channel bonding or MIMO) channel can offer 21Mbps in the downlink and 5.8Mbps in the uplink. In real word applications the speed advantage for HSDPA+ is much less because HSDPA+ carries voice calls over the same channel which reduces the data speeds. Verizon Wireless and AT&T have always used different network technologies with Verizon choosing the CDMA (3GPP2) path while AT&T electing the GSM (3GPP) route. While both technologies offer their share of advantages and disadvantages, going forward both companies will use the same network technology – Long Term Evolution or LTE.
LTE is not a one size fit all technology, but instead a technology that allows for a variety of different configurations which greatly impact how it is deployed and its performance. Both AT&T and Verizon Wireless utilize frequency division duplex (FDD) mode which means that the upload and download channels are on two separate frequencies. LTE also offers the capability to use time division duplex (TDD) which allows for both the download and upload channels to use one frequency with the download and upload being allocated different time slots.
Another similarity between AT&T and Verizon Wireless is that they are both utilizing 2×2 MIMO antenna technology. While LTE supports MIMO is an extremely complicated topic, but basically it allows double the amount of data to be transferred in a single channel by utilizing two transmit and receive antennas instead of one.
LTE release 8 supports the options for one, two, or four antenna configurations where the highest performance is achieved utilizing a 4×4 MIMO solution. Almost all wireless carriers are choosing the 2×2 MIMO route as it offers the best performance/price ratio. To go with a 4×4 MIMO solution over that of 2×2 MIMO means that double the number of antennas and amplifiers are needed along with more powerful processors in mobile handsets and base stations to decode the additional data streams. Additionally according research by Ericsson Communication, MIMO only provides performance improvements when a receiver has a signal to noise ratio (SNR) of approximately 10dB or better. This means that MIMO is most beneficial when a user is close to the cell site, which for most cell sites is only a small percent of the users.
The main difference between AT&T’s and Verizon Wireless’ 4G LTE network is the bandwidth that each channel uses, and this is based on the spectrum allocation that both companies own. AT&T is using a mixture of 10MHz and 5MHz channels while Verizon Wireless is solely using 10MHz channels. In areas where AT&T uses 5MHz channels, Verizon Wireless’ network will theoretically offer double the performance of that of AT&T’s. Theoretically a 10MHz channel utilizing 2×2 MIMO supports peak downlink data rates of 73Mbps while a 5MHz channel will only support 37Mbps. As with any wireless technology reaching anywhere near these theoretically numbers is extremely unlikely. AT&T knows that its 5MHz channels will put it at a large capacity and speed disadvantage compared to Verizon Wireless, so in markets where it has both 700MHz and AWS spectrum it will try to utilize two 5MHz channels instead of just one. The two channels become beneficial when a large number of users are on the network and the load is distributed across two 5MHz channels instead of crowding everyone into one 5MHz channel. Currently, these two channels can’t be bonded for higher throughput, but this technology will come available in LTE Advanced and is known as carrier aggregation.
The final difference between the two 4G LTE networks is the base station radios used. A growing trend in the wireless industry is to mount the base station’s radio and amplifier at the top of the tower. This is known as remote radio heads (RRH) and this technology minimizes the cable attenuation experienced by an antenna system. In traditional base station deployments, the radios and amplifiers are mounted on the ground, where they can be easily upgraded and repaired, and thick coax runs up the tower to the antennas. The issue with this is that long runs of coax cable experience attenuation. According to this spec sheet, 100FT of 1 ¼ coax cable has a loss of 1.6dB or roughly 31% less power at the top of the tower compared to when the signal left the amplifier. Clearly reducing the antenna’s output power by 31% not only reduces coverage and degrades downlink throughput, but it also affects the uplink. The antenna at the top of the power sees 31% more power from the mobile handset that what actually makes it to the base station. This result in decreases coverage, reduced uplink throughput, and diminished battery life for handsets. By mounting the radio and amplifier at the top of the top the 1.6dB cable loss is practically eliminated, greatly improving performance over traditional base station deployments. Instead of running thick coax up the tower, a much thinner combined fiber optic cable and power cable are run to each remote radio head.
AT&T 700MHz LTE RRH
Clearwire’s 4G WiMAX network was the first wireless operator to solely use RRH and they can be easily spotted by the large boxes connecting to the antenna. AT&T is following in Clearwire’s footsteps by primarily using RRH for its LTE deployment. Currently RRH can only support one technology and frequency band, so with AT&T dual frequency (700MHz and AWS) LTE deployment this means two RRH are needed for each face of the tower (most towers have three faces). Most of the Verizon Wireless tower I have observed do not have any RRH mounted, so it is probable they are going with the conventional base station deployment model with the radios and amplifiers mounted at the bottom of the tower. The benefit of this solution is that equipment can be quickly and easily repaired and upgraded while staying protected from the outdoor elements. When a RRH goes bad or needs to be upgraded it requires someone to climb the tower which is time consuming and can become very costly. Given that RRH technology is still very new, it will take time to see whether the performance gains of RRH make up for the limitations in repair and upgradability.
Overall, the technology is similar for both Verizon Wireless’ and AT&T’s LTE networks. While the technology might be similar, Verizon Wireless is the clear leader already having half the US population covered compared to just five cities for ATt&T. In the end regardless of whether one chooses Verizon Wireless or AT&T, US consumers are the true winners having access to multiple advanced 4G LTE networks.