ESB: retransmit_delay setting

Question

Hi, In my project we are using the ESB between a nrf52811 and a NRF5340dk and are trying to evaluate the maximum speed we can reach with ESB packet transmission without acknowledge. 
 
 For this we have the following configuration of the ESB an ptx code: 
 
 int esb_initialize(void)
{
 int err;
 /* These are arbitrary default addresses. In end user products
 * different addresses should be used for each set of devices.
 */
 uint8_t base_addr_0[4] = {0xE7, 0xE7, 0xE7, 0xE7};
 uint8_t base_addr_1[4] = {0xC2, 0xC2, 0xC2, 0xC2};
 uint8_t addr_prefix[8] = {0xE7, 0xC2, 0xC3, 0xC4, 0xC5, 0xC6, 0xC7, 0xC8};

 struct esb_config config = ESB_DEFAULT_CONFIG;

 config.protocol = ESB_PROTOCOL_ESB_DPL;
 config.retransmit_delay = 20;
 config.bitrate = ESB_BITRATE_2MBPS;
 config.event_handler = event_handler;
 config.mode = ESB_MODE_PTX;
 config.selective_auto_ack = true; 
 config.retransmit_count = 0;
 config.crc = ESB_CRC_8BIT; 
 config.use_fast_ramp_up = true;

 err = esb_init(&config);

 if (err) {
 return err;
 }

 err = esb_set_base_address_0(base_addr_0);
 if (err) {
 return err;
 }

 err = esb_set_base_address_1(base_addr_1);
 if (err) {
 return err;
 }

 err = esb_set_prefixes(addr_prefix, ARRAY_SIZE(addr_prefix));
 if (err) {
 return err;
 }

 return 0;
}
 
 int main(void)
{
 int err;

 LOG_INF("Enhanced ShockBurst ptx sample");

 err = clocks_start();
 if (err) {
 return 0;
 }

 // err = dk_leds_init();
 // if (err) {
 // LOG_ERR("LEDs initialization failed, err %d", err);
 // return 0;
 // }

 err = debug_pin_init();
 if (err) {
 return;
 }

 err = esb_initialize();
 if (err) {
 LOG_ERR("ESB initialization failed, err %d", err);
 return 0;
 }

 LOG_INF("Initialization complete");
 LOG_INF("Sending test packet");

 tx_payload.noack = true;
 while (1) {
 err = esb_write_payload(&tx_payload);
 tx_payload.data[1]++;
 while(esb_tx_full());
 }
}
 
 We have set some debug pin to monitor the Tx and Rx and the ESB configuration is similar for ptx and prx ( except on is in PRX mode ans the other in PTX mode). 
 
 We have done test with SDK v2.5.0 and SDK v2.9.0 
 
 We don’t observe what has been said in the documentation. We have configured the CONFIG_ESB_NEVER_DISABLE_TX to yes and when we change the value of the retransmit_delay its value is not linked to the delay between two consecutives transmissions. Here are the value of delay between two transmission according to the rentransmit_delay value. Why?

Retransmit_Delay

Delay between 2 Tx

20

113

40

113

50

Only One Tx

Here is what is said in the esb.h of the SDK. 
 uint16_t retransmit_delay; /**< The delay between each retransmission of
 * unacknowledged packets.
 * If the CONFIG_ESB_NEVER_DISABLE_TX Kconfig option is enabled,
 * this is the delay between two consecutive transmissions.
 * Depending on the reception processing time, a minimal
 * value might be required (for example, a typical value
 * for 32-bit payload is 20 &#181;s).

For some values of this retransmit_delay we don’t have a continous transmission of data even if the code is a loop of transmission. When debbugging it appear that the esb_state was set to ESB_STATE_PTX_TX_ACK even if our payload was configured with the field noack set to true. And this should not be possible if we look into the start_tx_transaction function of the esb.c in the sdk. Do anyone can explain me this strange behaviour? 
 
 Globally my test shown me that the setting of this retransmit_delay is really difficult and can have many different effect on the quality of the transmission so I would like to understand how it's used in my configuration with: CONFIG_ESB_NEVER_DISABLE_TX set to yes, the fast ramp up activated and no ack requested. Best regards Alexandre