Introduction

Getting multiple radios communicating in the same device can be very challenging. This document describes a few coexistence scenarios that we have seen recently, and the steps taken to help mitigate them.

Coexistence challenges between Cellular and 2.4GHz Wi-Fi/Bluetooth are inherent in the technology due to the frequency bands in which they operate. These challenges exist regardless of the specific chipset/module implementation.

Scenarios:

Scenario 1

Description

When the LTE antenna is close to the Wi-Fi antenna (around 30dB of isolation) and the LTE radio is transmitting on Band 12 (707.5 MHz) the LTE signal is received by the Wi-Fi antenna. This will cause the radio to generate a 3rd harmonic at around 2.1 GHz which could potentially cause the LTE module to fail certification due to spurious emissions. This issue can be present even if the Wi-Fi radio is turned off and would be an inherent problem exhibited by any 2.4 GHz Wi-Fi radio.

Possible Effects

  • The spurious emissions could cause the device to fail in-country cellular carrier and/or EMC regulatory certification
  • Problems with interference and Packet Error Rate on either the Wi-Fi or the Cellular radio may also be present.
  • Band 12 is used by all 4 major US carriers as well as Bell and Rogers in Canada.

Addition of an L-C Notch Filter to reject the unwanted frequencies on the RF path will solve this issue. If a customer uses an LTE module in their system, adding the L-C notch filter is a requirement in order to pass carrier certifications. This holds true for all designs incorporating any 2.4GHz Wi-Fi solution including the Sona IF573.

image

Scenario 2

Description

The Wi-Fi radio may exhibit issues with sensitivity when the Wi-Fi and LTE signals are co-located (around 20dB of isolation) and are operated simultaneously.

Possible Effects

  • The device could see problems sending or receiving 2.4 GHz (802.11 b/g/n) Wi-Fi data while the cellular radio is transmitting. This would likely result in an increased Bit Error Rate or lost Wi-Fi packets decreasing total throughput. These effects are reduced if the Wi-Fi and LTE radios only transmit simultaneously on an infrequent basis or are generally mutually exclusive in their operation.
  • This issue only occurs in LTE bands whose Uplink frequency is located between 1800 MHz and 2700 MHz. In our testing it occurred in LTE Band 1 Uplink (1920-1980 MHz), LTE Band 2 Uplink (1850-1910 MHz), and LTE Band 7 Uplink (2500-2570 MHz).
    • Band 1 is currently used by T-Mobile, Orange, O2, Vodafone, 3, EE and others in Europe and Asia.
      • Band 1 is currently not used in North America.
    • Band 2 is currently used by AT&T, T-Mobile, Verizon Bell Canada and others in North America.
      • Band 2 is currently not used in Europe or Asia.
    • Band 7 is currently used by T-Mobile, Orange, O2, Vodafone, 3, EE, China Telecom, Chunghwa, and others in Europe and Asia.
      • Band 7 is currently not used in North America.

Hardware Solution

  • A hardware only solution would be to add a diplexer to the RF path to split the 2.4 GHz signal from the 5 GHz and 6 GHz signals, then insert a BAW filter on the 2.4 GHz chain. An additional diplexer would be needed to merge the 2.4 GHZ back to the 5 GHZ and 6 GHz signals.

image

  • This solution will reduce the Wi-Fi TX power and RX sensitivity by 2dB due to the added loss of the components.
  • The 2.4 GHz chain could remain split from the 5 GHz and 6 GHz signals by not adding the 2nd diplexer This would then minimize the 2dB loss on the 5 and 6 GHz signals however it would require the addition of separate antennas for each band.

Software/Application Solution

  • Design the SW architecture to mitigate the likelihood that both 2.4GHz Wi-Fi & Cellular will transmit simultaneously and buffer Cellular or Wi-Fi data accordingly.
  • Focus design around TCP-like (vs UDP) protocols to ensure ACKs and error checks on data transmissions. Any coexistence issues with 2.4GHz Wi-Fi exhibited while the Cellular radio is transmitting will subside/cease to exist the moment the Cellular radio stops transmitting.

Effectively, for infrequent and non-streaming Wi-Fi/Cellular transmission use-cases, when ACKs are implemented using the TCP protocol, the system will recover quickly. In many customer applications, the perceived use-case issues will be transparent to the end-user.