Comparison of Bluetooth and Wi-Fi coexistence solutions on smartphones

Designing consumer electronics products that include both Wi-Fi and Bluetooth functionality presents many challenges when simultaneous operation is required . Bluetooth and 802.11b/g WLAN systems operate in the 2.4 GHz band, and both technologies use a significant portion of the available spectrum (see Figure 1).

Interference occurs when WLAN devices and Bluetooth devices are in close proximity and attempt to transmit and receive wireless signals at the same time. The two technologies use different methods to transmit signals: carrier sense multiple access (CSMA) and frequency hopping spread spectrum. The former is used in 802.11b/g transceivers, which listen for an idle channel before transmitting a signal. The transmitted signal has a bandwidth of approximately 20MHz and will be transmitted on one of up to three non-overlapping channels spaced 25MHz apart.

Bluetooth uses frequency hopping spread spectrum technology to hop between 79 channels with a bandwidth of 1MHz at a rate of 1,600 hops per second, sending shorter time-division multiplexed data packets at each hopping point. When a device initiates a connection and becomes the master device of the subnet, the Bluetooth connection is successfully established. If the target address is known, the device directly sends a page message. If the target address is unknown, a query message will be sent first, followed by a page message. Once the two devices are synchronized, the two Bluetooth-enabled devices are in a connected state, and each device will be set with a unique MAC address. A simple calculation can confirm that the signal output by the Bluetooth transmitter will conflict with the 802.11b/g signal about 25% of the time.

This co-channel interference effect is closely related to the relative strength of the signals and the length and duty cycle of the data packets sent. Various analyses and simulations of this interference phenomenon have shown that interference can severely affect one or both signals. The relevant standards provide different signal design methods - direct sequence spread spectrum (DSSS) used by 802.11b, orthogonal frequency division multiplexing (OFDM) used by 802.11g, and the degree of interference rebound caused by frequency hopping used by Bluetooth. These standards also use protocols based on packet retransmission and data rate reduction. However, these countermeasures can greatly reduce data throughput, which can seriously affect the performance of some devices. For example, in Bluetooth audio transmission or VoIP on WLAN, a packet error rate of more than a few percentage points will cause unbearable audio delays or even call interruptions. The following are two methods to solve coexistence interference - AFH (adaptive frequency hopping) and AFH combined with three-line coexistence (time division multiplexing) technology.

2 AFH (Adaptive Frequency Hopping) Technology Introduction and Test Results

AFH technology is an improvement on the original Bluetooth frequency hopping sequence. It allows Bluetooth devices to reduce the number of hopping points. Its basic principle is to distinguish between good and degraded channels in the ISM band, thereby avoiding the use of degraded channels and reducing the degree of interference. When the Bluetooth piconet enters the AFH state, the number of hopping points N that can be used in its frequency hopping sequence changes dynamically, and its maximum value does not exceed 79. AFH is only used in the connection state, and will not change the frequency hopping sequence in the paging, inquiry and other states.

The implementation of the adaptive frequency hopping selection mechanism is based on the frequency selection core of the original 79-hop system (the Bluetooth 1.2 protocol stipulates that the 23-hop system is no longer used), and two parameters AFH_mode and AFH_channel_map are added on its basis.

AFH_mode indicates whether the current frequency selection core can use the adaptive frequency hopping sequence; AFH_channel_map indicates which channels are available and which are unavailable. First, the original frequency selection core generates a channel. If this channel is an available channel defined in AFH_channel_map, it will not be adjusted and will be directly used as the output of the frequency hopping sequence; if this channel is included in the unavailable channels, it will be mapped to an available channel through the relocation function. This mapping relationship is one-to-one, that is, if the Bluetooth address, clock and AFH_channel_map are given, an unavailable RF channel will be uniquely converted into an available channel, which ensures that the master and slave devices using the AFH mechanism in the same micro-network can maintain the synchronization of the frequency hopping sequence.

Under this implementation mechanism, the frequency hopping sequence of the non-adaptive 79-hop system is equal to the frequency sequence generated by the AFH frequency selection core with all signals set to available. This property makes it easy to maintain compatibility with the original non-AFH equipment.

Another change in AFH technology is that in the original frequency hopping system, the master and slave nodes use different frequencies to send data; when in the AFH state, during a master-slave dialogue, the slave node uses the same RF channel as the master node to respond to the data packet to the master node, which is called the same channel mechanism of AFH. The main reason for using the same channel mechanism is that in the case of interference in the network, reducing frequency hopping can prevent the slave node from jumping to a channel that may conflict when sending a response packet, ensuring that the data is not easily interfered with during at least one master-slave dialogue, thereby achieving the purpose of improving throughput.

Unfortunately, technologies such as AFH are designed specifically for 2.4GHz devices to detect and avoid interference and are not sufficient to achieve coexistence of Bluetooth and WLAN. AFH as a standalone technology is far from sufficient when Bluetooth and 802.11 devices coexist in the same design, mainly because WLAN devices must provide higher output power to support long-range, high-data-rate, reliable Internet, voice, data, and video transmission. Figure 2 shows a simulation diagram of a mobile phone using Bluetooth and Wi-Fi at the same time, when Wi-Fi is transmitting data and the Bluetooth headset is answering a call from a PHS.

Using AFH technology alone resulted in a 20% drop in Wi-Fi throughput and a lot of noise when answering calls on PHS. This shows that the Wi-Fi transmission of the mobile phone will interfere with the Bluetooth reception of the mobile phone.