Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Adaptive Frequency Hopping for FHSS SystemsDate Submitted: March 13, 2001 Source: Oren Eliezer & Dan Michael Company: Texas Instruments Israel Address: POB 5133, Kfar-Saba 44150, Israel Phone: +972-9-7476962, Fax: +972-9-7430951, E-Mail: OrenE@ti.com Re: Non-collaborative means for enhancing coexistence performance of FHSS systems (802.15-01/092) Abstract: In continuation of 802.15-01/092 (presented in January), where various principles for coexistence performance enhancement were presented, this presentation provides further detail, some parameter values, and the principles of a proposed algorithm for the realization of adaptive frequency hopping. A future revision of this will provide a description of the proposed algorithm in greater detail. Purpose: This submission is to be considered by the committee in its process of approving non-collaborative coexistence mechanisms, subsequently to become part of the recommended practice set forth by Task Group 2. Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Oren Eliezer & Dan Michael, Texas Instruments

2. IEEE 802.15 TG2Adaptive Frequency Hoppingfor FHSS systemsHilton Head Island March 13, 2001 Oren Eliezer & Dan Michael Texas Instruments Israel OrenE@ti.com Oren Eliezer & Dan Michael, Texas Instruments

3. Outline • Properties of the optimal hopping sequence • Number of hopping channels • Frequencies to be selected for the hopping channels • Order of frequencies within the sequence • Length of the hopping sequence (cycle) • The two regulatory approaches to adaptive FH • The adaptive frequency hopping algorithm (AFH) • Realization implications in Bluetooth Oren Eliezer & Dan Michael, Texas Instruments

4. What are the optimal properties for the hopping sequence in an adaptive frequency hopping system? Oren Eliezer & Dan Michael, Texas Instruments

5. What is the optimal number of frequencies to be used assuming they are selected wisely? • If too many frequencies are used: • A greater portion of the available band would suffer interference, so it becomes more difficult to place other users of the band in clear portions of it (example: an 802.11b wireless LAN will not be able to avoid a Bluetooth system using 79 hopping channels of 1MHz bandwidth). • The FHSS system is also more likely to suffer interference from other users of the band. • If not enough frequencies are used: • Potential frequency diversity gains (in terms of multipath fading) may be lost. • Interference caused at a specific channel might be too significant in terms of instantaneous packet-error-rate (user-perceived performance degradation is application dependent). • The chances of a large portion of the channels being simultaneously interfered with are higher. Oren Eliezer & Dan Michael, Texas Instruments

6. Example of Band Occupancy in the Absence of Bluetooth Power 802.11b microwave oven cordless Frequency Oren Eliezer & Dan Michael, Texas Instruments

7. With Current Bluetooth Band Occupancy Power 802.11b microwave oven cordless Frequency Oren Eliezer & Dan Michael, Texas Instruments

8. Band Occupancy with Fewer Hopping Channels - co-existence with other systems is enhanced! Power 802.11b microwave oven cordless Frequency Oren Eliezer & Dan Michael, Texas Instruments

9. What are the optimal frequencies to be used by a frequency hopping system? • The optimal hopping channels are those that suffer the least amount of interference, and also cause the least amount of it to neighboring systems (the latter is harder to determine if the interfered device uses different frequencies in the two directions of its communications). • Interference could be local (possibly suffered by one end only), and the application may be asymmetric in its sensitivity to interference (e.g., wireless MP3), so frequency use does not have to be symmetric (i.e. a different hopping sequence may be used in each direction). • It might be preferable in certain applications or scenarios to not utilize asymmetry in the hopping channel selection (if a TX frequency of the hopper interferes with another user at that frequency, this might not be noticed unless the frequency is also used in the RX sequence and is interfered by that user). Oren Eliezer & Dan Michael, Texas Instruments

10. What is the optimal order for frequenciesin a hopping sequence? • Orthogonality between two different sequences, intended to minimize mutual interference, does not necessarily require pseudo-randomness. • Consecutive transmissions to a specific device should preferably be at distant frequencies, to allow for frequency diversity and reduce the chances of two consecutive packet failures. • If a specific frequency is used by the two ends of the communications system, it might as well be used in consecutive (strongly correlated) time slots, to more easily support antenna diversity in both directions (based on multiple antennas at one end only). Oren Eliezer & Dan Michael, Texas Instruments

11. What is the optimal period (cycle) for the hopping sequence? • A shorter period ensures faster acquisition • A shorter period allows faster adaptation in the presence of another frequency hopping system having the same slot timing (e.g. Bluetooth-Bluetooth coexistence) • If fast and simple acquisition is desired, and all hopping channels are to be used evenly, the length of the hopping sequence needs to be an integer multiple M of the number of hopping frequencies used. • The shortest period possible is equal to the number of hopping channels used (M=1). • It helps to have a prime number P of hopping channels, so that if a certain type of device is served every k slots (hops), then all P channels would be used with that device before a channel is repeated (P and k should not have a common devisor). Oren Eliezer & Dan Michael, Texas Instruments

12. Short Cycle System 1 System 2 Repetitive collisions at frequencies shared among the two hoppers lead to quicker replacement of them. Oren Eliezer & Dan Michael, Texas Instruments

13. Longer Cycle System 1 System 2 A “bad” frequency does not always appear as such, and will not be replaced as quickly. The situation of lower rate of collisions observed (2/15) above is only temporary. There’s drift… Oren Eliezer & Dan Michael, Texas Instruments

14. Same Longer Cycle After Drift System 1 System 2 The rate of collisions observed at this phase is 6/15. Oren Eliezer & Dan Michael, Texas Instruments

15. Conclusions Regarding Short Cycle Time • The averaged rate of collisions for two non-synchronized systems using the exact same set of N frequencies will always be 1/N, but the instantaneous collision rate changes according to the order of the frequencies in the coexisting sequences. • Having an equally short cycle, combined with identical slot times (as in Bluetooth), allows for capacity increase through the use of shared frequencies (to be replaced only once drift brings them together). • A short cycle (sequence length = number of hopping channels) ensures that collisions at the shared frequencies are repetitive, which results in their quick replacement. Oren Eliezer & Dan Michael, Texas Instruments

16. The Two Regulatory Approaches to AFH • Must the transmission comply with the current wording in FCC’s 15.247 for high power devices (use at least 75 hopping channels evenly, thus contaminating the entire band)? Or: • A reduction in hopping channels (or similar “violations”) can be assumed, if the associated benefits are proven, and they are in the spirit of the FCC’s regulations, targeting fair and maximized use of the spectrum amongst multiple users. This proposal follows the latter and selects 23 as the optimal number for 1MHz hopping channels. Oren Eliezer & Dan Michael, Texas Instruments

17. The AFH Algorithm Oren Eliezer & Dan Michael, Texas Instruments

18. Basic Principles (1/2) • Packet failure is detected (binary value) in every device (master and slaves), and the PER for each frequency received by it is monitored separately. • Distinction between packet loss due to propagation losses and that caused by interference (based on RSSI) is desirable, since they might require different treatment (primarily for power control). • No payload BER measurements, which are more difficult to handle, are not always available or statistically reliable, and offer little benefit in interference avoidance. • Frequency replacement policy is programmable and therefore flexible, so it can be set differently for different applications, having different needs (voice probably being the most sensitive application). Oren Eliezer & Dan Michael, Texas Instruments

19. Basic Principles (2/2) • Both the master and a slave could initiate a frequency replacement in their reception sequence (the hopping sequence used by the other party’s transmitter). • Can accommodate asymmetric use of frequencies, which is helpful when the interference suffered at one end is not identical to that of the other (e.g. in separate distant rooms). • Uses same (or close) frequencies in consecutive TX-RX slots, to better exploit antenna diversity in both directions based on a dual-antenna radio in the master only (best antenna for RX would also be the best one for TX). transmission from slave at frequency k Master with antenna-selection diversity next transmission from master at frequency k or near it Slave Oren Eliezer & Dan Michael, Texas Instruments

20. System Parameters (1/2) • channel failure counter (for each frequency) – when this counter reaches the threshold value, then the corresponding channel is marked as a “bad” frequency and put into a “need to replace” queue. • threshold – the minimal count to be reached in the channel failure counter before a frequency replacement request is invoked for the corresponding channel (the threshold value should correspond to the desired QoS, which is application dependent). • increment(INC) – the number added to the channel failure counter when a packet failure is detected in its channel. • decrement (DEC) – the number subtracted from the channel failure counter when a packet is successfully received at that channel (with the counter’s lower bound being zero). Oren Eliezer & Dan Michael, Texas Instruments

21. System Parameters (2/2) • penalty (PEN) – the additional increment (penalty) to be added to the channel failure counter when a predetermined event occurs, intended to accelerate the replacement of those channels causing the more noticeable performance degradation in terms of user perception. • Example of such event: Two consecutive reception failures in a specific device (due to two consecutive interfered channels in the hopping sequence) could be more harmful to certain applications (e.g. voice quality in a telephony device vs. data throughput in file transfer). In such case, it is desirable to quickly improve the situation by accelerating the replacement (reaching threshold in fewer steps). Oren Eliezer & Dan Michael, Texas Instruments

22. Channel Replacement (1/2) • When a frequency is found in the “need to replace” queue, after its counter exceeded the threshold value, then a Change_Freq(Bad_Freq, New_ Freq) message is sent, whereas: • Bad_Freq = the Bad Frequency Number (5 bit) to be replaced • New_Freq = the New Frequency Number (5 bit) to be used instead • The initiator of the message may replace the frequency immediately after sending the message. • The recipient of the message will replace the frequency immediately after receiving the message, and will send the Accepted acknowledgmentmessage. • The frequency change procedure will end after successful reception of the Accepted message, at which time the next frequency replacement request is allowed to be sent. Oren Eliezer & Dan Michael, Texas Instruments

23. Channel Replacement (2/2) • The initiator will not initiate another frequency change till the previous frequency change procedure has ended. • During the replacement procedure the two parties will still be communicating through at least 22 frequencies, and at the end of the process all 23 frequencies will be aligned. • The replacement scheme is robust, since only one change is done at a time! The communications cannot be lost as a result of possible failure in the replacement process. • Since the AFH can be asymmetric, the frequency replacement procedure in a device will affect the frequencies in one direction only (the direction of transmission to it). Oren Eliezer & Dan Michael, Texas Instruments

24. Realization Implications in Bluetooth • Complexity: • The proposed mechanism is simple, and therefore requires an insignificant amount of logic and development time. • If backwards compatibility with legacy devices is required (a 79-channel hopping sequence), the original complex sequence generator needs to be implemented too, being dominant in gate count. • Simultaneous support of legacy devices and enhanced devices by a single master: • Run both hopping sequence machines with same clock. • Toggle between the two in the selection of the relevant hopping channel, depending on type of device addressed by the transmitted packet (known to the master based on indications sent and acknowledged between the master and all its slaves). Oren Eliezer & Dan Michael, Texas Instruments