Choose 4.9 GHz airwaves

When building a wireless network, the 4.9 GHz public-safety spectrum is strongly recommended as the primary band, in order to avoid interference and maximize performance. This licensed band is available at no charge to municipalities and public-safety organizations nationwide and, in most instances, is still highly underutilized. In the 4.9 GHz band, a total of 50 MHz of spectrum is available. With a typical channel size of 10 MHz, 5 channels are available, and frequency re-use is now fairly easy to implement. When using the most recent wireless technology and when operating in line-of-sight (LOS) conditions without interference, typical sustained throughput per 10 MHz channel is about 50 Mb/s.

The 4.9 GHz band also could be augmented with unlicensed bands. While the 2.4 GHz and 5.8 GHz bands generally are very crowded and normally should be avoided, the more recently introduced 5.4 GHz band—with 255 MHz of available spectrum—is typically the best bet. Its vast amount of available spectrum enables the use of larger (20 or 40 MHz) channels, with throughput per channel of roughly 100 or 200 Mb/s, respectively.

The current generation of outdoor radios generally based on Wi-Fi (11n) chipsets and protocols. Lower-cost radio products largely will use the Wi-Fi protocols without modifications. However, one major issue with the standard Wi-Fi protocol comes into play when two or more wireless cameras stream simultaneously to a shared base station radio. The two client radios can send a video data packet in any timeslot; however, in streaming video, packets will be sent on a continuous basis, so there is a very good chance that the two wireless cameras will send their data packets at the same time, in the same timeslot. Logically, in this case, a collision will occur and the base station will not receive either packet. When this occurs, the base station will not acknowledge receipt of the packet and the wireless cameras will realize the packet is lost after a certain time lapse. The camera will then wait a random number of timeslots before retransmitting the data packet.

Although the packet eventually gets through the link, the result is an unpredictable time delay, which is not ideal at all for our killer application of steaming video. Use of a scheduled protocol mitigates this problem, by providing pre-assigned timeslots for each wireless camera to transmit its packets individually, avoiding the aforementioned collisions.

In order to build a stable wireless network for video applications, use of higher-cost radios also is recommended. Features of such radios would include a scheduled protocol, 2×2 MIMO (multiple input multiple output), synchronization for collocated radios, support of the 4.9 GHz and 5 GHz bands in a single radio, and support of full feature sets in 5, 10, 20 and 40 MHz bandwidth. Integrating the radio electronics with the antenna device also is advised, in order to avoid cable losses, which improves the receive sensitivity of a radio significantly. The use of directional antennas (in a multi-radio configuration, if needed) generally is preferred, to reduce self-interference and to focus the antenna gain—for both transmit and receive modes—specifically in the direction of the other radio end.

Building out a wireless network to reach remote camera sites stillcan be expensive or difficult. If distances are simply too long or lots of obstructions exist, making it difficult to create LOS connections between radios in the network, cellular technology could be considered as an alternative. In addition, the use of cellular technology makes a system more suitable for changes. For example, it is possible to redeploy a wireless camera unit without having to worry about adjustments to the wireless network, assuming of course proper cellular coverage.

When using cellular technology, it is important to ensure that LTE-based (4G) cellular service is available in the target area. Also, selection of an unlimited data plan is a crucial factor; for instance, at 2 Mb/s throughput, a single camera will stream about 20 GB in just one day. In our experience, at this moment, cities and other government entities are in a unique position to negotiate such unlimited plans at very reasonable monthly rates in support of their citywide surveillance systems. However, the end user should realize that resolution and frame rate may need to be compromised when using cellular technology.

Ultimately, designing a citywide video surveillance network is complex and a careful design process is required. The first step is a review of the desired camera locations, suitable camera types and bandwidth requirements. In the next step, one should match any existing fiber and networking resources. Finally, one should select the wireless solution and design best suited to reach remote sites. Over recent years, both wireless and cellular technologies have made a considerable leap forward, resulting in a multifold increase of bandwidth in support of surveillance cameras. Although wireless network bandwidth is not directly comparable with that of fiber connections, when designed correctly and when expectations are aligned with reality from the beginning, wireless citywide surveillance solutions can be developed as a powerful tool in support of public safety.