StarPlus Hybrid Approach to Avoid and Reduce the Impact of Interference in Congested Unlicensed Radio Bands
EION Wireless Engineering: D.J. Reid, Professional Engineer, Senior Systems Architect
In licensed frequency bands, use of synchronization is accepted as a preferred method to deal with interference problems associated with; numerous base stations and CPEs. Generally speaking, competing operators respect each other’s “space” and frequencies by implementing frequency planning techniques, to reduce the effects of interference. In WiMax implementations, operators prefer to use GPS based synchronization methods. In short, GPS synchronization allows all base station radios to transmit simultaneously and listen for CPE receive signals when the base station is not transmitting. It is the high level of frequency planning and signal synchronization inherent in WiMAX system implementation which provides for an order and less disruptive communication methodology.
In unlicensed frequency bands, the rules for communication are not so clear. Frequency planning is typically not implemented. Power levels are also not usually coordinated. Channel bandwidths are not coordinated. Some systems may use bandwidths as narrow as 3.5 MHz, while others may use much larger bandwidths. Given that in most cases, there is a mix and mash of disparate radio systems operating in dissimilar fashion, the rules for “playing nice” are usually non-existent. This usually makes it impossible to synchronize communications effectively.
There are other inherent problems, some of which include; fixed WiMAX 802.16d, systems use fixed polling of CPEs for traffic often with synchronization, while WiFi, 802.11a and 802.11n systems, use CSMA-CA – and employ on demand schemes to serve CPEs where the CPE will transmit a Request to Send whenever a quiet slot appears on the frequency. On a larger scale, when there are unsynced stations, whether WiFi or other systems, base station or AP and CPE transmissions are quite random and in-system collisions, as well as collisions with same-space 802.16d synchronized signals, are inevitable. Given these factors as well as years of experience in both the licensed and unlicensed wireless communications, EION Wireless would like to proceed as documented in the next few pages.
Hybrid Solution for overcoming Adjacent and Co-Channel Interference in Crowded Unsynchronized Radio Bands
Interference consists essentially of two major elements. The first is from adjacent channels activity affecting the performance of a radio system when standard isolation techniques are notadequate. The second is from co-channel interfering signals that directly impact the desiredradio system. To successfully mitigate the effects on a service both adjacent and co-channel interference must be addressed with appropriate techniques. EION’s solution uses a hybrid approach through application of segment filters combined with a Trade Marked, robust protocol, working in tandem - to reduce the overall detrimental impact on channel availability and system performance.
Techniques Reducing Adjacent Channel Interference problems
Even though the offending station(s) are not on the same channel, the broadband design of contemporary radios makes them susceptible to high-level adjacent channel signals. The mechanism that creates the interference is the Low Noise Amplifier or LNA in the analog front end of the receiver. This is very sensitive and covers a wide frequency range. As a direct result any signal(s) within the band the amplifier covers if too high in level will overload the amplifier resulting in the creation of spectrum by-products that will interfere with the desired signal also being received by the radio but at a much lower level. The spectral by products disrupt the desired signal and at the same time produce inter-modulation “noise” which contributes to the morass of distorted signals making the carrier to interference ratio unacceptable ( C/I < 6 dB).
Much of the offending signals that will impact a radio’s performance are usually located in the immediate vicinity of the Base Station being impacted. In fact it can be the other operators’ radios or sector radios in the same network mounted on the same tower. Several methods used singly or in combination can effectively reduce adjacent channels levels to an acceptable level.
Radio-Antenna Placement, Antenna Alignment and Frequency Co-ordination
Before adding hardware such as filters to an installation, several tactics should be employed as standard operating procedure or SOP in the establishment of a base radio. In order of application the radio and antenna placement, antenna alignment and frequency co-ordination should be done to ensure minimal adjacent channel levels impinging on the radio. These techniques have been field tested and demonstrated a number of times successfully eliminating any impairments from adjacent channel radio systems co-located on the tower.
Separation of at least 1 to 2 meters between antennas is the first tactic to improve C/I levels. This effectively reduces the impact of interfering side lobes from the adjacent antenna. Also use of sector antennas in stead of Omni –directional units greatly increases isolation. Alignment is another important consideration. The sector antenna constricted antenna pattern can be utilized to null out the side lobes of adjacent channel radiation from another close by antenna. These tactics will be able to mitigate most interference when used along with good performing radio with good IF channel response.
Though difficult to do in unlicensed bands, ensuring that there is no co-located radio on using an immediately adjacent channel is good avoidance tactic to prevent excessive C/I. This requires at least one channel spacing between the co-located radios frequency assignments. In doing this, both radios benefit with less in band interference occurring. The adjacent channel C/I has to be quite low ( 3 dB or less) before interference to be significant. Weaker adjacent channel interference form more distant co-located radios should not cause any significant issues
Using filters to block out the adjacent signals for example effectively eliminates the impact of co-located radios or near-by Base Stations operating in the same band. The filters in deployment split the operating radio band into segments so interfering signals can be effectively blocked (>30 dB stop-band) from impeding the desired weaker signals from remote CPEs. The use of channel or segment filters is common in microwave systems and filters are available with the required characteristics. This is a tactic reserved for difficult situations where all other tactics have failed to adequately reduce interference and can be used uniquely or in combination with other tactics.